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Banjo tutorial

Banjo is a "transpiler" (like FIDL's fidlc) — a program that converts an interface definition language (IDL) into target language specific files.

This tutorial is structured as follows:

  • brief overview of Banjo
  • simple example (I2C)
  • explanation of generated code from example

There's also a reference section that includes:

  • a list of builtin keywords and primitive types.


Banjo generates C and C++ code that can be used by both the protocol implementer and the protocol user.

A simple example

As a first step, let's take a look at a relatively simple Banjo specification. This is the file //sdk/banjo/fuchsia.hardware.i2cimpl/i2cimpl.fidl:

Note that the line numbers in the code samples throughout this tutorial are not part of the files.

[01] // Copyright 2018 The Fuchsia Authors. All rights reserved.
[02] // Use of this source code is governed by a BSD-style license that can be
[03] // found in the LICENSE file.
[04] @available(added=7)
[05] library fuchsia.hardware.i2cimpl;
[07] using zx;
[09] const I2C_IMPL_10_BIT_ADDR_MASK uint32 = 0xF000;
[10] /// The maximum number of I2cImplOp's that may be passed to Transact.
[11] const I2C_IMPL_MAX_RW_OPS uint32 = 8;
[12] /// The maximum length of all read or all write transfers in bytes.
[13] const I2C_IMPL_MAX_TOTAL_TRANSFER uint32 = 4096;
[15] /// See `Transact` below for usage.
[16] type I2cImplOp = struct {
[17]     address uint16;
[18]     @buffer
[19]     @mutable
[20]     data vector<uint8>:MAX;
[21]     is_read bool;
[22]     stop bool;
[23] };
[25] /// Low-level protocol for i2c drivers.
[26] @transport("Banjo")
[27] @banjo_layout("ddk-protocol")
[28] protocol I2cImpl {
[29]     /// First bus ID that this I2cImpl controls, zero-indexed.
[30]     GetBusBase() -> (struct {
[31]         base uint32;
[32]     });
[33]     /// Number of buses that this I2cImpl supports.
[34]     GetBusCount() -> (struct {
[35]         count uint32;
[36]     });
[37]     GetMaxTransferSize(struct {
[38]         bus_id uint32;
[39]     }) -> (struct {
[40]         s zx.status;
[41]         size uint64;
[42]     });
[43]     /// Sets the bitrate for the i2c bus in KHz units.
[44]     SetBitrate(struct {
[45]         bus_id uint32;
[46]         bitrate uint32;
[47]     }) -> (struct {
[48]         s zx.status;
[49]     });
[50]     /// |Transact| assumes that all ops buf are not null.
[51]     /// |Transact| assumes that all ops length are not zero.
[52]     /// |Transact| assumes that at least the last op has stop set to true.
[53]     Transact(struct {
[54]         bus_id uint32;
[55]         op vector<I2cImplOp>:MAX;
[56]     }) -> (struct {
[57]         status zx.status;
[58]     });
[59] };

It defines an interface that allows an application to read and write data on an I2C bus. In the I2C bus, data must first be written to the device in order to solicit a response. If a response is desired, the response can be read from the device. (A response might not be required when setting a write-only register, for example.)

Let's look at the individual components, line-by-line:

  • [05] — the library directive tells the Banjo compiler what prefix it should use on the generated output; think of it as a namespace specifier.
  • [07] — the using directive tells Banjo to include the zx library.
  • [09] [11] and [13] — these introduce two constants for use by the programmer.
  • [16 .. 23] — these define a structure, called I2cImplOp, that the programmer will then use for transferring data to and from the bus.
  • [26 .. 59] — these lines define the interface methods that are provided by this Banjo specification; we'll discuss this in greater detail below.

Don't be confused by the comments on [50 .. 52] (and elsewhere) — they're "flow through" comments that are intended to be emitted into the generated source. Any comment that starts with "///" (three! slashes) is a "flow through" comment. Ordinary comments (that is, "//") are intended for the current module. This will become clear when we look at the generated code.

The operation structure

In our I2C sample, the struct I2cImplOp structure defines four elements:

Element Type Use
address uint16 the address of the chip to interact with on the bus
data vector<voidptr> contains the data sent to, and optionally received from, the bus
is_read bool flag indicating read functionality desired
stop bool flag indicating a stop byte should be sent after the operation

The structure defines the communications area that will be used between the protocol implementation (the driver) and the protocol user (the program that's using the bus).

The interface

The more interesting part is the protocol specification.

We'll skip the @transport("Banjo") (line [26]) and @banjo_layout("ddk-protocol") (line [27]) attributes for now, but will return to them below, in Attributes.

The protocol section defines five interface methods:

  • GetBusBase
  • GetBusCount
  • GetMaxTransferSize
  • SetBitrate
  • Transact

Without going into details about their internal operations (this isn't a tutorial on I2C, after all), let's see how they translate into the target language. We'll look at the C and C++ implementations separately, using the C description to include the structure definition that's common to the C++ version as well.

Currently, generation of C and C++ code is supported, with Rust support planned in the future.


The C implementation is relatively straightforward:

  • structs and unions map almost directly into their C language counterparts.
  • enums and constants are generated as #define macros.
  • protocols are generated as two structs:
    • a function table, and
    • a struct with pointers to the function table and a context.
  • Some helper functions are also generated.

The C version is generated into $BUILD_DIR/fidling/gen/sdk/banjo/fuchsia.hardware.i2cimpl/fuchsia.hardware.i2cimpl_banjo_c/fuchsia/hardware/i2cimpl/c/banjo.h

The file is relatively long, so we'll look at it in several parts.


The first part has some boilerplate, which we'll show without further comment:

[01] // Copyright 2018 The Fuchsia Authors. All rights reserved.
[02] // Use of this source code is governed by a BSD-style license that can be
[03] // found in the LICENSE file.
[06] // Generated from the fuchsia.hardware.i2cimpl banjo file
[08] #pragma once
[11] #include <zircon/compiler.h>
[12] #include <zircon/types.h>

Forward declarations

Next are forward declarations for our structures and functions:

[16] // Forward declarations
[17] typedef struct i2c_impl_op i2c_impl_op_t;
[18] typedef struct i2c_impl_protocol i2c_impl_protocol_t;
[19] typedef struct i2c_impl_protocol_ops i2c_impl_protocol_ops_t;
[26] // Declarations
[27] // See `Transact` below for usage.
[28] struct i2c_impl_op {
[29]     uint16_t address;
[30]     uint8_t* data_buffer;
[31]     size_t data_size;
[32]     bool is_read;
[33]     bool stop;
[34] };

Note that lines [17 .. 19] only declare types, they don't actually define structures or prototypes for functions.

Notice how the "flow through" comments (original .fidl file line [15], for example) got emitted into the generated code (line [27] above), with one slash stripped off to make them look like normal comments.

Lines [28 .. 34] are, as advertised, an almost direct mapping of the struct I2cImplOp from the .fidl file above (lines [16 .. 23]).

Astute C programmers will immediately see how the C++ style vector<voidptr> data (original .fidl file line [20]) is handled in C: it gets converted to a pointer ("data_buffer") and a size ("data_size").

As far as the naming goes, the base name is data (as given in the .fidl file). For a vector of voidptr, the transpiler appends _buffer and _size to convert the vector into a C compatible structure. For all other vector types, the transpiler appends _list and _count instead (for code readability).


Next, we see our const uint32 constants converted into #define statements:

[20] // The maximum length of all read or all write transfers in bytes.
[21] #define I2C_IMPL_MAX_TOTAL_TRANSFER UINT32_C(4096)
[22] // The maximum number of I2cImplOp's that may be passed to Transact.
[23] #define I2C_IMPL_MAX_RW_OPS UINT32_C(8)
[24] #define I2C_IMPL_10_BIT_ADDR_MASK UINT32_C(0xF000)

In the C version, We chose #define instead of "passing through" the const uint32_t representation because:

  • #define statements only exist at compile time, and get inlined at every usage site, whereas a const uint32_t would get embedded in the binary, and
  • #define allows for more compile time optimizations (e.g., doing math with the constant value).

The downside is that we don't get type safety, which is why you see the helper macros (like UINT32_C() above); they just cast the constant to the appropriate type.

Protocol structures

And now we get into the good parts.

[36] struct i2c_impl_protocol_ops {
[37]     uint32_t (*get_bus_base)(void* ctx);
[38]     uint32_t (*get_bus_count)(void* ctx);
[39]     zx_status_t (*get_max_transfer_size)(void* ctx, uint32_t bus_id, uint64_t* out_size);
[40]     zx_status_t (*set_bitrate)(void* ctx, uint32_t bus_id, uint32_t bitrate);
[41]     zx_status_t (*transact)(void* ctx, uint32_t bus_id, const i2c_impl_op_t* op_list, size_t op_count);
[42] };

This creates a structure definition that contains the five protocol methods that were defined in the original .fidl file at lines [30], [34], [37], [44], and [43].

Notice the name mangling that has occurred — this is how you can map the protocol method names to the C function pointer names so that you know what they're called:

Banjo C Rule
Transact transact Convert leading uppercase to lowercase
GetBusBase get_bus_base As above, and convert camel-case to underscore-separated style
GetBusCount get_bus_count Same as above
SetBitrate set_bitrate Same as above
GetMaxTransferSize get_max_transfer_size Same as above

Next, the interface definitions are wrapped in a context-bearing structure:

[45] struct i2c_impl_protocol {
[46]     i2c_impl_protocol_ops_t* ops;
[47]     void* ctx;
[48] };

Finally, we see the actual generated code for the five methods:

[53] static inline uint32_t i2c_impl_get_bus_base(const i2c_impl_protocol_t* proto) {
[54]     return proto->ops->get_bus_base(proto->ctx);
[55] }
[57] // Number of buses that this I2cImpl supports.
[58] static inline uint32_t i2c_impl_get_bus_count(const i2c_impl_protocol_t* proto) {
[59]     return proto->ops->get_bus_count(proto->ctx);
[60] }
[62] static inline zx_status_t i2c_impl_get_max_transfer_size(const i2c_impl_protocol_t* proto, uint32_t bus_id, uint64_t* out_size) {
[63]     return proto->ops->get_max_transfer_size(proto->ctx, bus_id, out_size);
[64] }
[66] // Sets the bitrate for the i2c bus in KHz units.
[67] static inline zx_status_t i2c_impl_set_bitrate(const i2c_impl_protocol_t* proto, uint32_t bus_id, uint32_t bitrate) {
[68]     return proto->ops->set_bitrate(proto->ctx, bus_id, bitrate);
[69] }
[71] // |Transact| assumes that all ops buf are not null.
[72] // |Transact| assumes that all ops length are not zero.
[73] // |Transact| assumes that at least the last op has stop set to true.
[74] static inline zx_status_t i2c_impl_transact(const i2c_impl_protocol_t* proto, uint32_t bus_id, const i2c_impl_op_t* op_list, size_t op_count) {
[75]     return proto->ops->transact(proto->ctx, bus_id, op_list, op_count);
[76] }

Prefixes and paths

Notice how the prefix i2c_impl_ (from the interface name, .fidl file line [28]) got added to the method names; thus, Transact became i2c_impl_transact, and so on. This is part of the mapping between .fidl names and their C equivalents.

Also, the library name (line [05] in the .fidl file) is transformed into the include path: so library fuchsia.hardware.i2cimpl implies a path of <fuchsia/hardware/i2cimpl/c/banjo.h>.


The C++ code is slightly more complex than the C version. Let's take a look.

The Banjo transpiler generates three files: the first is the C file discussed above, and the other two are under $BUILD_DIR/fidling/gen/sdk/banjo/fuchsia.hardware.i2cimpl/fuchsia.hardware.i2cimpl_banjo_c/fuchsia/hardware/i2cimpl/cpp/

  • i2cimpl.h — the file your program should include, and
  • i2cimpl-internal.h — an internal file, included by i2cimpl.h

The "internal" file contains declarations and assertions, which we can safely skip.

The C++ version of i2cimpl.h is fairly long, so we'll look at it in smaller pieces. Here's an overview "map" of what we'll be looking at, showing the starting line number of each piece:

Line Section
1 boilerplate
20 auto generated usage comments
61 class I2cImplProtocol
112 class I2cImplProtocolClient


The boilerplate is pretty much what you'd expect:

[001] // Copyright 2018 The Fuchsia Authors. All rights reserved.
[002] // Use of this source code is governed by a BSD-style license that can be
[003] // found in the LICENSE file.
[006] // Generated from the fuchsia.hardware.i2cimpl banjo file
[008] #pragma once
[010] #include <ddktl/device-internal.h>
[011] #include <fuchsia/hardware/i2cimpl/c/banjo.h>
[012] #include <lib/ddk/device.h>
[013] #include <lib/ddk/driver.h>
[014] #include <zircon/assert.h>
[015] #include <zircon/compiler.h>
[016] #include <zircon/types.h>
[018] #include "banjo-internal.h"

It #includes a bunch of DDK and OS headers, including:

  • the C version of the header (line [011], which means that everything discussed above in the C section applies here as well), and
  • the generated i2cimpl-internal.h file (line [018]).

Next is the "auto generated usage comments" section; we'll come back to that later as it will make more sense once we've seen the actual class declarations.

The two class declarations are wrapped in the DDK namespace:

[057] namespace ddk {
[214] } // namespace ddk

The I2cImplProtocolClient wrapper class

The I2cImplProtocolClient class is a simple wrapper around the i2c_impl_protocol_t structure (defined in the C include file, line [45], which we discussed in Protocol structures, above).

[112] class I2cImplProtocolClient {
[113] public:
[114]     I2cImplProtocolClient()
[115]         : ops_(nullptr), ctx_(nullptr) {}
[116]     I2cImplProtocolClient(const i2c_impl_protocol_t* proto)
[117]         : ops_(proto->ops), ctx_(proto->ctx) {}
[119]     I2cImplProtocolClient(zx_device_t* parent) {
[120]         i2c_impl_protocol_t proto;
[121]         if (device_get_protocol(parent, ZX_PROTOCOL_I2C_IMPL, &proto) == ZX_OK) {
[122]             ops_ = proto.ops;
[123]             ctx_ = proto.ctx;
[124]         } else {
[125]             ops_ = nullptr;
[126]             ctx_ = nullptr;
[127]         }
[128]     }
[130]     I2cImplProtocolClient(zx_device_t* parent, const char* fragment_name) {
[131]         i2c_impl_protocol_t proto;
[132]         if (device_get_fragment_protocol(parent, fragment_name, ZX_PROTOCOL_I2C_IMPL, &proto) == ZX_OK) {
[133]             ops_ = proto.ops;
[134]             ctx_ = proto.ctx;
[135]         } else {
[136]             ops_ = nullptr;
[137]             ctx_ = nullptr;
[138]         }
[139]     }
[141]     // Create a I2cImplProtocolClient from the given parent device + "fragment".
[142]     //
[143]     // If ZX_OK is returned, the created object will be initialized in |result|.
[144]     static zx_status_t CreateFromDevice(zx_device_t* parent,
[145]                                         I2cImplProtocolClient* result) {
[146]         i2c_impl_protocol_t proto;
[147]         zx_status_t status = device_get_protocol(
[148]                 parent, ZX_PROTOCOL_I2C_IMPL, &proto);
[149]         if (status != ZX_OK) {
[150]             return status;
[151]         }
[152]         *result = I2cImplProtocolClient(&proto);
[153]         return ZX_OK;
[154]     }
[156]     // Create a I2cImplProtocolClient from the given parent device.
[157]     //
[158]     // If ZX_OK is returned, the created object will be initialized in |result|.
[159]     static zx_status_t CreateFromDevice(zx_device_t* parent, const char* fragment_name,
[160]                                         I2cImplProtocolClient* result) {
[161]         i2c_impl_protocol_t proto;
[162]         zx_status_t status = device_get_fragment_protocol(parent, fragment_name,
[163]                                  ZX_PROTOCOL_I2C_IMPL, &proto);
[164]         if (status != ZX_OK) {
[165]             return status;
[166]         }
[167]         *result = I2cImplProtocolClient(&proto);
[168]         return ZX_OK;
[169]     }
[171]     void GetProto(i2c_impl_protocol_t* proto) const {
[172]         proto->ctx = ctx_;
[173]         proto->ops = ops_;
[174]     }
[175]     bool is_valid() const {
[176]         return ops_ != nullptr;
[177]     }
[178]     void clear() {
[179]         ctx_ = nullptr;
[180]         ops_ = nullptr;
[181]     }
[183]     // First bus ID that this I2cImpl controls, zero-indexed.
[184]     uint32_t GetBusBase() const {
[185]         return ops_->get_bus_base(ctx_);
[186]     }
[188]     // Number of buses that this I2cImpl supports.
[189]     uint32_t GetBusCount() const {
[190]         return ops_->get_bus_count(ctx_);
[191]     }
[193]     zx_status_t GetMaxTransferSize(uint32_t bus_id, uint64_t* out_size) const {
[194]         return ops_->get_max_transfer_size(ctx_, bus_id, out_size);
[195]     }
[197]     // Sets the bitrate for the i2c bus in KHz units.
[198]     zx_status_t SetBitrate(uint32_t bus_id, uint32_t bitrate) const {
[199]         return ops_->set_bitrate(ctx_, bus_id, bitrate);
[200]     }
[202]     // |Transact| assumes that all ops buf are not null.
[203]     // |Transact| assumes that all ops length are not zero.
[204]     // |Transact| assumes that at least the last op has stop set to true.
[205]     zx_status_t Transact(uint32_t bus_id, const i2c_impl_op_t* op_list, size_t op_count) const {
[206]         return ops_->transact(ctx_, bus_id, op_list, op_count);
[207]     }
[209] private:
[210]     i2c_impl_protocol_ops_t* ops_;
[211]     void* ctx_;
[212] };

There are four constructors:

  • the default one ([114]) that sets ops_ and ctx_ to nullptr,
  • an initializer ([116]) that takes a pointer to an i2c_impl_protocol_t structure and populates the ops_ and ctx_ fields from their namesakes in the structure, and
  • an initializer ([119]) that extracts the ops_ and ctx_ information from a zx_device_t.
  • an initializer ([130]) like above but gets ops_ and ctx_ from a device fragment.

The last two constructors are preferred, and can be used like this:

ddk::I2cImplProtocolClient i2cimpl(parent);
if (!i2cimpl.is_valid()) {
  return ZX_ERR_*; // return an appropriate error
ddk::I2cImplProtocolClient i2cimpl(parent, "i2c-impl-fragment");
if (!i2cimpl.is_valid()) {
  return ZX_ERR_*; // return an appropriate error

Three convenience member functions are provided:

  • [171] GetProto() fetches the ctx_ and ops_ members into a protocol structure,
  • [175] is_valid() returns a bool indicating if the class has been initialized with a protocol, and
  • [178] clear() invalidates the ctx_ and ops_ pointers.

Next we find the four member functions that were specified in the .fidl file:

  • [138] GetBusBase(), and
  • [138] GetBusCount(), and
  • [138] GetMaxTransferSize(), and
  • [138] SetBitrate(), and
  • [134] Transact().

These work just liked the four wrapper functions from the C version of the include file — that is, they pass their arguments into a call through the respective function pointer.

In fact, compare i2c_impl_get_max_transfer_size() from the C version:

[138] zx_status_t GetMaxTransferSize(size_t* out_size) const {
[139]     return ops_->get_max_transfer_size(ctx_, out_size);
[140] }

with the C++ version above:

[138] zx_status_t GetMaxTransferSize(size_t* out_size) const {
[139]     return ops_->get_max_transfer_size(ctx_, out_size);
[140] }

As advertised, all that this class does is store the operations and context pointers for later use, so that the call through the wrapper is more elegant.

You'll also notice that the C++ wrapper function doesn't have any name mangling — to use a tautology, GetMaxTransferSize() is GetMaxTransferSize().

The I2cImplProtocol mixin class

Ok, that was the easy part. For this next part, we're going to talk about mixins and CRTPs — or Curiously Recurring Template Patterns.

Let's understand the "shape" of the class first (comment lines deleted for outlining purposes):

[060] template <typename D, typename Base = internal::base_mixin>
[061] class I2cImplProtocol : public Base {
[062] public:
[063]     I2cImplProtocol() {
[064]         internal::CheckI2cImplProtocolSubclass<D>();
[065]         i2c_impl_protocol_ops_.get_bus_base = I2cImplGetBusBase;
[066]         i2c_impl_protocol_ops_.get_bus_count = I2cImplGetBusCount;
[067]         i2c_impl_protocol_ops_.get_max_transfer_size = I2cImplGetMaxTransferSize;
[068]         i2c_impl_protocol_ops_.set_bitrate = I2cImplSetBitrate;
[069]         i2c_impl_protocol_ops_.transact = I2cImplTransact;
[071]         if constexpr (internal::is_base_proto<Base>::value) {
[072]             auto dev = static_cast<D*>(this);
[073]             // Can only inherit from one base_protocol implementation.
[074]             ZX_ASSERT(dev->ddk_proto_id_ == 0);
[075]             dev->ddk_proto_id_ = ZX_PROTOCOL_I2C_IMPL;
[076]             dev->ddk_proto_ops_ = &i2c_impl_protocol_ops_;
[077]         }
[078]     }
[080] protected:
[081]     i2c_impl_protocol_ops_t i2c_impl_protocol_ops_ = {};
[083] private:
[085]     static uint32_t I2cImplGetBusBase(void* ctx) {
[086]         auto ret = static_cast<D*>(ctx)->I2cImplGetBusBase();
[087]         return ret;
[088]     }
[090]     static uint32_t I2cImplGetBusCount(void* ctx) {
[091]         auto ret = static_cast<D*>(ctx)->I2cImplGetBusCount();
[092]         return ret;
[093]     }
[094]     static zx_status_t I2cImplGetMaxTransferSize(void* ctx, uint32_t bus_id, uint64_t* out_size) {
[095]         auto ret = static_cast<D*>(ctx)->I2cImplGetMaxTransferSize(bus_id, out_size);
[096]         return ret;
[097]     }
[099]     static zx_status_t I2cImplSetBitrate(void* ctx, uint32_t bus_id, uint32_t bitrate) {
[100]         auto ret = static_cast<D*>(ctx)->I2cImplSetBitrate(bus_id, bitrate);
[101]         return ret;
[102]     }
[106]     static zx_status_t I2cImplTransact(void* ctx, uint32_t bus_id, const i2c_impl_op_t* op_list, size_t op_count) {
[107]         auto ret = static_cast<D*>(ctx)->I2cImplTransact(bus_id, op_list, op_count);
[108]         return ret;
[109]     }
[110] };

The I2CImplProtocol class inherits from a base class, specified by the second template parameter. If it's left unspecified, it defaults to internal::base_mixin, and no special magic happens. If, however, the base class is explicitly specified, it should be ddk::base_protocol, in which case additional asserts are added (to double check that only one mixin is the base protocol). In addition, special DDKTL fields are set to automatically register this protocol as the base protocol when the driver triggers DdkAdd().

The constructor calls an internal validation function, CheckI2cImplProtocolSubclass() [32] (defined in the generated i2c-impl-internal.h file), which has several static_assert() calls. The class D is expected to implement the five member functions (I2cImplGetBusBase(), I2cIImplGetBusCount(), I2cImplGetMaxTransferSize(), I2cImplSetBitrate(), and I2cImplTransact()) in order for the static methods to work. If they're not provided by D, then the compiler would (in the absence of the static asserts) produce gnarly templating errors. The static asserts serve to produce diagnostic errors that are understandable by mere humans.

Next, the five pointer-to-function operations members (get_bus_base, get_bus_count, get_max_transfer_size, set_bitrate, and transact) are bound (lines [065 .. 069]).

Finally, the constexpr expression provides a default initialization if required.

Using the mixin class

The I2cImplProtocol class can be used as follows (from //src/devices/i2c/drivers/intel-i2c/intel-i2c-controller.h):

[135] class IntelI2cController : public IntelI2cControllerType,
[136]                            public ddk::I2cImplProtocol<IntelI2cController, ddk::base_protocol> {
[137]  public:
[138]   explicit IntelI2cController(zx_device_t* parent)
[139]       : IntelI2cControllerType(parent), pci_(parent, "pci") {}
[141]   static zx_status_t Create(void* ctx, zx_device_t* parent);
[143]   void DdkInit(ddk::InitTxn txn);
[170]   uint32_t I2cImplGetBusBase();
[171]   uint32_t I2cImplGetBusCount();
[172]   zx_status_t I2cImplGetMaxTransferSize(const uint32_t bus_id, size_t* out_size);
[173]   zx_status_t I2cImplSetBitrate(const uint32_t bus_id, const uint32_t bitrate);
[174]   zx_status_t I2cImplTransact(const uint32_t bus_id, const i2c_impl_op_t* op_list,
[175]                               const size_t op_count);
[177]   void DdkUnbind(ddk::UnbindTxn txn);
[178]   void DdkRelease();
[180]  private:

Here we see that class IntelI2cController inherits from the DDK's I2cImplProtocol and provides itself as the argument to the template — this is the "mixin" concept. This causes the IntelI2cController type to be substituted for D in the template definition of the class (from the i2c-impl.h header file above, lines [086], [091], [95], [100], and [107]).

Taking a look at just the I2cImplGetMaxTransferSize() function as an example, it's effectively as if the source code read:

[094] static zx_status_t I2cImplGetMaxTransferSize(void* ctx, uint32_t bus_id, uint64_t* out_size) {
[095]     auto ret = static_cast<IntelI2cController*>(ctx)->I2cImplGetMaxTransferSize(bus_id, out_size);
[096]     return ret;
[097] }

This ends up eliminating the cast-to-self boilerplate in your code. This casting is necessary because the type information is erased at the DDK boundary — recall that the context ctx is a void * pointer.

Auto-generated comments

Banjo automatically generates comments in the include file that basically summarize what we talked about above:

[020] // DDK i2cimpl-protocol support
[021] //
[022] // :: Proxies ::
[023] //
[024] // ddk::I2cImplProtocolClient is a simple wrapper around
[025] // i2c_impl_protocol_t. It does not own the pointers passed to it.
[026] //
[027] // :: Mixins ::
[028] //
[029] // ddk::I2cImplProtocol is a mixin class that simplifies writing DDK drivers
[030] // that implement the i2c-impl protocol. It doesn't set the base protocol.
[031] //
[032] // :: Examples ::
[033] //
[034] // // A driver that implements a ZX_PROTOCOL_I2C_IMPL device.
[035] // class I2cImplDevice;
[036] // using I2cImplDeviceType = ddk::Device<I2cImplDevice, /* ddk mixins */>;
[037] //
[038] // class I2cImplDevice : public I2cImplDeviceType,
[039] //                      public ddk::I2cImplProtocol<I2cImplDevice> {
[040] //   public:
[041] //     I2cImplDevice(zx_device_t* parent)
[042] //         : I2cImplDeviceType(parent) {}
[043] //
[044] //     uint32_t I2cImplGetBusBase();
[045] //
[046] //     uint32_t I2cImplGetBusCount();
[047] //
[048] //     zx_status_t I2cImplGetMaxTransferSize(uint32_t bus_id, uint64_t* out_size);
[049] //
[050] //     zx_status_t I2cImplSetBitrate(uint32_t bus_id, uint32_t bitrate);
[051] //
[052] //     zx_status_t I2cImplTransact(uint32_t bus_id, const i2c_impl_op_t* op_list, size_t op_count);
[053] //
[054] //     ...
[055] // };

Using Banjo

Now that we've seen the generated code for the I2C driver, let's take a look at how we would use it.

@@@ to be completed


@@@ This is where we should list all builtin keywords and primitive types


Recall from the example above that the protocol section had two attributes; a @transport("Banjo") and a @banjo_layout("ddk-protocol") attribute.

The transport attribute

All Banjo protocols must have @transport("Banjo") to indicate that Banjo is being used instead of FIDL.

The banjo_layout attribute

The line just before the protocol is the banjo_layout attribute:

[27] @banjo_layout("ddk-protocol")
[28] protocol I2cImpl {

The attribute applies to the next item; so in this case, the entire protocol. Only one layout is allowed per interface.

There are in fact 3 BanjoLayout attribute types currently supported:

  • ddk-protocol
  • ddk-interface
  • ddk-callback

In order to understand how these layout types work, let's assume we have two drivers, A and B. Driver A spawns a device, which B then attaches to, (making B a child of A).

If B then queries the DDK for its parent's "protocol" through device_get_protocol(), it'll get a ddk-protocol. A ddk-protocol is a set of callbacks that a parent provides to its child.

One of the protocol functions can be to register a "reverse-protocol", whereby the child provides a set of callbacks for the parent to trigger instead. This is a ddk-interface.

From a code generation perspective, these two (ddk-protocol and ddk-interface) look almost identical, except for some slight naming differences (ddk-protocol automatically appends the word "protocol" to the end of generated structs / classes, whereas ddk-interface doesn't).

ddk-callback is a slight optimization over ddk-interface, and is used when an interface has just one single function. Instead of generating two structures, like:

struct interface {
   void* ctx;
   inteface_function_ptr_table* callbacks;

struct interface_function_ptr_table {
   void (*one_function)(...);

a ddk-callback will generate a single structure with the function pointer inlined:

struct callback {
  void* ctx;
  void (*one_function)(...);

The async attribute

For an example of the @async attribute, see the fuchsia.hardware.block Block protocol.

Within the protocol section, we see the @async attribute:

[254] protocol Block {
...       /// comments (removed)
[268]     @async

The @async attribute is a way to make protocol messages not be synchronous. It autogenerates a callback type in which the output arguments are inputs to the callback. The original method will not have any of the output parameters specified in its signatures.

In the protocol above there is a Queue method declared as:

[268] @async
[269] Queue(resource struct {
[270]     @in_out
[271]     txn BlockOp;
[272] }) -> (resource struct {
[273]     status zx.status;
[274]     @mutable
[275]     op BlockOp;
[276] });

When used (as above) in conjunction with the @async attribute, it means that we want Banjo to invoke a callback function, so that we can handle the output data (the second BlockOp above, representing the data from the block device).

Here's how it works. We send data to the block device through the first BlockOp argument. Some time later, the block device may generate data in response to our request. Because we specified @async, Banjo generates the functions to take a callback function as input.

In C, these two lines (from the block.h file) are important:

[085] typedef void (*block_queue_callback)(void* ctx, zx_status_t status, block_op_t* op);
[211] void (*queue)(void* ctx, block_op_t* txn, block_queue_callback callback, void* cookie);

In C++, we have two place where the callback is referenced:

[113] static void BlockQueue(void* ctx, block_op_t* txn, block_queue_callback callback, void* cookie) {
[114]     static_cast<D*>(ctx)->BlockQueue(txn, callback, cookie);
[115] }


[201] void Queue(block_op_t* txn, block_queue_callback callback, void* cookie) const {
[202]     ops_->queue(ctx_, txn, callback, cookie);
[203] }

Notice how the C++ is similar to the C: that's because the generated code includes the C header file as part of the C++ header file.

The transaction callback has the following arguments:

Argument Meaning
ctx the cookie
status status of the asynchronous response (provided by callee)
op the data from the transfer

How is this different than just using the @banjo_layout("ddk-callback") attribute we discussed above?

First, there's no struct with the callback and cookie value in it, they're inlined as arguments instead.

Second, the callback provided is a "one time use" function. That is to say, it should be called once, and only once, for each invocation of the protocol method it was supplied to. For contrast, a method provided by a ddk-callback is a "register once, call many times" type of function (similar to ddk-interface and ddk-protocol). For this reason, ddk-callback and ddk-interface structures usually have paired register() and unregister() calls in order to tell the parent device when it should stop calling those callbacks.

One more caveat with @async is that its callback MUST be called for each protocol method invocation, and the accompanying cookie must be provided. Failure to do so will result in undefined behavior (likely a leak, deadlock, timeout, or crash).

Although not the case currently, C++ and future language bindings (like Rust) will provide "future" / "promise" style based APIs in the generated code, built on top of these callbacks in order to prevent mistakes.

Ok, one more caveat with @async — the @async attribute applies only to the immediately following method; not any other methods.

The buffer attribute

This attribute applies to protocol method parameters of the vector type to convey that they are used as buffers. In practice, it only affects the names of the generated parameters.

The callee_allocated attribute

When applied to a protocol method output parameter of type vector, the attribute conveys the fact that the contents of the vector should be allocated by the receiver of the method call.

The derive_debug attribute (C bindings only)

When applied to an enum declaration, a helper *_to_str() function will be generated for C bindings which returns a const char* for each value of the enum. For example, an enum declared with this attribute such as

enum ExampleEnum {
    VAL_ONE = 1;
    VAL_TWO = 2;

will result in the following generated definition.

static inline const char* example_enum_to_str(example_enum_t value) {
  switch (value) {
      return "EXAMPLE_ENUM_VAL_ONE";
      return "EXAMPLE_ENUM_VAL_TWO";
  return "UNKNOWN";

The inner_pointer attribute

In the context of a protocol input parameter of type vector, this attribute turns the contents of the vector into pointers to objects instead of objects themselves.

The in_out attribute

Adding this attribute to a protocol method input parameter makes the parameter mutable, effectively turning it into an "in-out" parameter.

The mutable attribute

This attribute should be used to make struct/union fields of type vector or string mutable.

The namespaced attribute

This attribute applies to const declarations and makes it so that the C backend prefaces the constant name with the snake-cased FIDL library name, e.g. library_name_CONSTANT_K instead of CONSTANT_K. This attribute may be required to avoid name conflicts with FIDL hlcpp constant bindings in the same build target.

The out_of_line_contents attribute

This attribute allows the contents of a vector field in a struct/union to be stored outside of the container.

The preserve_c_names attribute

This attribute applies to struct declarations and makes it so that their fields' names remain unchanged when run through the C backend.

Banjo Mocks

Banjo generates a C++ mock class for each protocol. This mock can be passed to protocol users in tests.


Tests in Zircon get the mock headers automatically. Tests outsize of Zircon must depend on the protocol target with a _mock suffix, e.g. //sdk/banjo/fuchsia.hardware.gpio:fuchsia.hardware.gpio_banjo_cpp_mock.

Using the mocks

Test code must include the protocol header with a -mock suffix, e.g. #include <fuchsia/hardware/gpio/cpp/banjo-mock.h>.

Consider the following Banjo protocol snippet:

[20] @transport("Banjo")
[21] @banjo_layout("ddk-protocol")
[22] protocol Gpio {
[53]     /// Gets an interrupt object pertaining to a particular GPIO pin.
[54]     GetInterrupt(struct {
[55]         flags uint32;
[56]     }) -> (resource struct {
[57]         s zx.status;
[58]         irq zx.handle:INTERRUPT;
[59]     });
[82] };

Here are the corresponding bits of the mock class generated by Banjo:

[034] class MockGpio : ddk::GpioProtocol<MockGpio> {
[035] public:
[036]     MockGpio() : proto_{&gpio_protocol_ops_, this} {}
[038]    virtual ~MockGpio() {}
[040]     const gpio_protocol_t* GetProto() const { return &proto_; }
[067]     virtual MockGpio& ExpectGetInterrupt(zx_status_t out_s, uint32_t flags, zx::interrupt out_irq) {
[068]         mock_get_interrupt_.ExpectCall({out_s, std::move(out_irq)}, flags);
[069]         return *this;
[070]     }
[092]     void VerifyAndClear() {
[098]         mock_get_interrupt_.VerifyAndClear();
[103]     }
[131]     virtual zx_status_t GpioGetInterrupt(uint32_t flags, zx::interrupt* out_irq) {
[132]         std::tuple<zx_status_t, zx::interrupt> ret = mock_get_interrupt_.Call(flags);
[133]         *out_irq = std::move(std::get<1>(ret));
[134]         return std::get<0>(ret);
[135]     }

The MockGpio class implements the GPIO protocol. ExpectGetInterrupt is used to set expectations on how GpioGetInterrupt is called. GetProto is used to get the gpio_protocol_t that can be passed to the code under test. This code will call GpioGetInterrupt which will ensure that it got called with the correct arguments and will return the value specified by ExpectGetInterrupt. Finally, the test can call VerifyAndClear to verify that all expectations were satisfied. Here is an example test using this mock:

TEST(SomeTest, SomeTestCase) {
    ddk::MockGpio gpio;

    zx::interrupt interrupt;
    gpio.ExpectGetInterrupt(ZX_OK, 0, zx::move(interrupt))
        .ExpectGetInterrupt(ZX_ERR_INTERNAL, 100, zx::interrupt());

    CodeUnderTest dut(gpio.GetProto());


Equality operator overrides

Tests using Banjo mocks with structure types will have to define equality operator overrides. For example, for a struct type some_struct_type the test will have to define a function with the signature

bool operator==(const some_struct_type& lhs, const some_struct_type& rhs);

in the top-level namespace.

Custom mocks

It is expected that some tests may need to alter the default mock behavior. To help with this, all expectation and protocol methods are virtual, and all MockFunction members are protected.

Async methods

The Banjo mocks issue callbacks from all async methods by default.