RAMdisk Device


In this section, we'll examine a simplified RAM-disk driver.

This driver introduces:

  • the block protocol's query() and queue() ops
  • Virtual Memory Address Regions (VMARs) and Virtual Memory Objects (VMOs)

The source is in //examples/drivers//ramdisk/demo-ramdisk.c.

As with all drivers, the first thing to look at is how the driver initializes itself:

static zx_status_t ramdisk_driver_bind(void* ctx, zx_device_t* parent) {
    zx_status_t status = ZX_OK;

    // (1) create the device context block
    ramdisk_device_t* ramdev = calloc(1, sizeof((*ramdev)));
    if (ramdev == NULL) {
        return ZX_ERR_NO_MEMORY;

    // (2) create a VMO
    status = zx_vmo_create(RAMDISK_SIZE, 0, &ramdev->vmo);
    if (status != ZX_OK) {
        goto cleanup;

    // (3) map the VMO into our address space
    status = zx_vmar_map(zx_vmar_root_self(), 0, ramdev->vmo, 0, RAMDISK_SIZE,
                         ZX_VM_FLAG_PERM_READ | ZX_VM_FLAG_PERM_WRITE, &ramdev->mapped_addr);
    if (status != ZX_OK) {
        goto cleanup;

    // (4) add the device
    device_add_args_t args = {
        .version = DEVICE_ADD_ARGS_VERSION,
        .name = "demo-ramdisk",
        .ctx = ramdev,
        .ops = &ramdisk_proto,
        .proto_id = ZX_PROTOCOL_BLOCK_IMPL,
        .proto_ops = &block_ops,

    if ((status = device_add(parent, &args, &ramdev->zxdev)) != ZX_OK) {
    return status;

    // (5) clean up after ourselves
    return status;

static zx_driver_ops_t ramdisk_driver_ops = {
    .version = DRIVER_OPS_VERSION,
    .bind = ramdisk_driver_bind,

ZIRCON_DRIVER_BEGIN(ramdisk, ramdisk_driver_ops, "zircon", "0.1", 1)

At the bottom, you can see that this driver binds to a ZX_PROTOCOL_MISC_PARENT type of protocol, and provides ramdisk_driver_ops as the list of operations supported. This is no different than any of the other drivers we've seen so far.

The binding function, ramdisk_driver_bind(), does the following:

  1. Allocates the device context block.
  2. Creates a VMO. The VMO is a kernel object that represents a chunk of memory. In this simplified RAM-disk driver, we're creating a VMO that's RAMDISK_SIZE bytes long. This chunk of memory is the RAM-disk — that's where the data is stored. The VMO creation call, zx_vmo_create(), returns the VMO handle through its third argument, which is a member in our context block.
  3. Maps the VMO into our address space with zx_vmar_map(). This function returns a pointer to a VMAR that points to the entire VMO (because we specified RAMDISK_SIZE as the mapping size argument) and gives us read and write access (because of the ZX_VM_FLAG_PERM_* flags). The pointer is stored in our context block's mapped_addr member.
  4. Adds our device with device_add(), just like all the examples we've seen above. The difference here, though is that we see two new members: proto_id and proto_ops. These are defined as "optional custom protocol" members. As usual, we store the newly created device in the zxdev member of our context block.
  5. Cleans up resources if there were any problems along the way.

For completeness, here's the context block:

typedef struct ramdisk_device {
    zx_device_t*    zxdev;
    uintptr_t       mapped_addr;
    uint32_t        flags;
    zx_handle_t     vmo;
    bool            dead;
} ramdisk_device_t;

The fields are:

Type Field Description
zx_device_t* zxdev the ramdisk device
uintptr_t mapped_addr address of the VMAR
uin32_t flags device flags
zx_handle_t vmo a handle to our VMO
bool dead indicates if the device is still alive


Where this device is different from the others that we've seen, though, is that the device_add() function adds two sets of operations; the "regular" one, and an optional "protocol specific" one:

static zx_protocol_device_t ramdisk_proto = {
    .version = DEVICE_OPS_VERSION,
    .message = ramdisk_message,
    .get_size = ramdisk_getsize,
    .unbind = ramdisk_unbind,
    .release = ramdisk_release,

static block_protocol_ops_t block_ops = {
    .query = ramdisk_query,
    .queue = ramdisk_queue,

The zx_protocol_device_t one handles control messages (ramdisk_message()), device size queries (ramdisk_getsize()), and device cleanups (ramdisk_unbind() and ramdisk_release()).

The block_protocol_ops_t one contains protocol operations particular to the block protocol. We bound these to the device in the device_add_args_t structure (step (4) above through the .proto_ops field. We also set the .proto_id field to ZX_PROTOCOL_BLOCK_IMPL — this is what identifies this driver as being able to handle block protocol operations.

Let's tackle the trivial functions first:

static zx_off_t ramdisk_getsize(void* ctx) {
    return RAMDISK_SIZE;

static void ramdisk_unbind(void* ctx) {
    ramdisk_device_t* ramdev = ctx;
    ramdev->dead = true;

static void ramdisk_release(void* ctx) {
    ramdisk_device_t* ramdev = ctx;

    if (ramdev->vmo != ZX_HANDLE_INVALID) {
        zx_vmar_unmap(zx_vmar_root_self(), ramdev->mapped_addr, RAMDISK_SIZE);

static void ramdisk_query(void* ctx, block_info_t* bi, size_t* bopsz) {
    ramdisk_get_info(ctx, bi);
    *bopsz = sizeof(block_op_t);

ramdisk_getsize() is the easiest — it simply returns the size of the resource, in bytes. In our simplified RAM-disk driver, this is hardcoded as a #define near the top of the file.

Next, ramdisk_unbind() and ramdisk_release() work together. When the driver is being shut down, the ramdisk_unbind() hook is called. It sets the dead flag to indicate that the driver is shutting down (this is checked in the ramdisk_queue() handler, below). It's expected that the driver will finish up any I/O operations that are in progress (there won't be any in our RAM-disk), and it should call device_unbind_reply() to indicate unbinding is complete.

Sometime after device_unbind_reply() is called, the driver's ramdisk_release() will be called. Here we unmap the VMAR, with zx_vmar_unmap(), and close the VMO, with zx_handle_close(). As our final act, we release the device context block. At this point, the device is finished.

Block Operations

The ramdisk_query() function is called by the block protocol in order to get information about the device. There's a data structure (the block_info_t) that's filled out by the driver:

// from fuchsia/hardware/block/driver/c/banjo.h:
typedef struct {
    uint64_t    block_count;        // The number of blocks in this block device
    uint32_t    block_size;         // The size of a single block
    uint32_t    max_transfer_size;  // Max size in bytes per transfer.
                                    // May be BLOCK_MAX_TRANSFER_UNBOUNDED if there
                                    // is no restriction.
    uint32_t    flags;
    uint32_t    reserved;
} block_info_t;

// our helper function
static void ramdisk_get_info(void* ctx, block_info_t* info) {
    ramdisk_device_t* ramdev = ctx;
    memset(info, 0, sizeof(*info));
    info->block_size = BLOCK_SIZE;
    info->block_count = BLOCK_COUNT;
    // Arbitrarily set, but matches the SATA driver for testing
    info->max_transfer_size = fuchsia_hardware_block::wire::kMaxTransferUnbounded;
    info->flags = ramdev->flags;

In this simplified driver, the block_size, block_count, and max_transfer_size fields are hardcoded numbers.

The flags member is used to identify if the device is read-only (FLAG_READONLY, otherwise it's read/write), removable (FLAG_REMOVABLE, otherwise it's not removable) or has a bootable partition (FLAG_BOOTPART, otherwise it doesn't).

The final value that ramdisk_query() returns is the "block operation size" value through the pointer to bopsz. This is a host-maintained block that's big enough to contain the block_op_t plus any additional data the driver wants (appended to the block_op_t), like an extended context block.

Reading and writing

Finally, it's time to discuss the actual "block" data transfers; that is, how does data get read from / written to the device?

The second block protocol handler, ramdisk_queue(), performs that function.

As you might suspect from the name, it's intended that this hook starts whatever transfer operation (a read or a write) is requested, but doesn't require that the operation completes before the hook returns. This is a little like what we saw in earlier chapters in the read() and write() handlers for devices like /dev/misc/demo-fifo — there, we could either return data immediately, or put the client to sleep, waking it up later when data (or room for data) became available.

With ramdisk_queue() we get passed a block operations structure that indicates the expected operation: BLOCK_OP_READ, BLOCK_OP_WRITE, or BLOCK_OP_FLUSH. The structure also contains additional fields telling us the offset and size of the transfer (from //src/lib/ddk/include/ddk/protocol/block.h):

// simplified from original
struct block_op {
    struct {
        uint32_t    command;    // command and flags
        uint32_t    extra;      // available for temporary use
        zx_handle_t vmo;        // vmo of data to read or write
        uint32_t    length;     // transfer length in blocks (0 is invalid)
        uint64_t    offset_dev; // device offset in blocks
        uint64_t    offset_vmo; // vmo offset in blocks
        uint64_t*   pages;      // optional physical page list
    } rw;

    void (*completion_cb)(block_op_t* block, zx_status_t status);

The transfer takes place to or from the vmo in the structure — in the case of a read, we transfer data to the VMO, and vice versa for a write. The length indicates the number of blocks (not bytes) to transfer, and the two offset fields, offset_dev and offset_vmo, indicate the relative offsets (again, in blocks not bytes) into the device and the VMO of where the transfer should take place.

The implementation is straightforward:

static void ramdisk_queue(void* ctx, block_op_t* bop) {
    ramdisk_device_t* ramdev = ctx;

    // (1) see if we should still be handling requests
    if (ramdev->dead) {
        bop->completion_cb(bop, ZX_ERR_IO_NOT_PRESENT);

    // (2) what operation are we performing?
    switch ((bop->command &= BLOCK_OP_MASK)) {
    case BLOCK_OP_READ:
    case BLOCK_OP_WRITE: {
        // (3) perform validation common for both
        if ((bop->rw.offset_dev >= BLOCK_COUNT)
            || ((BLOCK_COUNT - bop->rw.offset_dev) < bop->rw.length)
            || bop->rw.length * BLOCK_SIZE > MAX_TRANSFER_BYTES) {
            bop->completion_cb(bop, ZX_ERR_OUT_OF_RANGE);

        // (4) compute address
        void* addr = (void*) ramdev->mapped_addr + bop->rw.offset_dev * BLOCK_SIZE;
        zx_status_t status;

        // (5) now perform actions specific to each
        if (bop->command == BLOCK_OP_READ) {
            status = zx_vmo_write(bop->rw.vmo, addr, bop->rw.offset_vmo * BLOCK_SIZE,
                                  bop->rw.length * BLOCK_SIZE);
        } else {
            status = zx_vmo_read(bop->rw.vmo, addr, bop->rw.offset_vmo * BLOCK_SIZE,
                                 bop->rw.length * BLOCK_SIZE);

        // (6) indicate completion
        bop->completion_cb(bop, status);

    case BLOCK_OP_FLUSH:
        bop->completion_cb(bop, ZX_OK);

        bop->completion_cb(bop, ZX_ERR_NOT_SUPPORTED);

As usual, we establish a context block at the top by casting the ctx argument. The bop argument is the "block operation" structure we saw above. The command field indicates what the ramdisk_queue() function should do.

In step (1), we check to see if we've set the dead flag (ramdisk_unbind() sets it when required). If so, it means that our device is no longer accepting new requests, so we return ZX_ERR_IO_NOT_PRESENT in order to encourage clients to close the device.

In step (3), we handle some common validation for both read and write — neither should allow offsets that exceed the size of the device, nor transfer more than the maximum transfer size.

Similarly, in step (4) we compute the device address (that is, we establish a pointer to our VMAR that's offset by the appropriate number of blocks as per the request).

In step (5) we perform either a zx_vmo_read() or a zx_vmo_write(), depending on the command. This is what transfers data between a pointer within our VMAR (addr) and the client's VMO (bop->rw.vmo). Notice that in the read case, we write to the VMO, and in the write case, we read from the VMO.

Finally, in step (6) (and the other two cases), we signal completion with the completion callback in the block ops structure.

The interesting thing about completion is that:

  • it doesn't have to happen right away — we could have queued this operation and signalled completion some time later,
  • it is allowed to be called before this function returns (like we did).

The last point simply means that we are not forced to defer completion until after the queuing function returns. This allows us to complete the operation directly in the function. For our trivial RAM-disk example, this makes sense — we have the ability to do the data transfer to or from media instantly; no need to defer.

How is the real one more complicated?

The RAM-disk presented above is somewhat simplified from the "real" RAM-disk device (present at //src/devices/block/drivers/ramdisk/ramdisk.c).

The real one adds the following functionality:

  • dynamic device creation through a new VMO
  • ability to use an existing VMO
  • background thread
  • sleep mode