Registers overview

The registers driver should be used for registers that may be accessed by multiple drivers.


Drivers often communicate with hardware via register-based interfaces. Conceptually, registers are groups of 8/16/32/64 bits that can be read or written via an address space. MMIO registers are accessed via the CPU's memory address space, while I2C registers are accessed via an I2C bus.

Most hardware only supports atomic register writes. So, drivers that want to change a few bits in a register must do so via read/modify/write operations. For example, for a 32-bit register, if you were to change the 0-1st bits both to 0, you would need to read all 32 bits of that register, say 0xFFFFFFFF, then write back to that register 0xFFFFFFFC.


Read/modify/write operations introduce the potential for data races. The classic transaction example "withdraw $10 from a bank account" is effectively a read/modify/write operation. See Data Race in the Examples section for a concrete illustration.

When a register is "exclusively owned" by a driver, the driver is expected to synchronize its accesses to the register to avoid data races. (The potential for races still exists, but it's outside the scope of the register driver).

When a register is "shared" (may be accessed) by multiple drivers, we need a global coordinator to avoid races. The register driver can act as this global coordinator, when each driver only needs to access a disjoint subset of the register's bits. The global coordinator will provide the needed synchronization between reads/writes of a register.


The registers driver not only prevents races by providing synchronization, but also provides isolation of bit fields. The registers driver splits a register up into multiple resources and ensures that each driver only has access to those bits. A driver may not accidentally read or write into bits that it does not own.

Theory of operation

In the board driver, a register which is accessed by multiple drivers declares the different bit fields that it wants to expose. The registers driver creates devices for each of the bit fields. A driver that wants to access those bit fields needs to bind to the corresponding device.

Within the registers driver, a lock for each register is used to ensure that only one read/write can access the register at once. The interface of the registers driver is defined by registers-util.fidl.

How to use

Currently the Reset Registers are the only registers that are migrated to use the registers driver. This section will use them as an example on how to use the registers driver.

  1. Board driver changes

    Metadata format is declared in metadata.fidl.

    In the board driver (i.e. nelson-registers), make the following changes.

    a. MMIO

    Ensure that the registers driver has access to the MMIO. If not, add MMIO and corresponding MMIO index:

      enum MmioMetadataIdx {
      static const std::vector<fpbus::Mmio> registers_mmios{
              .base = A311D_RESET_BASE,
              .length = A311D_RESET_LENGTH,

    Note that the MmioMetadataIndex must correspond to the MMIO's index in registers_mmios.

    b. Declare bitfields

    The RegistersMetadataToFidl helper function can be used to declare bitfields:

      auto metadata_bytes = fidl_metadata::registers::RegistersMetadataToFidl<uint32_t>(kRegisters);
      if (metadata_bytes.is_error()) {
        zxlogf(ERROR, "%s: Failed to FIDL encode registers metadata %s\n", __func__,
        return metadata_bytes.error_value();
      const std::vector<fpbus::Metadata> registers_metadata{
              .type = DEVICE_METADATA_REGISTERS,
              .data = metadata_bytes.value(),

    where the bitfields are defined in the kRegisters field:

      static const fidl_metadata::registers::Register<uint32_t> kRegisters[]{
              .bind_id = aml_registers::REGISTER_USB_PHY_V2_RESET,
              .mmio_id = kResetMmio,
              .masks =
                          .value = aml_registers::USB_RESET1_REGISTER_UNKNOWN_1_MASK |
                          .mmio_offset = A311D_RESET1_REGISTER,
                          .value = aml_registers::USB_RESET1_LEVEL_MASK,
                          .mmio_offset = A311D_RESET1_LEVEL,
    • bind_id: The unique ID for each bitfield definition that is used to identify it during binding.
    • mmio_id: The ID corresponding to the MMIO that this bitfield is in reference to.
    • masks: A list of masks describing the specific bitfields that should be accessible through this register device.
      • value: The bitmask that is available to access, with 1 meaning accessible and 0 inaccessible. For example for a 32-bit register, a bitmask may be 0xFFFF0000, which means the higher 16 bits of the register are accessible through this register device and the lower 16 are not.
      • mmio_offset: The starting offset from the start of the MMIO identified by mmio_id that these bitfields define.
      • count: The number of register address this bitfield mask applies to following mmio_offset. Defaults to 1.
      • overlap_check_on: If true, the registers driver will verify that these bitfields do not overlap with any other defined bitfields. Otherwise, the check is skipped. Defaults to true.

    The registers driver will create a device that serves registers-util.fidl with bind properties: {:.devsite-disable-click-to-copy} bind_fuchsia_register::NAME == bind_id

  2. Binding to the registers driver

    The driver that wishes to access these bitfields must then bind to the device created above.

  3. Using the registers driver interface

    After successfully connecting to the registers device, say through fidl::WireSyncClient<fuchsia_hardware_registers::Device> register, you may call the corresponding FIDL read/write according to the FIDL interface defined in registers-util.fidl. E.g., for 32-bit registers,

    auto result =
        reset_register_->WriteRegister32(RESET1_LEVEL_OFFSET, aml_registers::USB_RESET1_LEVEL_MASK,
    if ((result.status() != ZX_OK) || result->is_error()) {
      zxlogf(ERROR, "Write failed\n");
      return ZX_ERR_INTERNAL;
  4. Enjoy isolated synchronized registers!


Data Race

Say we have a register that is accessed by both driver A and driver B. Now driver A wants to write b01 to bits 0-1 of the register and driver B wants to write b001 to bits 8-10 of the register. Depending on timing driver A may read the original value of the register, say 0xFFFFFFFF and driver B may also read that same value. Driver A then writes to bits 0-1 and puts into memory 0xFFFFFFFD. Driver B then writes to memory 0xFFFFF9FF. In this sequence of events, the register now holds the value of 0xFFFFF9FF, which driver B had written last. However, the next time driver A comes to read bits 0-1 of this register, it will not get the value it expects as it was previously written.

AMLogic SoCs

This section gives some examples of where the registers driver should and should not be used. The registers driver definitely should be used when it is needed for synchronization between multiple drivers, and may or may not be used only for isolation of resources. Note that the following is not a complete list of shared registers in the AMLogic SoCs.

Reset Registers

AMLogic SoCs have the reset functionality for various hardware units concentrated in a few 32-bit registers. This is exemplified by RESET1_REGISTER in S905D3 Table 6-186. For example, USB reset is controlled by bit 2 and SD_EMMC by bits 12-14. The hardware design forces us to share RESET1_REGISTER among multiple drivers including the EMMC driver and the USB driver.

See nelson-registers for the board file changes made for reset registers and nelson-usb - aml_usb_phy device for adding reset register fragments. AmlUsbPhy::InitPhy() of aml-usb-phy uses the FIDL client to write to the reset register.

Power Registers

Similar to the reset registers described above, AO_RTI_GEN_PWR_SLEEP0 (S905D3 Table 6-17) and AO_RTI_GEN_PWR_ISO0 (S905D3 Table 6-18) are shared by multiple drivers, including display, USB, and ML, and should be managed by the registers driver because they are writable and need coordination. This migration is currently in progress.

On the other hand, although AO_RTI_GEN_PWR_ACK0 (S905D3 Table 6-19) will be shared by multiple drivers (PCIE, USB, display, etc.), it is readonly. Data from this register can be accessed concurrently without any risk of races, so it is not required to use the registers driver for synchronization. If desired, we may still use the registers driver for isolation of resources.