In this section, we look at composite devices. A composite device is a device composed of other devices.
These devices address the case of hardware-level composition, in which a “device” (from the user's perspective) is implemented by several distinct hardware blocks.
Examples include:
In these situations, the relationship of the hardware is known to the board driver at boot time (either statically or through a dynamic means, such as ACPI).
We'll use the astro-audio
device for our examples:
This device features:
Note that the ZX_PROTOCOL_I2C
and ZX_PROTOCOL_GPIO
protocols are used to transfer data; that is, I2C messages, and GPIO pin status are sent and received through the respective drivers.
The ZX_PROTOCOL_PDEV
part is different. Here, the protocol is used only to grant access (the green checkmarks in the diagram) to the MMIO and IRQ; the actual MMIO data and interrupts are not handled by the PDEV
; they're handled directly by the astro-audio
driver itself.
To create a composite device, a number of data structures need to be set up.
We need a number of binding instructions (zx_bind_inst_t
) that tell us which devices we match. These binding instructions are the ones discussed in the “Registration” topic in the introduction section.
For the astro-audio
device, we have:
static const zx_bind_inst_t root_match[] = { BI_MATCH(), }; static const zx_bind_inst_t i2c_match[] = { BI_ABORT_IF(NE, BIND_PROTOCOL, ZX_PROTOCOL_I2C), BI_ABORT_IF(NE, BIND_I2C_BUS_ID, ASTRO_I2C_3), BI_MATCH_IF(EQ, BIND_I2C_ADDRESS, I2C_AUDIO_CODEC_ADDR), }; static const zx_bind_inst_t fault_gpio_match[] = { BI_ABORT_IF(NE, BIND_PROTOCOL, ZX_PROTOCOL_GPIO), BI_MATCH_IF(EQ, BIND_GPIO_PIN, GPIO_AUDIO_SOC_FAULT_L), }; static const zx_bind_inst_t enable_gpio_match[] = { BI_ABORT_IF(NE, BIND_PROTOCOL, ZX_PROTOCOL_GPIO), BI_MATCH_IF(EQ, BIND_GPIO_PIN, GPIO_SOC_AUDIO_EN), };
These binding instructions are used to find the devices.
We have four binding instruction arrays; a root_match[]
, which contains common information for the other three, and then the three devices: the I2C (i2c_match[]
) device and the two GPIOs (fault_gpio_match[]
and enable_gpio_match[]
).
These instructions are then placed into an array of structures (device_fragment_part_t
), which defines each fragment:
In the astro-audio
device, we have:
static const device_fragment_part_t i2c_fragment[] = { { countof(root_match), root_match }, { countof(i2c_match), i2c_match }, }; static const device_fragment_part_t fault_gpio_fragment[] = { { countof(root_match), root_match }, { countof(fault_gpio_match), fault_gpio_match }, }; static const device_fragment_part_t enable_gpio_fragment[] = { { countof(root_match), root_match }, { countof(enable_gpio_match), enable_gpio_match }, };
At this point, we have three fragment devices, i2c_fragment[]
, fault_gpio_fragment[]
, and enable_gpio_fragment[]
.
The following rules apply:
root_match
identifier. Note that this requirement is likely to change, since most users provide an “always match” anyway.BIND_TOPO_START
through BIND_TOPO_END
(basically buses, like I2C and PCI) must be matched. These sequences of matches must be unique.Finally, we combine them into an aggregate called fragments[]
of type device_fragment_t
:
This now gives us a single identifier, fragments[]
, that we can use when creating the composite device.
In astro-audio
, this looks like:
static const device_fragment_t fragments[] = { { "i2c", countof(i2c_fragment), i2c_fragment }, { "gpio-fault", countof(fault_gpio_fragment), fault_gpio_fragment }, { "gpio-enable", countof(enable_gpio_fragment), enable_gpio_fragment }, };
For simple (non-composite) devices, we used device_add() (which we saw in the “Registration” section previously).
For composite devices, we use device_add_composite():
zx_status_t device_add_composite( zx_device_t* dev, const char* name, const zx_device_prop_t* props, size_t props_count, const device_fragment_t* fragments, size_t fragments_count, uint32_t coresident_device_index);
The arguments are as follows:
Argument | Meaning |
---|---|
dev | Parent device |
name | The name of the device |
props | Properties (see “Declaring a Driver”) |
props_count | How many entries are in props |
fragments | The individual fragment devices |
fragments_count | How many entries are in fragments |
coresident_device_index | Which driver host to use |
The dev
value must be the zx_device_t
corresponding to the “sys
” device (i.e., the platform bus driver's device).
Note that the coresident_device_index
is used to indicate which driver host the new device should use. If you specify UINT32_MAX
, the device will reside in a new driver host.
Note that
astro-audio
uses pbus_composite_device_add() rather than device_add_composite(). The difference is that pbus_composite_device_add() is an API provided by the platform bus driver that wraps device_add_composite() and inserts an additional fragment for ferrying over direct-access resources such as MMIO, IRQs, and BTIs.
From a programming perspective, a composite device acts like an ordinary device, but it has no banjo protocol. This allows you to access all of the individual fragments that make up the composite device.
The first thing to retrieve a device for each fragment. This is done with device_get_fragment():
bool device_get_fragment ( zx_device_t* parent, const char* fragment_name, zx_device_t** fragment);
The arguments are as follows:
Argument | Meaning |
---|---|
parent | Pointer to zx_device_t representing parent |
fragment_name | The name of the fragment you wish to fetch |
fragment | Pointer to zx_device_t representing the fragment |
The program starts by declaring an array of zx_device_t*
pointers to hold the devices, and call device_get_fragment():
zx_device_t* fragment; bool found = device_get_fragment(&composite, "fragment-name", &fragment); if (!found) { zxlogf(ERROR, "could not get fragment-name"); return ZX_ERR_INTERNAL; }
The name of fragment supplied to device_get_fragment() is the same as the one in device_fragment_t entries supplied to the device_add_composite() call by the board driver.
Here we discuss some specialized / advanced topics.
What's actually going on in the astro-audio
driver is a little more complex than initially shown:
The fragments are bound to an internal driver (located in the //src/devices/internal/drivers/fragment directory).
The driver handles proxying across process boundaries if necessary. This proxying uses the DEVICE_ADD_MUST_ISOLATE
mechanism (introduced in the Isolate devices section).
When a device is added with DEVICE_ADD_MUST_ISOLATE
, two devices end up being created: the normal device, in the same process as its parent, and a proxy.
The proxy is created in a new driver host; if the normal device‘s driver is normal.so
, then its driver is normal.proxy.so
. This driver is expected to implement a create() method, which calls device_add() and stashes the IPC channel it’s given. That channel will be used later for communicating with the normal device in order to satisfy the proxy‘s children’s requests.
The normal device implements the rxrpc
hook, which is invoked by the driver runtime each time a message is received from the channel shared with the proxy.
So, in order to implement a new protocol proxy, one must modify the fragment.proxy.so
drivers to handle the desired protocol by sending messages to the normal device, and modify the fragment.so
driver to service those messages appropriately.
The fragment proxy is implemented in /src/devices/internal/drivers/fragment/fragment-proxy.cc, and the other half in /src/devices/internal/drivers/fragment/fragment.cc.