342 lines
9.6 KiB
Plaintext
342 lines
9.6 KiB
Plaintext
ACPI based device enumeration
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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ACPI 5 introduced a set of new resources (UartTSerialBus, I2cSerialBus,
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SpiSerialBus, GpioIo and GpioInt) which can be used in enumerating slave
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devices behind serial bus controllers.
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In addition we are starting to see peripherals integrated in the
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SoC/Chipset to appear only in ACPI namespace. These are typically devices
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that are accessed through memory-mapped registers.
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In order to support this and re-use the existing drivers as much as
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possible we decided to do following:
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o Devices that have no bus connector resource are represented as
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platform devices.
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o Devices behind real busses where there is a connector resource
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are represented as struct spi_device or struct i2c_device
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(standard UARTs are not busses so there is no struct uart_device).
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As both ACPI and Device Tree represent a tree of devices (and their
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resources) this implementation follows the Device Tree way as much as
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possible.
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The ACPI implementation enumerates devices behind busses (platform, SPI and
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I2C), creates the physical devices and binds them to their ACPI handle in
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the ACPI namespace.
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This means that when ACPI_HANDLE(dev) returns non-NULL the device was
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enumerated from ACPI namespace. This handle can be used to extract other
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device-specific configuration. There is an example of this below.
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Platform bus support
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~~~~~~~~~~~~~~~~~~~~
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Since we are using platform devices to represent devices that are not
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connected to any physical bus we only need to implement a platform driver
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for the device and add supported ACPI IDs. If this same IP-block is used on
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some other non-ACPI platform, the driver might work out of the box or needs
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some minor changes.
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Adding ACPI support for an existing driver should be pretty
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straightforward. Here is the simplest example:
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#ifdef CONFIG_ACPI
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static struct acpi_device_id mydrv_acpi_match[] = {
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/* ACPI IDs here */
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{ }
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};
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MODULE_DEVICE_TABLE(acpi, mydrv_acpi_match);
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#endif
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static struct platform_driver my_driver = {
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...
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.driver = {
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.acpi_match_table = ACPI_PTR(mydrv_acpi_match),
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},
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};
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If the driver needs to perform more complex initialization like getting and
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configuring GPIOs it can get its ACPI handle and extract this information
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from ACPI tables.
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DMA support
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~~~~~~~~~~~
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DMA controllers enumerated via ACPI should be registered in the system to
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provide generic access to their resources. For example, a driver that would
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like to be accessible to slave devices via generic API call
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dma_request_slave_channel() must register itself at the end of the probe
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function like this:
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err = devm_acpi_dma_controller_register(dev, xlate_func, dw);
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/* Handle the error if it's not a case of !CONFIG_ACPI */
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and implement custom xlate function if needed (usually acpi_dma_simple_xlate()
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is enough) which converts the FixedDMA resource provided by struct
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acpi_dma_spec into the corresponding DMA channel. A piece of code for that case
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could look like:
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#ifdef CONFIG_ACPI
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struct filter_args {
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/* Provide necessary information for the filter_func */
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...
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};
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static bool filter_func(struct dma_chan *chan, void *param)
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{
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/* Choose the proper channel */
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...
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}
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static struct dma_chan *xlate_func(struct acpi_dma_spec *dma_spec,
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struct acpi_dma *adma)
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{
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dma_cap_mask_t cap;
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struct filter_args args;
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/* Prepare arguments for filter_func */
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...
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return dma_request_channel(cap, filter_func, &args);
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}
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#else
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static struct dma_chan *xlate_func(struct acpi_dma_spec *dma_spec,
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struct acpi_dma *adma)
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{
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return NULL;
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}
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#endif
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dma_request_slave_channel() will call xlate_func() for each registered DMA
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controller. In the xlate function the proper channel must be chosen based on
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information in struct acpi_dma_spec and the properties of the controller
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provided by struct acpi_dma.
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Clients must call dma_request_slave_channel() with the string parameter that
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corresponds to a specific FixedDMA resource. By default "tx" means the first
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entry of the FixedDMA resource array, "rx" means the second entry. The table
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below shows a layout:
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Device (I2C0)
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{
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...
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Method (_CRS, 0, NotSerialized)
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{
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Name (DBUF, ResourceTemplate ()
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{
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FixedDMA (0x0018, 0x0004, Width32bit, _Y48)
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FixedDMA (0x0019, 0x0005, Width32bit, )
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})
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...
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}
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}
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So, the FixedDMA with request line 0x0018 is "tx" and next one is "rx" in
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this example.
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In robust cases the client unfortunately needs to call
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acpi_dma_request_slave_chan_by_index() directly and therefore choose the
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specific FixedDMA resource by its index.
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SPI serial bus support
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~~~~~~~~~~~~~~~~~~~~~~
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Slave devices behind SPI bus have SpiSerialBus resource attached to them.
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This is extracted automatically by the SPI core and the slave devices are
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enumerated once spi_register_master() is called by the bus driver.
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Here is what the ACPI namespace for a SPI slave might look like:
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Device (EEP0)
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{
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Name (_ADR, 1)
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Name (_CID, Package() {
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"ATML0025",
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"AT25",
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})
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...
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Method (_CRS, 0, NotSerialized)
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{
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SPISerialBus(1, PolarityLow, FourWireMode, 8,
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ControllerInitiated, 1000000, ClockPolarityLow,
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ClockPhaseFirst, "\\_SB.PCI0.SPI1",)
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}
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...
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The SPI device drivers only need to add ACPI IDs in a similar way than with
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the platform device drivers. Below is an example where we add ACPI support
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to at25 SPI eeprom driver (this is meant for the above ACPI snippet):
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#ifdef CONFIG_ACPI
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static struct acpi_device_id at25_acpi_match[] = {
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{ "AT25", 0 },
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{ },
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};
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MODULE_DEVICE_TABLE(acpi, at25_acpi_match);
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#endif
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static struct spi_driver at25_driver = {
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.driver = {
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...
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.acpi_match_table = ACPI_PTR(at25_acpi_match),
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},
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};
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Note that this driver actually needs more information like page size of the
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eeprom etc. but at the time writing this there is no standard way of
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passing those. One idea is to return this in _DSM method like:
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Device (EEP0)
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{
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...
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Method (_DSM, 4, NotSerialized)
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{
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Store (Package (6)
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{
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"byte-len", 1024,
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"addr-mode", 2,
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"page-size, 32
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}, Local0)
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// Check UUIDs etc.
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Return (Local0)
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}
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Then the at25 SPI driver can get this configuration by calling _DSM on its
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ACPI handle like:
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struct acpi_buffer output = { ACPI_ALLOCATE_BUFFER, NULL };
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struct acpi_object_list input;
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acpi_status status;
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/* Fill in the input buffer */
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status = acpi_evaluate_object(ACPI_HANDLE(&spi->dev), "_DSM",
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&input, &output);
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if (ACPI_FAILURE(status))
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/* Handle the error */
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/* Extract the data here */
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kfree(output.pointer);
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I2C serial bus support
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~~~~~~~~~~~~~~~~~~~~~~
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The slaves behind I2C bus controller only need to add the ACPI IDs like
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with the platform and SPI drivers. The I2C core automatically enumerates
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any slave devices behind the controller device once the adapter is
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registered.
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Below is an example of how to add ACPI support to the existing mpu3050
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input driver:
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#ifdef CONFIG_ACPI
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static struct acpi_device_id mpu3050_acpi_match[] = {
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{ "MPU3050", 0 },
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{ },
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};
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MODULE_DEVICE_TABLE(acpi, mpu3050_acpi_match);
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#endif
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static struct i2c_driver mpu3050_i2c_driver = {
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.driver = {
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.name = "mpu3050",
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.owner = THIS_MODULE,
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.pm = &mpu3050_pm,
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.of_match_table = mpu3050_of_match,
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.acpi_match_table ACPI_PTR(mpu3050_acpi_match),
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},
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.probe = mpu3050_probe,
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.remove = mpu3050_remove,
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.id_table = mpu3050_ids,
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};
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GPIO support
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~~~~~~~~~~~~
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ACPI 5 introduced two new resources to describe GPIO connections: GpioIo
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and GpioInt. These resources are used be used to pass GPIO numbers used by
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the device to the driver. For example:
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Method (_CRS, 0, NotSerialized)
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{
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Name (SBUF, ResourceTemplate()
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{
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...
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// Used to power on/off the device
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GpioIo (Exclusive, PullDefault, 0x0000, 0x0000,
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IoRestrictionOutputOnly, "\\_SB.PCI0.GPI0",
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0x00, ResourceConsumer,,)
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{
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// Pin List
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0x0055
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}
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// Interrupt for the device
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GpioInt (Edge, ActiveHigh, ExclusiveAndWake, PullNone,
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0x0000, "\\_SB.PCI0.GPI0", 0x00, ResourceConsumer,,)
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{
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// Pin list
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0x0058
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}
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...
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}
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Return (SBUF)
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}
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These GPIO numbers are controller relative and path "\\_SB.PCI0.GPI0"
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specifies the path to the controller. In order to use these GPIOs in Linux
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we need to translate them to the corresponding Linux GPIO descriptors.
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There is a standard GPIO API for that and is documented in
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Documentation/gpio/.
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In the above example we can get the corresponding two GPIO descriptors with
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a code like this:
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#include <linux/gpio/consumer.h>
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...
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struct gpio_desc *irq_desc, *power_desc;
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irq_desc = gpiod_get_index(dev, NULL, 1);
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if (IS_ERR(irq_desc))
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/* handle error */
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power_desc = gpiod_get_index(dev, NULL, 0);
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if (IS_ERR(power_desc))
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/* handle error */
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/* Now we can use the GPIO descriptors */
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There are also devm_* versions of these functions which release the
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descriptors once the device is released.
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MFD devices
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~~~~~~~~~~~
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The MFD devices register their children as platform devices. For the child
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devices there needs to be an ACPI handle that they can use to reference
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parts of the ACPI namespace that relate to them. In the Linux MFD subsystem
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we provide two ways:
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o The children share the parent ACPI handle.
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o The MFD cell can specify the ACPI id of the device.
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For the first case, the MFD drivers do not need to do anything. The
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resulting child platform device will have its ACPI_COMPANION() set to point
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to the parent device.
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If the ACPI namespace has a device that we can match using an ACPI id,
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the id should be set like:
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static struct mfd_cell my_subdevice_cell = {
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.name = "my_subdevice",
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/* set the resources relative to the parent */
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.acpi_pnpid = "XYZ0001",
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};
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The ACPI id "XYZ0001" is then used to lookup an ACPI device directly under
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the MFD device and if found, that ACPI companion device is bound to the
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resulting child platform device.
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