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+ACPI based device enumeration
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+ACPI 5 introduced a set of new resources (UartTSerialBus, I2cSerialBus,
+SpiSerialBus, GpioIo and GpioInt) which can be used in enumerating slave
+devices behind serial bus controllers.
+
+In addition we are starting to see peripherals integrated in the
+SoC/Chipset to appear only in ACPI namespace. These are typically devices
+that are accessed through memory-mapped registers.
+
+In order to support this and re-use the existing drivers as much as
+possible we decided to do following:
+
+ o Devices that have no bus connector resource are represented as
+ platform devices.
+
+ o Devices behind real busses where there is a connector resource
+ are represented as struct spi_device or struct i2c_device
+ (standard UARTs are not busses so there is no struct uart_device).
+
+As both ACPI and Device Tree represent a tree of devices (and their
+resources) this implementation follows the Device Tree way as much as
+possible.
+
+The ACPI implementation enumerates devices behind busses (platform, SPI and
+I2C), creates the physical devices and binds them to their ACPI handle in
+the ACPI namespace.
+
+This means that when ACPI_HANDLE(dev) returns non-NULL the device was
+enumerated from ACPI namespace. This handle can be used to extract other
+device-specific configuration. There is an example of this below.
+
+Platform bus support
+~~~~~~~~~~~~~~~~~~~~
+Since we are using platform devices to represent devices that are not
+connected to any physical bus we only need to implement a platform driver
+for the device and add supported ACPI IDs. If this same IP-block is used on
+some other non-ACPI platform, the driver might work out of the box or needs
+some minor changes.
+
+Adding ACPI support for an existing driver should be pretty
+straightforward. Here is the simplest example:
+
+ #ifdef CONFIG_ACPI
+ static struct acpi_device_id mydrv_acpi_match[] = {
+ /* ACPI IDs here */
+ { }
+ };
+ MODULE_DEVICE_TABLE(acpi, mydrv_acpi_match);
+ #endif
+
+ static struct platform_driver my_driver = {
+ ...
+ .driver = {
+ .acpi_match_table = ACPI_PTR(mydrv_acpi_match),
+ },
+ };
+
+If the driver needs to perform more complex initialization like getting and
+configuring GPIOs it can get its ACPI handle and extract this information
+from ACPI tables.
+
+Currently the kernel is not able to automatically determine from which ACPI
+device it should make the corresponding platform device so we need to add
+the ACPI device explicitly to acpi_platform_device_ids list defined in
+drivers/acpi/scan.c. This limitation is only for the platform devices, SPI
+and I2C devices are created automatically as described below.
+
+SPI serial bus support
+~~~~~~~~~~~~~~~~~~~~~~
+Slave devices behind SPI bus have SpiSerialBus resource attached to them.
+This is extracted automatically by the SPI core and the slave devices are
+enumerated once spi_register_master() is called by the bus driver.
+
+Here is what the ACPI namespace for a SPI slave might look like:
+
+ Device (EEP0)
+ {
+ Name (_ADR, 1)
+ Name (_CID, Package() {
+ "ATML0025",
+ "AT25",
+ })
+ ...
+ Method (_CRS, 0, NotSerialized)
+ {
+ SPISerialBus(1, PolarityLow, FourWireMode, 8,
+ ControllerInitiated, 1000000, ClockPolarityLow,
+ ClockPhaseFirst, "\\_SB.PCI0.SPI1",)
+ }
+ ...
+
+The SPI device drivers only need to add ACPI IDs in a similar way than with
+the platform device drivers. Below is an example where we add ACPI support
+to at25 SPI eeprom driver (this is meant for the above ACPI snippet):
+
+ #ifdef CONFIG_ACPI
+ static struct acpi_device_id at25_acpi_match[] = {
+ { "AT25", 0 },
+ { },
+ };
+ MODULE_DEVICE_TABLE(acpi, at25_acpi_match);
+ #endif
+
+ static struct spi_driver at25_driver = {
+ .driver = {
+ ...
+ .acpi_match_table = ACPI_PTR(at25_acpi_match),
+ },
+ };
+
+Note that this driver actually needs more information like page size of the
+eeprom etc. but at the time writing this there is no standard way of
+passing those. One idea is to return this in _DSM method like:
+
+ Device (EEP0)
+ {
+ ...
+ Method (_DSM, 4, NotSerialized)
+ {
+ Store (Package (6)
+ {
+ "byte-len", 1024,
+ "addr-mode", 2,
+ "page-size, 32
+ }, Local0)
+
+ // Check UUIDs etc.
+
+ Return (Local0)
+ }
+
+Then the at25 SPI driver can get this configation by calling _DSM on its
+ACPI handle like:
+
+ struct acpi_buffer output = { ACPI_ALLOCATE_BUFFER, NULL };
+ struct acpi_object_list input;
+ acpi_status status;
+
+ /* Fill in the input buffer */
+
+ status = acpi_evaluate_object(ACPI_HANDLE(&spi->dev), "_DSM",
+ &input, &output);
+ if (ACPI_FAILURE(status))
+ /* Handle the error */
+
+ /* Extract the data here */
+
+ kfree(output.pointer);
+
+I2C serial bus support
+~~~~~~~~~~~~~~~~~~~~~~
+The slaves behind I2C bus controller only need to add the ACPI IDs like
+with the platform and SPI drivers. However the I2C bus controller driver
+needs to call acpi_i2c_register_devices() after it has added the adapter.
+
+An I2C bus (controller) driver does:
+
+ ...
+ ret = i2c_add_numbered_adapter(adapter);
+ if (ret)
+ /* handle error */
+
+ of_i2c_register_devices(adapter);
+ /* Enumerate the slave devices behind this bus via ACPI */
+ acpi_i2c_register_devices(adapter);
+
+Below is an example of how to add ACPI support to the existing mpu3050
+input driver:
+
+ #ifdef CONFIG_ACPI
+ static struct acpi_device_id mpu3050_acpi_match[] = {
+ { "MPU3050", 0 },
+ { },
+ };
+ MODULE_DEVICE_TABLE(acpi, mpu3050_acpi_match);
+ #endif
+
+ static struct i2c_driver mpu3050_i2c_driver = {
+ .driver = {
+ .name = "mpu3050",
+ .owner = THIS_MODULE,
+ .pm = &mpu3050_pm,
+ .of_match_table = mpu3050_of_match,
+ .acpi_match_table ACPI_PTR(mpu3050_acpi_match),
+ },
+ .probe = mpu3050_probe,
+ .remove = mpu3050_remove,
+ .id_table = mpu3050_ids,
+ };
+
+GPIO support
+~~~~~~~~~~~~
+ACPI 5 introduced two new resources to describe GPIO connections: GpioIo
+and GpioInt. These resources are used be used to pass GPIO numbers used by
+the device to the driver. For example:
+
+ Method (_CRS, 0, NotSerialized)
+ {
+ Name (SBUF, ResourceTemplate()
+ {
+ GpioIo (Exclusive, PullDefault, 0x0000, 0x0000,
+ IoRestrictionOutputOnly, "\\_SB.PCI0.GPI0",
+ 0x00, ResourceConsumer,,)
+ {
+ // Pin List
+ 0x0055
+ }
+ ...
+
+ Return (SBUF)
+ }
+ }
+
+These GPIO numbers are controller relative and path "\\_SB.PCI0.GPI0"
+specifies the path to the controller. In order to use these GPIOs in Linux
+we need to translate them to the Linux GPIO numbers.
+
+The driver can do this by including <linux/acpi_gpio.h> and then calling
+acpi_get_gpio(path, gpio). This will return the Linux GPIO number or
+negative errno if there was no translation found.
+
+Other GpioIo parameters must be converted first by the driver to be
+suitable to the gpiolib before passing them.
+
+In case of GpioInt resource an additional call to gpio_to_irq() must be
+done before calling request_irq().