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Mika Westerberg
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Rafael J. Wysocki
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Dec 7, 2012
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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 = __devexit_p(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(). |
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trunk/Documentation/devicetree/bindings/cpufreq/cpufreq-spear.txt
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