cachepc-linux

Fork of AMDESE/linux with modifications for CachePC side-channel attack
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enumeration.rst (22278B)


      1.. SPDX-License-Identifier: GPL-2.0
      2
      3=============================
      4ACPI Based Device Enumeration
      5=============================
      6
      7ACPI 5 introduced a set of new resources (UartTSerialBus, I2cSerialBus,
      8SpiSerialBus, GpioIo and GpioInt) which can be used in enumerating slave
      9devices behind serial bus controllers.
     10
     11In addition we are starting to see peripherals integrated in the
     12SoC/Chipset to appear only in ACPI namespace. These are typically devices
     13that are accessed through memory-mapped registers.
     14
     15In order to support this and re-use the existing drivers as much as
     16possible we decided to do following:
     17
     18  - Devices that have no bus connector resource are represented as
     19    platform devices.
     20
     21  - Devices behind real busses where there is a connector resource
     22    are represented as struct spi_device or struct i2c_device. Note
     23    that standard UARTs are not busses so there is no struct uart_device,
     24    although some of them may be represented by sturct serdev_device.
     25
     26As both ACPI and Device Tree represent a tree of devices (and their
     27resources) this implementation follows the Device Tree way as much as
     28possible.
     29
     30The ACPI implementation enumerates devices behind busses (platform, SPI,
     31I2C, and in some cases UART), creates the physical devices and binds them
     32to their ACPI handle in the ACPI namespace.
     33
     34This means that when ACPI_HANDLE(dev) returns non-NULL the device was
     35enumerated from ACPI namespace. This handle can be used to extract other
     36device-specific configuration. There is an example of this below.
     37
     38Platform bus support
     39====================
     40
     41Since we are using platform devices to represent devices that are not
     42connected to any physical bus we only need to implement a platform driver
     43for the device and add supported ACPI IDs. If this same IP-block is used on
     44some other non-ACPI platform, the driver might work out of the box or needs
     45some minor changes.
     46
     47Adding ACPI support for an existing driver should be pretty
     48straightforward. Here is the simplest example::
     49
     50	static const struct acpi_device_id mydrv_acpi_match[] = {
     51		/* ACPI IDs here */
     52		{ }
     53	};
     54	MODULE_DEVICE_TABLE(acpi, mydrv_acpi_match);
     55
     56	static struct platform_driver my_driver = {
     57		...
     58		.driver = {
     59			.acpi_match_table = mydrv_acpi_match,
     60		},
     61	};
     62
     63If the driver needs to perform more complex initialization like getting and
     64configuring GPIOs it can get its ACPI handle and extract this information
     65from ACPI tables.
     66
     67DMA support
     68===========
     69
     70DMA controllers enumerated via ACPI should be registered in the system to
     71provide generic access to their resources. For example, a driver that would
     72like to be accessible to slave devices via generic API call
     73dma_request_chan() must register itself at the end of the probe function like
     74this::
     75
     76	err = devm_acpi_dma_controller_register(dev, xlate_func, dw);
     77	/* Handle the error if it's not a case of !CONFIG_ACPI */
     78
     79and implement custom xlate function if needed (usually acpi_dma_simple_xlate()
     80is enough) which converts the FixedDMA resource provided by struct
     81acpi_dma_spec into the corresponding DMA channel. A piece of code for that case
     82could look like::
     83
     84	#ifdef CONFIG_ACPI
     85	struct filter_args {
     86		/* Provide necessary information for the filter_func */
     87		...
     88	};
     89
     90	static bool filter_func(struct dma_chan *chan, void *param)
     91	{
     92		/* Choose the proper channel */
     93		...
     94	}
     95
     96	static struct dma_chan *xlate_func(struct acpi_dma_spec *dma_spec,
     97			struct acpi_dma *adma)
     98	{
     99		dma_cap_mask_t cap;
    100		struct filter_args args;
    101
    102		/* Prepare arguments for filter_func */
    103		...
    104		return dma_request_channel(cap, filter_func, &args);
    105	}
    106	#else
    107	static struct dma_chan *xlate_func(struct acpi_dma_spec *dma_spec,
    108			struct acpi_dma *adma)
    109	{
    110		return NULL;
    111	}
    112	#endif
    113
    114dma_request_chan() will call xlate_func() for each registered DMA controller.
    115In the xlate function the proper channel must be chosen based on
    116information in struct acpi_dma_spec and the properties of the controller
    117provided by struct acpi_dma.
    118
    119Clients must call dma_request_chan() with the string parameter that corresponds
    120to a specific FixedDMA resource. By default "tx" means the first entry of the
    121FixedDMA resource array, "rx" means the second entry. The table below shows a
    122layout::
    123
    124	Device (I2C0)
    125	{
    126		...
    127		Method (_CRS, 0, NotSerialized)
    128		{
    129			Name (DBUF, ResourceTemplate ()
    130			{
    131				FixedDMA (0x0018, 0x0004, Width32bit, _Y48)
    132				FixedDMA (0x0019, 0x0005, Width32bit, )
    133			})
    134		...
    135		}
    136	}
    137
    138So, the FixedDMA with request line 0x0018 is "tx" and next one is "rx" in
    139this example.
    140
    141In robust cases the client unfortunately needs to call
    142acpi_dma_request_slave_chan_by_index() directly and therefore choose the
    143specific FixedDMA resource by its index.
    144
    145Named Interrupts
    146================
    147
    148Drivers enumerated via ACPI can have names to interrupts in the ACPI table
    149which can be used to get the IRQ number in the driver.
    150
    151The interrupt name can be listed in _DSD as 'interrupt-names'. The names
    152should be listed as an array of strings which will map to the Interrupt()
    153resource in the ACPI table corresponding to its index.
    154
    155The table below shows an example of its usage::
    156
    157    Device (DEV0) {
    158        ...
    159        Name (_CRS, ResourceTemplate() {
    160            ...
    161            Interrupt (ResourceConsumer, Level, ActiveHigh, Exclusive) {
    162                0x20,
    163                0x24
    164            }
    165        })
    166
    167        Name (_DSD, Package () {
    168            ToUUID("daffd814-6eba-4d8c-8a91-bc9bbf4aa301"),
    169            Package () {
    170                Package () { "interrupt-names", Package () { "default", "alert" } },
    171            }
    172        ...
    173        })
    174    }
    175
    176The interrupt name 'default' will correspond to 0x20 in Interrupt()
    177resource and 'alert' to 0x24. Note that only the Interrupt() resource
    178is mapped and not GpioInt() or similar.
    179
    180The driver can call the function - fwnode_irq_get_byname() with the fwnode
    181and interrupt name as arguments to get the corresponding IRQ number.
    182
    183SPI serial bus support
    184======================
    185
    186Slave devices behind SPI bus have SpiSerialBus resource attached to them.
    187This is extracted automatically by the SPI core and the slave devices are
    188enumerated once spi_register_master() is called by the bus driver.
    189
    190Here is what the ACPI namespace for a SPI slave might look like::
    191
    192	Device (EEP0)
    193	{
    194		Name (_ADR, 1)
    195		Name (_CID, Package () {
    196			"ATML0025",
    197			"AT25",
    198		})
    199		...
    200		Method (_CRS, 0, NotSerialized)
    201		{
    202			SPISerialBus(1, PolarityLow, FourWireMode, 8,
    203				ControllerInitiated, 1000000, ClockPolarityLow,
    204				ClockPhaseFirst, "\\_SB.PCI0.SPI1",)
    205		}
    206		...
    207
    208The SPI device drivers only need to add ACPI IDs in a similar way than with
    209the platform device drivers. Below is an example where we add ACPI support
    210to at25 SPI eeprom driver (this is meant for the above ACPI snippet)::
    211
    212	static const struct acpi_device_id at25_acpi_match[] = {
    213		{ "AT25", 0 },
    214		{ }
    215	};
    216	MODULE_DEVICE_TABLE(acpi, at25_acpi_match);
    217
    218	static struct spi_driver at25_driver = {
    219		.driver = {
    220			...
    221			.acpi_match_table = at25_acpi_match,
    222		},
    223	};
    224
    225Note that this driver actually needs more information like page size of the
    226eeprom, etc. This information can be passed via _DSD method like::
    227
    228	Device (EEP0)
    229	{
    230		...
    231		Name (_DSD, Package ()
    232		{
    233			ToUUID("daffd814-6eba-4d8c-8a91-bc9bbf4aa301"),
    234			Package ()
    235			{
    236				Package () { "size", 1024 },
    237				Package () { "pagesize", 32 },
    238				Package () { "address-width", 16 },
    239			}
    240		})
    241	}
    242
    243Then the at25 SPI driver can get this configuration by calling device property
    244APIs during ->probe() phase like::
    245
    246	err = device_property_read_u32(dev, "size", &size);
    247	if (err)
    248		...error handling...
    249
    250	err = device_property_read_u32(dev, "pagesize", &page_size);
    251	if (err)
    252		...error handling...
    253
    254	err = device_property_read_u32(dev, "address-width", &addr_width);
    255	if (err)
    256		...error handling...
    257
    258I2C serial bus support
    259======================
    260
    261The slaves behind I2C bus controller only need to add the ACPI IDs like
    262with the platform and SPI drivers. The I2C core automatically enumerates
    263any slave devices behind the controller device once the adapter is
    264registered.
    265
    266Below is an example of how to add ACPI support to the existing mpu3050
    267input driver::
    268
    269	static const struct acpi_device_id mpu3050_acpi_match[] = {
    270		{ "MPU3050", 0 },
    271		{ }
    272	};
    273	MODULE_DEVICE_TABLE(acpi, mpu3050_acpi_match);
    274
    275	static struct i2c_driver mpu3050_i2c_driver = {
    276		.driver	= {
    277			.name	= "mpu3050",
    278			.pm	= &mpu3050_pm,
    279			.of_match_table = mpu3050_of_match,
    280			.acpi_match_table = mpu3050_acpi_match,
    281		},
    282		.probe		= mpu3050_probe,
    283		.remove		= mpu3050_remove,
    284		.id_table	= mpu3050_ids,
    285	};
    286	module_i2c_driver(mpu3050_i2c_driver);
    287
    288Reference to PWM device
    289=======================
    290
    291Sometimes a device can be a consumer of PWM channel. Obviously OS would like
    292to know which one. To provide this mapping the special property has been
    293introduced, i.e.::
    294
    295    Device (DEV)
    296    {
    297        Name (_DSD, Package ()
    298        {
    299            ToUUID("daffd814-6eba-4d8c-8a91-bc9bbf4aa301"),
    300            Package () {
    301                Package () { "compatible", Package () { "pwm-leds" } },
    302                Package () { "label", "alarm-led" },
    303                Package () { "pwms",
    304                    Package () {
    305                        "\\_SB.PCI0.PWM",  // <PWM device reference>
    306                        0,                 // <PWM index>
    307                        600000000,         // <PWM period>
    308                        0,                 // <PWM flags>
    309                    }
    310                }
    311            }
    312        })
    313        ...
    314    }
    315
    316In the above example the PWM-based LED driver references to the PWM channel 0
    317of \_SB.PCI0.PWM device with initial period setting equal to 600 ms (note that
    318value is given in nanoseconds).
    319
    320GPIO support
    321============
    322
    323ACPI 5 introduced two new resources to describe GPIO connections: GpioIo
    324and GpioInt. These resources can be used to pass GPIO numbers used by
    325the device to the driver. ACPI 5.1 extended this with _DSD (Device
    326Specific Data) which made it possible to name the GPIOs among other things.
    327
    328For example::
    329
    330	Device (DEV)
    331	{
    332		Method (_CRS, 0, NotSerialized)
    333		{
    334			Name (SBUF, ResourceTemplate()
    335			{
    336				// Used to power on/off the device
    337				GpioIo (Exclusive, PullNone, 0, 0, IoRestrictionOutputOnly,
    338					"\\_SB.PCI0.GPI0", 0, ResourceConsumer) { 85 }
    339
    340				// Interrupt for the device
    341				GpioInt (Edge, ActiveHigh, ExclusiveAndWake, PullNone, 0,
    342					 "\\_SB.PCI0.GPI0", 0, ResourceConsumer) { 88 }
    343			}
    344
    345			Return (SBUF)
    346		}
    347
    348		// ACPI 5.1 _DSD used for naming the GPIOs
    349		Name (_DSD, Package ()
    350		{
    351			ToUUID("daffd814-6eba-4d8c-8a91-bc9bbf4aa301"),
    352			Package ()
    353			{
    354				Package () { "power-gpios", Package () { ^DEV, 0, 0, 0 } },
    355				Package () { "irq-gpios", Package () { ^DEV, 1, 0, 0 } },
    356			}
    357		})
    358		...
    359	}
    360
    361These GPIO numbers are controller relative and path "\\_SB.PCI0.GPI0"
    362specifies the path to the controller. In order to use these GPIOs in Linux
    363we need to translate them to the corresponding Linux GPIO descriptors.
    364
    365There is a standard GPIO API for that and is documented in
    366Documentation/admin-guide/gpio/.
    367
    368In the above example we can get the corresponding two GPIO descriptors with
    369a code like this::
    370
    371	#include <linux/gpio/consumer.h>
    372	...
    373
    374	struct gpio_desc *irq_desc, *power_desc;
    375
    376	irq_desc = gpiod_get(dev, "irq");
    377	if (IS_ERR(irq_desc))
    378		/* handle error */
    379
    380	power_desc = gpiod_get(dev, "power");
    381	if (IS_ERR(power_desc))
    382		/* handle error */
    383
    384	/* Now we can use the GPIO descriptors */
    385
    386There are also devm_* versions of these functions which release the
    387descriptors once the device is released.
    388
    389See Documentation/firmware-guide/acpi/gpio-properties.rst for more information
    390about the _DSD binding related to GPIOs.
    391
    392RS-485 support
    393==============
    394
    395ACPI _DSD (Device Specific Data) can be used to describe RS-485 capability
    396of UART.
    397
    398For example::
    399
    400	Device (DEV)
    401	{
    402		...
    403
    404		// ACPI 5.1 _DSD used for RS-485 capabilities
    405		Name (_DSD, Package ()
    406		{
    407			ToUUID("daffd814-6eba-4d8c-8a91-bc9bbf4aa301"),
    408			Package ()
    409			{
    410				Package () {"rs485-rts-active-low", Zero},
    411				Package () {"rs485-rx-active-high", Zero},
    412				Package () {"rs485-rx-during-tx", Zero},
    413			}
    414		})
    415		...
    416
    417MFD devices
    418===========
    419
    420The MFD devices register their children as platform devices. For the child
    421devices there needs to be an ACPI handle that they can use to reference
    422parts of the ACPI namespace that relate to them. In the Linux MFD subsystem
    423we provide two ways:
    424
    425  - The children share the parent ACPI handle.
    426  - The MFD cell can specify the ACPI id of the device.
    427
    428For the first case, the MFD drivers do not need to do anything. The
    429resulting child platform device will have its ACPI_COMPANION() set to point
    430to the parent device.
    431
    432If the ACPI namespace has a device that we can match using an ACPI id or ACPI
    433adr, the cell should be set like::
    434
    435	static struct mfd_cell_acpi_match my_subdevice_cell_acpi_match = {
    436		.pnpid = "XYZ0001",
    437		.adr = 0,
    438	};
    439
    440	static struct mfd_cell my_subdevice_cell = {
    441		.name = "my_subdevice",
    442		/* set the resources relative to the parent */
    443		.acpi_match = &my_subdevice_cell_acpi_match,
    444	};
    445
    446The ACPI id "XYZ0001" is then used to lookup an ACPI device directly under
    447the MFD device and if found, that ACPI companion device is bound to the
    448resulting child platform device.
    449
    450Device Tree namespace link device ID
    451====================================
    452
    453The Device Tree protocol uses device identification based on the "compatible"
    454property whose value is a string or an array of strings recognized as device
    455identifiers by drivers and the driver core.  The set of all those strings may be
    456regarded as a device identification namespace analogous to the ACPI/PNP device
    457ID namespace.  Consequently, in principle it should not be necessary to allocate
    458a new (and arguably redundant) ACPI/PNP device ID for a devices with an existing
    459identification string in the Device Tree (DT) namespace, especially if that ID
    460is only needed to indicate that a given device is compatible with another one,
    461presumably having a matching driver in the kernel already.
    462
    463In ACPI, the device identification object called _CID (Compatible ID) is used to
    464list the IDs of devices the given one is compatible with, but those IDs must
    465belong to one of the namespaces prescribed by the ACPI specification (see
    466Section 6.1.2 of ACPI 6.0 for details) and the DT namespace is not one of them.
    467Moreover, the specification mandates that either a _HID or an _ADR identification
    468object be present for all ACPI objects representing devices (Section 6.1 of ACPI
    4696.0).  For non-enumerable bus types that object must be _HID and its value must
    470be a device ID from one of the namespaces prescribed by the specification too.
    471
    472The special DT namespace link device ID, PRP0001, provides a means to use the
    473existing DT-compatible device identification in ACPI and to satisfy the above
    474requirements following from the ACPI specification at the same time.  Namely,
    475if PRP0001 is returned by _HID, the ACPI subsystem will look for the
    476"compatible" property in the device object's _DSD and will use the value of that
    477property to identify the corresponding device in analogy with the original DT
    478device identification algorithm.  If the "compatible" property is not present
    479or its value is not valid, the device will not be enumerated by the ACPI
    480subsystem.  Otherwise, it will be enumerated automatically as a platform device
    481(except when an I2C or SPI link from the device to its parent is present, in
    482which case the ACPI core will leave the device enumeration to the parent's
    483driver) and the identification strings from the "compatible" property value will
    484be used to find a driver for the device along with the device IDs listed by _CID
    485(if present).
    486
    487Analogously, if PRP0001 is present in the list of device IDs returned by _CID,
    488the identification strings listed by the "compatible" property value (if present
    489and valid) will be used to look for a driver matching the device, but in that
    490case their relative priority with respect to the other device IDs listed by
    491_HID and _CID depends on the position of PRP0001 in the _CID return package.
    492Specifically, the device IDs returned by _HID and preceding PRP0001 in the _CID
    493return package will be checked first.  Also in that case the bus type the device
    494will be enumerated to depends on the device ID returned by _HID.
    495
    496For example, the following ACPI sample might be used to enumerate an lm75-type
    497I2C temperature sensor and match it to the driver using the Device Tree
    498namespace link::
    499
    500	Device (TMP0)
    501	{
    502		Name (_HID, "PRP0001")
    503		Name (_DSD, Package () {
    504			ToUUID("daffd814-6eba-4d8c-8a91-bc9bbf4aa301"),
    505			Package () {
    506				Package () { "compatible", "ti,tmp75" },
    507			}
    508		})
    509		Method (_CRS, 0, Serialized)
    510		{
    511			Name (SBUF, ResourceTemplate ()
    512			{
    513				I2cSerialBusV2 (0x48, ControllerInitiated,
    514					400000, AddressingMode7Bit,
    515					"\\_SB.PCI0.I2C1", 0x00,
    516					ResourceConsumer, , Exclusive,)
    517			})
    518			Return (SBUF)
    519		}
    520	}
    521
    522It is valid to define device objects with a _HID returning PRP0001 and without
    523the "compatible" property in the _DSD or a _CID as long as one of their
    524ancestors provides a _DSD with a valid "compatible" property.  Such device
    525objects are then simply regarded as additional "blocks" providing hierarchical
    526configuration information to the driver of the composite ancestor device.
    527
    528However, PRP0001 can only be returned from either _HID or _CID of a device
    529object if all of the properties returned by the _DSD associated with it (either
    530the _DSD of the device object itself or the _DSD of its ancestor in the
    531"composite device" case described above) can be used in the ACPI environment.
    532Otherwise, the _DSD itself is regarded as invalid and therefore the "compatible"
    533property returned by it is meaningless.
    534
    535Refer to Documentation/firmware-guide/acpi/DSD-properties-rules.rst for more
    536information.
    537
    538PCI hierarchy representation
    539============================
    540
    541Sometimes could be useful to enumerate a PCI device, knowing its position on the
    542PCI bus.
    543
    544For example, some systems use PCI devices soldered directly on the mother board,
    545in a fixed position (ethernet, Wi-Fi, serial ports, etc.). In this conditions it
    546is possible to refer to these PCI devices knowing their position on the PCI bus
    547topology.
    548
    549To identify a PCI device, a complete hierarchical description is required, from
    550the chipset root port to the final device, through all the intermediate
    551bridges/switches of the board.
    552
    553For example, let us assume to have a system with a PCIe serial port, an
    554Exar XR17V3521, soldered on the main board. This UART chip also includes
    55516 GPIOs and we want to add the property ``gpio-line-names`` [1] to these pins.
    556In this case, the ``lspci`` output for this component is::
    557
    558	07:00.0 Serial controller: Exar Corp. XR17V3521 Dual PCIe UART (rev 03)
    559
    560The complete ``lspci`` output (manually reduced in length) is::
    561
    562	00:00.0 Host bridge: Intel Corp... Host Bridge (rev 0d)
    563	...
    564	00:13.0 PCI bridge: Intel Corp... PCI Express Port A #1 (rev fd)
    565	00:13.1 PCI bridge: Intel Corp... PCI Express Port A #2 (rev fd)
    566	00:13.2 PCI bridge: Intel Corp... PCI Express Port A #3 (rev fd)
    567	00:14.0 PCI bridge: Intel Corp... PCI Express Port B #1 (rev fd)
    568	00:14.1 PCI bridge: Intel Corp... PCI Express Port B #2 (rev fd)
    569	...
    570	05:00.0 PCI bridge: Pericom Semiconductor Device 2404 (rev 05)
    571	06:01.0 PCI bridge: Pericom Semiconductor Device 2404 (rev 05)
    572	06:02.0 PCI bridge: Pericom Semiconductor Device 2404 (rev 05)
    573	06:03.0 PCI bridge: Pericom Semiconductor Device 2404 (rev 05)
    574	07:00.0 Serial controller: Exar Corp. XR17V3521 Dual PCIe UART (rev 03) <-- Exar
    575	...
    576
    577The bus topology is::
    578
    579	-[0000:00]-+-00.0
    580	           ...
    581	           +-13.0-[01]----00.0
    582	           +-13.1-[02]----00.0
    583	           +-13.2-[03]--
    584	           +-14.0-[04]----00.0
    585	           +-14.1-[05-09]----00.0-[06-09]--+-01.0-[07]----00.0 <-- Exar
    586	           |                               +-02.0-[08]----00.0
    587	           |                               \-03.0-[09]--
    588	           ...
    589	           \-1f.1
    590
    591To describe this Exar device on the PCI bus, we must start from the ACPI name
    592of the chipset bridge (also called "root port") with address::
    593
    594	Bus: 0 - Device: 14 - Function: 1
    595
    596To find this information is necessary disassemble the BIOS ACPI tables, in
    597particular the DSDT (see also [2])::
    598
    599	mkdir ~/tables/
    600	cd ~/tables/
    601	acpidump > acpidump
    602	acpixtract -a acpidump
    603	iasl -e ssdt?.* -d dsdt.dat
    604
    605Now, in the dsdt.dsl, we have to search the device whose address is related to
    6060x14 (device) and 0x01 (function). In this case we can find the following
    607device::
    608
    609	Scope (_SB.PCI0)
    610	{
    611	... other definitions follow ...
    612		Device (RP02)
    613		{
    614			Method (_ADR, 0, NotSerialized)  // _ADR: Address
    615			{
    616				If ((RPA2 != Zero))
    617				{
    618					Return (RPA2) /* \RPA2 */
    619				}
    620				Else
    621				{
    622					Return (0x00140001)
    623				}
    624			}
    625	... other definitions follow ...
    626
    627and the _ADR method [3] returns exactly the device/function couple that
    628we are looking for. With this information and analyzing the above ``lspci``
    629output (both the devices list and the devices tree), we can write the following
    630ACPI description for the Exar PCIe UART, also adding the list of its GPIO line
    631names::
    632
    633	Scope (_SB.PCI0.RP02)
    634	{
    635		Device (BRG1) //Bridge
    636		{
    637			Name (_ADR, 0x0000)
    638
    639			Device (BRG2) //Bridge
    640			{
    641				Name (_ADR, 0x00010000)
    642
    643				Device (EXAR)
    644				{
    645					Name (_ADR, 0x0000)
    646
    647					Name (_DSD, Package ()
    648					{
    649						ToUUID("daffd814-6eba-4d8c-8a91-bc9bbf4aa301"),
    650						Package ()
    651						{
    652							Package ()
    653							{
    654								"gpio-line-names",
    655								Package ()
    656								{
    657									"mode_232",
    658									"mode_422",
    659									"mode_485",
    660									"misc_1",
    661									"misc_2",
    662									"misc_3",
    663									"",
    664									"",
    665									"aux_1",
    666									"aux_2",
    667									"aux_3",
    668								}
    669							}
    670						}
    671					})
    672				}
    673			}
    674		}
    675	}
    676
    677The location "_SB.PCI0.RP02" is obtained by the above investigation in the
    678dsdt.dsl table, whereas the device names "BRG1", "BRG2" and "EXAR" are
    679created analyzing the position of the Exar UART in the PCI bus topology.
    680
    681References
    682==========
    683
    684[1] Documentation/firmware-guide/acpi/gpio-properties.rst
    685
    686[2] Documentation/admin-guide/acpi/initrd_table_override.rst
    687
    688[3] ACPI Specifications, Version 6.3 - Paragraph 6.1.1 _ADR Address)
    689    https://uefi.org/sites/default/files/resources/ACPI_6_3_May16.pdf,
    690    referenced 2020-11-18