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 .. SPDX-License-Identifier: GPL-2.0
 
 ======================================
 _DSD Device Properties Related to GPIO
 ======================================
 
 With the release of ACPI 5.1, the _DSD configuration object finally
 allows names to be given to GPIOs (and other things as well) returned
 by _CRS.  Previously, we were only able to use an integer index to find
 the corresponding GPIO, which is pretty error prone (it depends on
 the _CRS output ordering, for example).
 
 With _DSD we can now query GPIOs using a name instead of an integer
 index, like the ASL example below shows::
 
   // Bluetooth device with reset and shutdown GPIOs
   Device (BTH)
   {
       Name (_HID, ...)
 
       Name (_CRS, ResourceTemplate ()
       {
           GpioIo (Exclusive, PullUp, 0, 0, IoRestrictionOutputOnly,
                   "\\_SB.GPO0", 0, ResourceConsumer) { 15 }
           GpioIo (Exclusive, PullUp, 0, 0, IoRestrictionOutputOnly,
                   "\\_SB.GPO0", 0, ResourceConsumer) { 27, 31 }
       })
 
       Name (_DSD, Package ()
       {
           ToUUID("daffd814-6eba-4d8c-8a91-bc9bbf4aa301"),
           Package ()
           {
               Package () { "reset-gpios", Package () { ^BTH, 1, 1, 0 } },
               Package () { "shutdown-gpios", Package () { ^BTH, 0, 0, 0 } },
           }
       })
   }
 
 The format of the supported GPIO property is::
 
   Package () { "name", Package () { ref, index, pin, active_low }}
 
 ref
   The device that has _CRS containing GpioIo()/GpioInt() resources,
   typically this is the device itself (BTH in our case).
 index
   Index of the GpioIo()/GpioInt() resource in _CRS starting from zero.
 pin
   Pin in the GpioIo()/GpioInt() resource. Typically this is zero.
 active_low
   If 1, the GPIO is marked as active_low.
 
 Since ACPI GpioIo() resource does not have a field saying whether it is
 active low or high, the "active_low" argument can be used here.  Setting
 it to 1 marks the GPIO as active low.
 
 Note, active_low in _DSD does not make sense for GpioInt() resource and
 must be 0. GpioInt() resource has its own means of defining it.
 
 In our Bluetooth example the "reset-gpios" refers to the second GpioIo()
 resource, second pin in that resource with the GPIO number of 31.
 
 The GpioIo() resource unfortunately doesn't explicitly provide an initial
 state of the output pin which driver should use during its initialization.
 
 Linux tries to use common sense here and derives the state from the bias
 and polarity settings. The table below shows the expectations:
 
 =========  =============  ==============
 Pull Bias     Polarity     Requested...
 =========  =============  ==============
 Implicit     x            AS IS (assumed firmware configured for us)
 Explicit     x (no _DSD)  as Pull Bias (Up == High, Down == Low),
                           assuming non-active (Polarity = !Pull Bias)
 Down         Low          as low, assuming active
 Down         High         as low, assuming non-active
 Up           Low          as high, assuming non-active
 Up           High         as high, assuming active
 =========  =============  ==============
 
 That said, for our above example the both GPIOs, since the bias setting
 is explicit and _DSD is present, will be treated as active with a high
 polarity and Linux will configure the pins in this state until a driver
 reprograms them differently.
 
 It is possible to leave holes in the array of GPIOs. This is useful in
 cases like with SPI host controllers where some chip selects may be
 implemented as GPIOs and some as native signals. For example a SPI host
 controller can have chip selects 0 and 2 implemented as GPIOs and 1 as
 native::
 
   Package () {
       "cs-gpios",
       Package () {
           ^GPIO, 19, 0, 0, // chip select 0: GPIO
           0,               // chip select 1: native signal
           ^GPIO, 20, 0, 0, // chip select 2: GPIO
       }
   }
 
 Note, that historically ACPI has no means of the GPIO polarity and thus
 the SPISerialBus() resource defines it on the per-chip basis. In order
 to avoid a chain of negations, the GPIO polarity is considered being
 Active High. Even for the cases when _DSD() is involved (see the example
 above) the GPIO CS polarity must be defined Active High to avoid ambiguity.
 
 Other supported properties
 ==========================
 
 Following Device Tree compatible device properties are also supported by
 _DSD device properties for GPIO controllers:
 
 - gpio-hog
 - output-high
 - output-low
 - input
 - line-name
 
 Example::
 
   Name (_DSD, Package () {
       // _DSD Hierarchical Properties Extension UUID
       ToUUID("dbb8e3e6-5886-4ba6-8795-1319f52a966b"),
       Package () {
           Package () { "hog-gpio8", "G8PU" }
       }
   })
 
   Name (G8PU, Package () {
       ToUUID("daffd814-6eba-4d8c-8a91-bc9bbf4aa301"),
       Package () {
           Package () { "gpio-hog", 1 },
           Package () { "gpios", Package () { 8, 0 } },
           Package () { "output-high", 1 },
           Package () { "line-name", "gpio8-pullup" },
       }
   })
 
 - gpio-line-names
 
 The ``gpio-line-names`` declaration is a list of strings ("names"), which
 describes each line/pin of a GPIO controller/expander. This list, contained in
 a package, must be inserted inside the GPIO controller declaration of an ACPI
 table (typically inside the DSDT). The ``gpio-line-names`` list must respect the
 following rules (see also the examples):
 
   - the first name in the list corresponds with the first line/pin of the GPIO
     controller/expander
   - the names inside the list must be consecutive (no "holes" are permitted)
   - the list can be incomplete and can end before the last GPIO line: in
     other words, it is not mandatory to fill all the GPIO lines
   - empty names are allowed (two quotation marks ``""`` correspond to an empty
     name)
   - names inside one GPIO controller/expander must be unique
 
 Example of a GPIO controller of 16 lines, with an incomplete list with two
 empty names::
 
   Package () {
       "gpio-line-names",
       Package () {
           "pin_0",
           "pin_1",
           "",
           "",
           "pin_3",
           "pin_4_push_button",
       }
   }
 
 At runtime, the above declaration produces the following result (using the
 "libgpiod" tools)::
 
   root@debian:~# gpioinfo gpiochip4
   gpiochip4 - 16 lines:
           line   0:      "pin_0"       unused   input  active-high
           line   1:      "pin_1"       unused   input  active-high
           line   2:      unnamed       unused   input  active-high
           line   3:      unnamed       unused   input  active-high
           line   4:      "pin_3"       unused   input  active-high
           line   5: "pin_4_push_button" unused input active-high
           line   6:      unnamed       unused   input  active-high
           line   7       unnamed       unused   input  active-high
           line   8:      unnamed       unused   input  active-high
           line   9:      unnamed       unused   input  active-high
           line  10:      unnamed       unused   input  active-high
           line  11:      unnamed       unused   input  active-high
           line  12:      unnamed       unused   input  active-high
           line  13:      unnamed       unused   input  active-high
           line  14:      unnamed       unused   input  active-high
           line  15:      unnamed       unused   input  active-high
   root@debian:~# gpiofind pin_4_push_button
   gpiochip4 5
   root@debian:~#
 
 Another example::
 
   Package () {
       "gpio-line-names",
       Package () {
           "SPI0_CS_N", "EXP2_INT", "MUX6_IO", "UART0_RXD",
           "MUX7_IO", "LVL_C_A1", "MUX0_IO", "SPI1_MISO",
       }
   }
 
 See Documentation/devicetree/bindings/gpio/gpio.txt for more information
 about these properties.
 
 ACPI GPIO Mappings Provided by Drivers
 ======================================
 
 There are systems in which the ACPI tables do not contain _DSD but provide _CRS
 with GpioIo()/GpioInt() resources and device drivers still need to work with
 them.
 
 In those cases ACPI device identification objects, _HID, _CID, _CLS, _SUB, _HRV,
 available to the driver can be used to identify the device and that is supposed
 to be sufficient to determine the meaning and purpose of all of the GPIO lines
 listed by the GpioIo()/GpioInt() resources returned by _CRS.  In other words,
 the driver is supposed to know what to use the GpioIo()/GpioInt() resources for
 once it has identified the device.  Having done that, it can simply assign names
 to the GPIO lines it is going to use and provide the GPIO subsystem with a
 mapping between those names and the ACPI GPIO resources corresponding to them.
 
 To do that, the driver needs to define a mapping table as a NULL-terminated
 array of struct acpi_gpio_mapping objects that each contains a name, a pointer
 to an array of line data (struct acpi_gpio_params) objects and the size of that
 array.  Each struct acpi_gpio_params object consists of three fields,
 crs_entry_index, line_index, active_low, representing the index of the target
 GpioIo()/GpioInt() resource in _CRS starting from zero, the index of the target
 line in that resource starting from zero, and the active-low flag for that line,
 respectively, in analogy with the _DSD GPIO property format specified above.
 
 For the example Bluetooth device discussed previously the data structures in
 question would look like this::
 
   static const struct acpi_gpio_params reset_gpio = { 1, 1, false };
   static const struct acpi_gpio_params shutdown_gpio = { 0, 0, false };
 
   static const struct acpi_gpio_mapping bluetooth_acpi_gpios[] = {
     { "reset-gpios", &reset_gpio, 1 },
     { "shutdown-gpios", &shutdown_gpio, 1 },
     { }
   };
 
 Next, the mapping table needs to be passed as the second argument to
 acpi_dev_add_driver_gpios() or its managed analogue that will
 register it with the ACPI device object pointed to by its first
 argument. That should be done in the driver's .probe() routine.
 On removal, the driver should unregister its GPIO mapping table by
 calling acpi_dev_remove_driver_gpios() on the ACPI device object where that
 table was previously registered.
 
 Using the _CRS fallback
 =======================
 
 If a device does not have _DSD or the driver does not create ACPI GPIO
 mapping, the Linux GPIO framework refuses to return any GPIOs. This is
 because the driver does not know what it actually gets. For example if we
 have a device like below::
 
   Device (BTH)
   {
       Name (_HID, ...)
 
       Name (_CRS, ResourceTemplate () {
           GpioIo (Exclusive, PullNone, 0, 0, IoRestrictionNone,
                   "\\_SB.GPO0", 0, ResourceConsumer) { 15 }
           GpioIo (Exclusive, PullNone, 0, 0, IoRestrictionNone,
                   "\\_SB.GPO0", 0, ResourceConsumer) { 27 }
       })
   }
 
 The driver might expect to get the right GPIO when it does::
 
   desc = gpiod_get(dev, "reset", GPIOD_OUT_LOW);
   if (IS_ERR(desc))
         ...error handling...
 
 but since there is no way to know the mapping between "reset" and
 the GpioIo() in _CRS desc will hold ERR_PTR(-ENOENT).
 
 The driver author can solve this by passing the mapping explicitly
 (this is the recommended way and it's documented in the above chapter).
 
 The ACPI GPIO mapping tables should not contaminate drivers that are not
 knowing about which exact device they are servicing on. It implies that
 the ACPI GPIO mapping tables are hardly linked to an ACPI ID and certain
 objects, as listed in the above chapter, of the device in question.
 
 Getting GPIO descriptor
 =======================
 
 There are two main approaches to get GPIO resource from ACPI::
 
   desc = gpiod_get(dev, connection_id, flags);
   desc = gpiod_get_index(dev, connection_id, index, flags);
 
 We may consider two different cases here, i.e. when connection ID is
 provided and otherwise.
 
 Case 1::
 
   desc = gpiod_get(dev, "non-null-connection-id", flags);
   desc = gpiod_get_index(dev, "non-null-connection-id", index, flags);
 
 Case 2::
 
   desc = gpiod_get(dev, NULL, flags);
   desc = gpiod_get_index(dev, NULL, index, flags);
 
 Case 1 assumes that corresponding ACPI device description must have
 defined device properties and will prevent to getting any GPIO resources
 otherwise.
 
 Case 2 explicitly tells GPIO core to look for resources in _CRS.
 
 Be aware that gpiod_get_index() in cases 1 and 2, assuming that there
 are two versions of ACPI device description provided and no mapping is
 present in the driver, will return different resources. That's why a
 certain driver has to handle them carefully as explained in the previous
 chapter.

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