Documentation: ACPI: move gpio-properties.txt to firmware-guide/acpi and convert to reST

This converts the plain text documentation to reStructuredText format
and adds it to Sphinx TOC tree.

No essential content change.

Signed-off-by: Changbin Du <changbin.du@gmail.com>
Reviewed-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

2 files changed, 234 insertions, 0 deletions

diff --git a/Documentation/firmware-guide/acpi/gpio-properties.rst b/Documentation/firmware-guide/acpi/gpio-properties.rst
new file mode 100644
index 000000000000..bb6d74f23ee0
--- /dev/null
+++ b/Documentation/firmware-guide/acpi/gpio-properties.rst
@@ -0,0 +1,233 @@
+.. 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, IoRestrictionInputOnly,
+                  "\\_SB.GPO0", 0, ResourceConsumer) {15}
+          GpioIo (Exclusive, PullUp, 0, 0, IoRestrictionInputOnly,
+                  "\\_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.
+
+In our Bluetooth example the "reset-gpios" refers to the second GpioIo()
+resource, second pin in that resource with the GPIO number of 31.
+
+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
+      }
+  }
+
+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
+
+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 contain 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() 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);
+
+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 explictly
+(the recommended way and 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 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 previous
+chapter.
diff --git a/Documentation/firmware-guide/acpi/index.rst b/Documentation/firmware-guide/acpi/index.rst
index 0e05b843521c..61d67763851b 100644
--- a/Documentation/firmware-guide/acpi/index.rst
+++ b/Documentation/firmware-guide/acpi/index.rst
@@ -11,3 +11,4 @@ ACPI Support
    enumeration
    osi
    DSD-properties-rules
+   gpio-properties