1.. SPDX-License-Identifier: GPL-2.0
2
3======================================
4_DSD Device Properties Related to GPIO
5======================================
6
7With the release of ACPI 5.1, the _DSD configuration object finally
8allows names to be given to GPIOs (and other things as well) returned
9by _CRS.  Previously, we were only able to use an integer index to find
10the corresponding GPIO, which is pretty error prone (it depends on
11the _CRS output ordering, for example).
12
13With _DSD we can now query GPIOs using a name instead of an integer
14index, like the ASL example below shows::
15
16  // Bluetooth device with reset and shutdown GPIOs
17  Device (BTH)
18  {
19      Name (_HID, ...)
20
21      Name (_CRS, ResourceTemplate ()
22      {
23          GpioIo (Exclusive, PullUp, 0, 0, IoRestrictionInputOnly,
24                  "\\_SB.GPO0", 0, ResourceConsumer) {15}
25          GpioIo (Exclusive, PullUp, 0, 0, IoRestrictionInputOnly,
26                  "\\_SB.GPO0", 0, ResourceConsumer) {27, 31}
27      })
28
29      Name (_DSD, Package ()
30      {
31          ToUUID("daffd814-6eba-4d8c-8a91-bc9bbf4aa301"),
32          Package ()
33	  {
34              Package () {"reset-gpios", Package() {^BTH, 1, 1, 0 }},
35              Package () {"shutdown-gpios", Package() {^BTH, 0, 0, 0 }},
36          }
37      })
38  }
39
40The format of the supported GPIO property is::
41
42  Package () { "name", Package () { ref, index, pin, active_low }}
43
44ref
45  The device that has _CRS containing GpioIo()/GpioInt() resources,
46  typically this is the device itself (BTH in our case).
47index
48  Index of the GpioIo()/GpioInt() resource in _CRS starting from zero.
49pin
50  Pin in the GpioIo()/GpioInt() resource. Typically this is zero.
51active_low
52  If 1 the GPIO is marked as active_low.
53
54Since ACPI GpioIo() resource does not have a field saying whether it is
55active low or high, the "active_low" argument can be used here.  Setting
56it to 1 marks the GPIO as active low.
57
58In our Bluetooth example the "reset-gpios" refers to the second GpioIo()
59resource, second pin in that resource with the GPIO number of 31.
60
61It is possible to leave holes in the array of GPIOs. This is useful in
62cases like with SPI host controllers where some chip selects may be
63implemented as GPIOs and some as native signals. For example a SPI host
64controller can have chip selects 0 and 2 implemented as GPIOs and 1 as
65native::
66
67  Package () {
68      "cs-gpios",
69      Package () {
70          ^GPIO, 19, 0, 0, // chip select 0: GPIO
71          0,               // chip select 1: native signal
72          ^GPIO, 20, 0, 0, // chip select 2: GPIO
73      }
74  }
75
76Other supported properties
77==========================
78
79Following Device Tree compatible device properties are also supported by
80_DSD device properties for GPIO controllers:
81
82- gpio-hog
83- output-high
84- output-low
85- input
86- line-name
87
88Example::
89
90  Name (_DSD, Package () {
91      // _DSD Hierarchical Properties Extension UUID
92      ToUUID("dbb8e3e6-5886-4ba6-8795-1319f52a966b"),
93      Package () {
94          Package () {"hog-gpio8", "G8PU"}
95      }
96  })
97
98  Name (G8PU, Package () {
99      ToUUID("daffd814-6eba-4d8c-8a91-bc9bbf4aa301"),
100      Package () {
101          Package () {"gpio-hog", 1},
102          Package () {"gpios", Package () {8, 0}},
103          Package () {"output-high", 1},
104          Package () {"line-name", "gpio8-pullup"},
105      }
106  })
107
108- gpio-line-names
109
110Example::
111
112  Package () {
113      "gpio-line-names",
114      Package () {
115          "SPI0_CS_N", "EXP2_INT", "MUX6_IO", "UART0_RXD", "MUX7_IO",
116          "LVL_C_A1", "MUX0_IO", "SPI1_MISO"
117      }
118  }
119
120See Documentation/devicetree/bindings/gpio/gpio.txt for more information
121about these properties.
122
123ACPI GPIO Mappings Provided by Drivers
124======================================
125
126There are systems in which the ACPI tables do not contain _DSD but provide _CRS
127with GpioIo()/GpioInt() resources and device drivers still need to work with
128them.
129
130In those cases ACPI device identification objects, _HID, _CID, _CLS, _SUB, _HRV,
131available to the driver can be used to identify the device and that is supposed
132to be sufficient to determine the meaning and purpose of all of the GPIO lines
133listed by the GpioIo()/GpioInt() resources returned by _CRS.  In other words,
134the driver is supposed to know what to use the GpioIo()/GpioInt() resources for
135once it has identified the device.  Having done that, it can simply assign names
136to the GPIO lines it is going to use and provide the GPIO subsystem with a
137mapping between those names and the ACPI GPIO resources corresponding to them.
138
139To do that, the driver needs to define a mapping table as a NULL-terminated
140array of struct acpi_gpio_mapping objects that each contain a name, a pointer
141to an array of line data (struct acpi_gpio_params) objects and the size of that
142array.  Each struct acpi_gpio_params object consists of three fields,
143crs_entry_index, line_index, active_low, representing the index of the target
144GpioIo()/GpioInt() resource in _CRS starting from zero, the index of the target
145line in that resource starting from zero, and the active-low flag for that line,
146respectively, in analogy with the _DSD GPIO property format specified above.
147
148For the example Bluetooth device discussed previously the data structures in
149question would look like this::
150
151  static const struct acpi_gpio_params reset_gpio = { 1, 1, false };
152  static const struct acpi_gpio_params shutdown_gpio = { 0, 0, false };
153
154  static const struct acpi_gpio_mapping bluetooth_acpi_gpios[] = {
155    { "reset-gpios", &reset_gpio, 1 },
156    { "shutdown-gpios", &shutdown_gpio, 1 },
157    { },
158  };
159
160Next, the mapping table needs to be passed as the second argument to
161acpi_dev_add_driver_gpios() that will register it with the ACPI device object
162pointed to by its first argument.  That should be done in the driver's .probe()
163routine.  On removal, the driver should unregister its GPIO mapping table by
164calling acpi_dev_remove_driver_gpios() on the ACPI device object where that
165table was previously registered.
166
167Using the _CRS fallback
168=======================
169
170If a device does not have _DSD or the driver does not create ACPI GPIO
171mapping, the Linux GPIO framework refuses to return any GPIOs. This is
172because the driver does not know what it actually gets. For example if we
173have a device like below::
174
175  Device (BTH)
176  {
177      Name (_HID, ...)
178
179      Name (_CRS, ResourceTemplate () {
180          GpioIo (Exclusive, PullNone, 0, 0, IoRestrictionNone,
181                  "\\_SB.GPO0", 0, ResourceConsumer) {15}
182          GpioIo (Exclusive, PullNone, 0, 0, IoRestrictionNone,
183                  "\\_SB.GPO0", 0, ResourceConsumer) {27}
184      })
185  }
186
187The driver might expect to get the right GPIO when it does::
188
189  desc = gpiod_get(dev, "reset", GPIOD_OUT_LOW);
190
191but since there is no way to know the mapping between "reset" and
192the GpioIo() in _CRS desc will hold ERR_PTR(-ENOENT).
193
194The driver author can solve this by passing the mapping explictly
195(the recommended way and documented in the above chapter).
196
197The ACPI GPIO mapping tables should not contaminate drivers that are not
198knowing about which exact device they are servicing on. It implies that
199the ACPI GPIO mapping tables are hardly linked to ACPI ID and certain
200objects, as listed in the above chapter, of the device in question.
201
202Getting GPIO descriptor
203=======================
204
205There are two main approaches to get GPIO resource from ACPI::
206
207  desc = gpiod_get(dev, connection_id, flags);
208  desc = gpiod_get_index(dev, connection_id, index, flags);
209
210We may consider two different cases here, i.e. when connection ID is
211provided and otherwise.
212
213Case 1::
214
215  desc = gpiod_get(dev, "non-null-connection-id", flags);
216  desc = gpiod_get_index(dev, "non-null-connection-id", index, flags);
217
218Case 2::
219
220  desc = gpiod_get(dev, NULL, flags);
221  desc = gpiod_get_index(dev, NULL, index, flags);
222
223Case 1 assumes that corresponding ACPI device description must have
224defined device properties and will prevent to getting any GPIO resources
225otherwise.
226
227Case 2 explicitly tells GPIO core to look for resources in _CRS.
228
229Be aware that gpiod_get_index() in cases 1 and 2, assuming that there
230are two versions of ACPI device description provided and no mapping is
231present in the driver, will return different resources. That's why a
232certain driver has to handle them carefully as explained in previous
233chapter.
234