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, IoRestrictionOutputOnly,
24                  "\\_SB.GPO0", 0, ResourceConsumer) {15}
25          GpioIo (Exclusive, PullUp, 0, 0, IoRestrictionOutputOnly,
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
58Note, active_low in _DSD does not make sense for GpioInt() resource and
59must be 0. GpioInt() resource has its own means of defining it.
60
61In our Bluetooth example the "reset-gpios" refers to the second GpioIo()
62resource, second pin in that resource with the GPIO number of 31.
63
64The GpioIo() resource unfortunately doesn't explicitly provide an initial
65state of the output pin which driver should use during its initialization.
66
67Linux tries to use common sense here and derives the state from the bias
68and polarity settings. The table below shows the expectations:
69
70=========  =============  ==============
71Pull Bias     Polarity     Requested...
72=========  =============  ==============
73Implicit     x            AS IS (assumed firmware configured for us)
74Explicit     x (no _DSD)  as Pull Bias (Up == High, Down == Low),
75                          assuming non-active (Polarity = !Pull Bias)
76Down         Low          as low, assuming active
77Down         High         as low, assuming non-active
78Up           Low          as high, assuming non-active
79Up           High         as high, assuming active
80=========  =============  ==============
81
82That said, for our above example the both GPIOs, since the bias setting
83is explicit and _DSD is present, will be treated as active with a high
84polarity and Linux will configure the pins in this state until a driver
85reprograms them differently.
86
87It is possible to leave holes in the array of GPIOs. This is useful in
88cases like with SPI host controllers where some chip selects may be
89implemented as GPIOs and some as native signals. For example a SPI host
90controller can have chip selects 0 and 2 implemented as GPIOs and 1 as
91native::
92
93  Package () {
94      "cs-gpios",
95      Package () {
96          ^GPIO, 19, 0, 0, // chip select 0: GPIO
97          0,               // chip select 1: native signal
98          ^GPIO, 20, 0, 0, // chip select 2: GPIO
99      }
100  }
101
102Other supported properties
103==========================
104
105Following Device Tree compatible device properties are also supported by
106_DSD device properties for GPIO controllers:
107
108- gpio-hog
109- output-high
110- output-low
111- input
112- line-name
113
114Example::
115
116  Name (_DSD, Package () {
117      // _DSD Hierarchical Properties Extension UUID
118      ToUUID("dbb8e3e6-5886-4ba6-8795-1319f52a966b"),
119      Package () {
120          Package () {"hog-gpio8", "G8PU"}
121      }
122  })
123
124  Name (G8PU, Package () {
125      ToUUID("daffd814-6eba-4d8c-8a91-bc9bbf4aa301"),
126      Package () {
127          Package () {"gpio-hog", 1},
128          Package () {"gpios", Package () {8, 0}},
129          Package () {"output-high", 1},
130          Package () {"line-name", "gpio8-pullup"},
131      }
132  })
133
134- gpio-line-names
135
136Example::
137
138  Package () {
139      "gpio-line-names",
140      Package () {
141          "SPI0_CS_N", "EXP2_INT", "MUX6_IO", "UART0_RXD",
142          "MUX7_IO", "LVL_C_A1", "MUX0_IO", "SPI1_MISO",
143      }
144  }
145
146See Documentation/devicetree/bindings/gpio/gpio.txt for more information
147about these properties.
148
149ACPI GPIO Mappings Provided by Drivers
150======================================
151
152There are systems in which the ACPI tables do not contain _DSD but provide _CRS
153with GpioIo()/GpioInt() resources and device drivers still need to work with
154them.
155
156In those cases ACPI device identification objects, _HID, _CID, _CLS, _SUB, _HRV,
157available to the driver can be used to identify the device and that is supposed
158to be sufficient to determine the meaning and purpose of all of the GPIO lines
159listed by the GpioIo()/GpioInt() resources returned by _CRS.  In other words,
160the driver is supposed to know what to use the GpioIo()/GpioInt() resources for
161once it has identified the device.  Having done that, it can simply assign names
162to the GPIO lines it is going to use and provide the GPIO subsystem with a
163mapping between those names and the ACPI GPIO resources corresponding to them.
164
165To do that, the driver needs to define a mapping table as a NULL-terminated
166array of struct acpi_gpio_mapping objects that each contains a name, a pointer
167to an array of line data (struct acpi_gpio_params) objects and the size of that
168array.  Each struct acpi_gpio_params object consists of three fields,
169crs_entry_index, line_index, active_low, representing the index of the target
170GpioIo()/GpioInt() resource in _CRS starting from zero, the index of the target
171line in that resource starting from zero, and the active-low flag for that line,
172respectively, in analogy with the _DSD GPIO property format specified above.
173
174For the example Bluetooth device discussed previously the data structures in
175question would look like this::
176
177  static const struct acpi_gpio_params reset_gpio = { 1, 1, false };
178  static const struct acpi_gpio_params shutdown_gpio = { 0, 0, false };
179
180  static const struct acpi_gpio_mapping bluetooth_acpi_gpios[] = {
181    { "reset-gpios", &reset_gpio, 1 },
182    { "shutdown-gpios", &shutdown_gpio, 1 },
183    { }
184  };
185
186Next, the mapping table needs to be passed as the second argument to
187acpi_dev_add_driver_gpios() or its managed analogue that will
188register it with the ACPI device object pointed to by its first
189argument. That should be done in the driver's .probe() routine.
190On removal, the driver should unregister its GPIO mapping table by
191calling acpi_dev_remove_driver_gpios() on the ACPI device object where that
192table was previously registered.
193
194Using the _CRS fallback
195=======================
196
197If a device does not have _DSD or the driver does not create ACPI GPIO
198mapping, the Linux GPIO framework refuses to return any GPIOs. This is
199because the driver does not know what it actually gets. For example if we
200have a device like below::
201
202  Device (BTH)
203  {
204      Name (_HID, ...)
205
206      Name (_CRS, ResourceTemplate () {
207          GpioIo (Exclusive, PullNone, 0, 0, IoRestrictionNone,
208                  "\\_SB.GPO0", 0, ResourceConsumer) {15}
209          GpioIo (Exclusive, PullNone, 0, 0, IoRestrictionNone,
210                  "\\_SB.GPO0", 0, ResourceConsumer) {27}
211      })
212  }
213
214The driver might expect to get the right GPIO when it does::
215
216  desc = gpiod_get(dev, "reset", GPIOD_OUT_LOW);
217
218but since there is no way to know the mapping between "reset" and
219the GpioIo() in _CRS desc will hold ERR_PTR(-ENOENT).
220
221The driver author can solve this by passing the mapping explicitly
222(this is the recommended way and it's documented in the above chapter).
223
224The ACPI GPIO mapping tables should not contaminate drivers that are not
225knowing about which exact device they are servicing on. It implies that
226the ACPI GPIO mapping tables are hardly linked to an ACPI ID and certain
227objects, as listed in the above chapter, of the device in question.
228
229Getting GPIO descriptor
230=======================
231
232There are two main approaches to get GPIO resource from ACPI::
233
234  desc = gpiod_get(dev, connection_id, flags);
235  desc = gpiod_get_index(dev, connection_id, index, flags);
236
237We may consider two different cases here, i.e. when connection ID is
238provided and otherwise.
239
240Case 1::
241
242  desc = gpiod_get(dev, "non-null-connection-id", flags);
243  desc = gpiod_get_index(dev, "non-null-connection-id", index, flags);
244
245Case 2::
246
247  desc = gpiod_get(dev, NULL, flags);
248  desc = gpiod_get_index(dev, NULL, index, flags);
249
250Case 1 assumes that corresponding ACPI device description must have
251defined device properties and will prevent to getting any GPIO resources
252otherwise.
253
254Case 2 explicitly tells GPIO core to look for resources in _CRS.
255
256Be aware that gpiod_get_index() in cases 1 and 2, assuming that there
257are two versions of ACPI device description provided and no mapping is
258present in the driver, will return different resources. That's why a
259certain driver has to handle them carefully as explained in the previous
260chapter.
261