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