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+-------------+-------------+-----------------------------------------------+ 71| Pull Bias | Polarity | Requested... | 72+=============+=============+===============================================+ 73| Implicit | 74+-------------+-------------+-----------------------------------------------+ 75| **Default** | x | AS IS (assumed firmware configured it for us) | 76+-------------+-------------+-----------------------------------------------+ 77| Explicit | 78+-------------+-------------+-----------------------------------------------+ 79| **None** | x | AS IS (assumed firmware configured it for us) | 80| | | with no Pull Bias | 81+-------------+-------------+-----------------------------------------------+ 82| **Up** | x (no _DSD) | | 83| +-------------+ as high, assuming non-active | 84| | Low | | 85| +-------------+-----------------------------------------------+ 86| | High | as high, assuming active | 87+-------------+-------------+-----------------------------------------------+ 88| **Down** | x (no _DSD) | | 89| +-------------+ as low, assuming non-active | 90| | High | | 91| +-------------+-----------------------------------------------+ 92| | Low | as low, assuming active | 93+-------------+-------------+-----------------------------------------------+ 94 95That said, for our above example the both GPIOs, since the bias setting 96is explicit and _DSD is present, will be treated as active with a high 97polarity and Linux will configure the pins in this state until a driver 98reprograms them differently. 99 100It is possible to leave holes in the array of GPIOs. This is useful in 101cases like with SPI host controllers where some chip selects may be 102implemented as GPIOs and some as native signals. For example a SPI host 103controller can have chip selects 0 and 2 implemented as GPIOs and 1 as 104native:: 105 106 Package () { 107 "cs-gpios", 108 Package () { 109 ^GPIO, 19, 0, 0, // chip select 0: GPIO 110 0, // chip select 1: native signal 111 ^GPIO, 20, 0, 0, // chip select 2: GPIO 112 } 113 } 114 115Note, that historically ACPI has no means of the GPIO polarity and thus 116the SPISerialBus() resource defines it on the per-chip basis. In order 117to avoid a chain of negations, the GPIO polarity is considered being 118Active High. Even for the cases when _DSD() is involved (see the example 119above) the GPIO CS polarity must be defined Active High to avoid ambiguity. 120 121Other supported properties 122========================== 123 124Following Device Tree compatible device properties are also supported by 125_DSD device properties for GPIO controllers: 126 127- gpio-hog 128- output-high 129- output-low 130- input 131- line-name 132 133Example:: 134 135 Name (_DSD, Package () { 136 // _DSD Hierarchical Properties Extension UUID 137 ToUUID("dbb8e3e6-5886-4ba6-8795-1319f52a966b"), 138 Package () { 139 Package () { "hog-gpio8", "G8PU" } 140 } 141 }) 142 143 Name (G8PU, Package () { 144 ToUUID("daffd814-6eba-4d8c-8a91-bc9bbf4aa301"), 145 Package () { 146 Package () { "gpio-hog", 1 }, 147 Package () { "gpios", Package () { 8, 0 } }, 148 Package () { "output-high", 1 }, 149 Package () { "line-name", "gpio8-pullup" }, 150 } 151 }) 152 153- gpio-line-names 154 155The ``gpio-line-names`` declaration is a list of strings ("names"), which 156describes each line/pin of a GPIO controller/expander. This list, contained in 157a package, must be inserted inside the GPIO controller declaration of an ACPI 158table (typically inside the DSDT). The ``gpio-line-names`` list must respect the 159following rules (see also the examples): 160 161 - the first name in the list corresponds with the first line/pin of the GPIO 162 controller/expander 163 - the names inside the list must be consecutive (no "holes" are permitted) 164 - the list can be incomplete and can end before the last GPIO line: in 165 other words, it is not mandatory to fill all the GPIO lines 166 - empty names are allowed (two quotation marks ``""`` correspond to an empty 167 name) 168 - names inside one GPIO controller/expander must be unique 169 170Example of a GPIO controller of 16 lines, with an incomplete list with two 171empty names:: 172 173 Package () { 174 "gpio-line-names", 175 Package () { 176 "pin_0", 177 "pin_1", 178 "", 179 "", 180 "pin_3", 181 "pin_4_push_button", 182 } 183 } 184 185At runtime, the above declaration produces the following result (using the 186"libgpiod" tools):: 187 188 root@debian:~# gpioinfo gpiochip4 189 gpiochip4 - 16 lines: 190 line 0: "pin_0" unused input active-high 191 line 1: "pin_1" unused input active-high 192 line 2: unnamed unused input active-high 193 line 3: unnamed unused input active-high 194 line 4: "pin_3" unused input active-high 195 line 5: "pin_4_push_button" unused input active-high 196 line 6: unnamed unused input active-high 197 line 7 unnamed unused input active-high 198 line 8: unnamed unused input active-high 199 line 9: unnamed unused input active-high 200 line 10: unnamed unused input active-high 201 line 11: unnamed unused input active-high 202 line 12: unnamed unused input active-high 203 line 13: unnamed unused input active-high 204 line 14: unnamed unused input active-high 205 line 15: unnamed unused input active-high 206 root@debian:~# gpiofind pin_4_push_button 207 gpiochip4 5 208 root@debian:~# 209 210Another example:: 211 212 Package () { 213 "gpio-line-names", 214 Package () { 215 "SPI0_CS_N", "EXP2_INT", "MUX6_IO", "UART0_RXD", 216 "MUX7_IO", "LVL_C_A1", "MUX0_IO", "SPI1_MISO", 217 } 218 } 219 220See Documentation/devicetree/bindings/gpio/gpio.txt for more information 221about these properties. 222 223ACPI GPIO Mappings Provided by Drivers 224====================================== 225 226There are systems in which the ACPI tables do not contain _DSD but provide _CRS 227with GpioIo()/GpioInt() resources and device drivers still need to work with 228them. 229 230In those cases ACPI device identification objects, _HID, _CID, _CLS, _SUB, _HRV, 231available to the driver can be used to identify the device and that is supposed 232to be sufficient to determine the meaning and purpose of all of the GPIO lines 233listed by the GpioIo()/GpioInt() resources returned by _CRS. In other words, 234the driver is supposed to know what to use the GpioIo()/GpioInt() resources for 235once it has identified the device. Having done that, it can simply assign names 236to the GPIO lines it is going to use and provide the GPIO subsystem with a 237mapping between those names and the ACPI GPIO resources corresponding to them. 238 239To do that, the driver needs to define a mapping table as a NULL-terminated 240array of struct acpi_gpio_mapping objects that each contains a name, a pointer 241to an array of line data (struct acpi_gpio_params) objects and the size of that 242array. Each struct acpi_gpio_params object consists of three fields, 243crs_entry_index, line_index, active_low, representing the index of the target 244GpioIo()/GpioInt() resource in _CRS starting from zero, the index of the target 245line in that resource starting from zero, and the active-low flag for that line, 246respectively, in analogy with the _DSD GPIO property format specified above. 247 248For the example Bluetooth device discussed previously the data structures in 249question would look like this:: 250 251 static const struct acpi_gpio_params reset_gpio = { 1, 1, false }; 252 static const struct acpi_gpio_params shutdown_gpio = { 0, 0, false }; 253 254 static const struct acpi_gpio_mapping bluetooth_acpi_gpios[] = { 255 { "reset-gpios", &reset_gpio, 1 }, 256 { "shutdown-gpios", &shutdown_gpio, 1 }, 257 { } 258 }; 259 260Next, the mapping table needs to be passed as the second argument to 261acpi_dev_add_driver_gpios() or its managed analogue that will 262register it with the ACPI device object pointed to by its first 263argument. That should be done in the driver's .probe() routine. 264On removal, the driver should unregister its GPIO mapping table by 265calling acpi_dev_remove_driver_gpios() on the ACPI device object where that 266table was previously registered. 267 268Using the _CRS fallback 269======================= 270 271If a device does not have _DSD or the driver does not create ACPI GPIO 272mapping, the Linux GPIO framework refuses to return any GPIOs. This is 273because the driver does not know what it actually gets. For example if we 274have a device like below:: 275 276 Device (BTH) 277 { 278 Name (_HID, ...) 279 280 Name (_CRS, ResourceTemplate () { 281 GpioIo (Exclusive, PullNone, 0, 0, IoRestrictionNone, 282 "\\_SB.GPO0", 0, ResourceConsumer) { 15 } 283 GpioIo (Exclusive, PullNone, 0, 0, IoRestrictionNone, 284 "\\_SB.GPO0", 0, ResourceConsumer) { 27 } 285 }) 286 } 287 288The driver might expect to get the right GPIO when it does:: 289 290 desc = gpiod_get(dev, "reset", GPIOD_OUT_LOW); 291 if (IS_ERR(desc)) 292 ...error handling... 293 294but since there is no way to know the mapping between "reset" and 295the GpioIo() in _CRS desc will hold ERR_PTR(-ENOENT). 296 297The driver author can solve this by passing the mapping explicitly 298(this is the recommended way and it's documented in the above chapter). 299 300The ACPI GPIO mapping tables should not contaminate drivers that are not 301knowing about which exact device they are servicing on. It implies that 302the ACPI GPIO mapping tables are hardly linked to an ACPI ID and certain 303objects, as listed in the above chapter, of the device in question. 304 305Getting GPIO descriptor 306======================= 307 308There are two main approaches to get GPIO resource from ACPI:: 309 310 desc = gpiod_get(dev, connection_id, flags); 311 desc = gpiod_get_index(dev, connection_id, index, flags); 312 313We may consider two different cases here, i.e. when connection ID is 314provided and otherwise. 315 316Case 1:: 317 318 desc = gpiod_get(dev, "non-null-connection-id", flags); 319 desc = gpiod_get_index(dev, "non-null-connection-id", index, flags); 320 321Case 2:: 322 323 desc = gpiod_get(dev, NULL, flags); 324 desc = gpiod_get_index(dev, NULL, index, flags); 325 326Case 1 assumes that corresponding ACPI device description must have 327defined device properties and will prevent to getting any GPIO resources 328otherwise. 329 330Case 2 explicitly tells GPIO core to look for resources in _CRS. 331 332Be aware that gpiod_get_index() in cases 1 and 2, assuming that there 333are two versions of ACPI device description provided and no mapping is 334present in the driver, will return different resources. That's why a 335certain driver has to handle them carefully as explained in the previous 336chapter. 337
