Based on kernel version 4.16.1. Page generated on 2018-04-09 11:52 EST.
1 _DSD Device Properties Related to GPIO 2 -------------------------------------- 3 4 With the release of ACPI 5.1, the _DSD configuration object finally 5 allows names to be given to GPIOs (and other things as well) returned 6 by _CRS. Previously, we were only able to use an integer index to find 7 the corresponding GPIO, which is pretty error prone (it depends on 8 the _CRS output ordering, for example). 9 10 With _DSD we can now query GPIOs using a name instead of an integer 11 index, 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 37 The 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 47 Since ACPI GpioIo() resource does not have a field saying whether it is 48 active low or high, the "active_low" argument can be used here. Setting 49 it to 1 marks the GPIO as active low. 50 51 In our Bluetooth example the "reset-gpios" refers to the second GpioIo() 52 resource, second pin in that resource with the GPIO number of 31. 53 54 It is possible to leave holes in the array of GPIOs. This is useful in 55 cases like with SPI host controllers where some chip selects may be 56 implemented as GPIOs and some as native signals. For example a SPI host 57 controller can have chip selects 0 and 2 implemented as GPIOs and 1 as 58 native: 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 69 Other supported properties 70 -------------------------- 71 72 Following 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 81 Example: 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 102 103 Example: 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 } 112 113 See Documentation/devicetree/bindings/gpio/gpio.txt for more information 114 about these properties. 115 116 ACPI GPIO Mappings Provided by Drivers 117 -------------------------------------- 118 119 There are systems in which the ACPI tables do not contain _DSD but provide _CRS 120 with GpioIo()/GpioInt() resources and device drivers still need to work with 121 them. 122 123 In those cases ACPI device identification objects, _HID, _CID, _CLS, _SUB, _HRV, 124 available to the driver can be used to identify the device and that is supposed 125 to be sufficient to determine the meaning and purpose of all of the GPIO lines 126 listed by the GpioIo()/GpioInt() resources returned by _CRS. In other words, 127 the driver is supposed to know what to use the GpioIo()/GpioInt() resources for 128 once it has identified the device. Having done that, it can simply assign names 129 to the GPIO lines it is going to use and provide the GPIO subsystem with a 130 mapping between those names and the ACPI GPIO resources corresponding to them. 131 132 To do that, the driver needs to define a mapping table as a NULL-terminated 133 array of struct acpi_gpio_mapping objects that each contain a name, a pointer 134 to an array of line data (struct acpi_gpio_params) objects and the size of that 135 array. Each struct acpi_gpio_params object consists of three fields, 136 crs_entry_index, line_index, active_low, representing the index of the target 137 GpioIo()/GpioInt() resource in _CRS starting from zero, the index of the target 138 line in that resource starting from zero, and the active-low flag for that line, 139 respectively, in analogy with the _DSD GPIO property format specified above. 140 141 For the example Bluetooth device discussed previously the data structures in 142 question would look like this: 143 144 static const struct acpi_gpio_params reset_gpio = { 1, 1, false }; 145 static const struct acpi_gpio_params shutdown_gpio = { 0, 0, false }; 146 147 static const struct acpi_gpio_mapping bluetooth_acpi_gpios[] = { 148 { "reset-gpios", &reset_gpio, 1 }, 149 { "shutdown-gpios", &shutdown_gpio, 1 }, 150 { }, 151 }; 152 153 Next, the mapping table needs to be passed as the second argument to 154 acpi_dev_add_driver_gpios() that will register it with the ACPI device object 155 pointed to by its first argument. That should be done in the driver's .probe() 156 routine. On removal, the driver should unregister its GPIO mapping table by 157 calling acpi_dev_remove_driver_gpios() on the ACPI device object where that 158 table was previously registered. 159 160 Using the _CRS fallback 161 ----------------------- 162 163 If a device does not have _DSD or the driver does not create ACPI GPIO 164 mapping, the Linux GPIO framework refuses to return any GPIOs. This is 165 because the driver does not know what it actually gets. For example if we 166 have 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 } 179 180 The driver might expect to get the right GPIO when it does: 181 182 desc = gpiod_get(dev, "reset", GPIOD_OUT_LOW); 183 184 but since there is no way to know the mapping between "reset" and 185 the GpioIo() in _CRS desc will hold ERR_PTR(-ENOENT). 186 187 The driver author can solve this by passing the mapping explictly 188 (the recommended way and documented in the above chapter). 189 190 The ACPI GPIO mapping tables should not contaminate drivers that are not 191 knowing about which exact device they are servicing on. It implies that 192 the ACPI GPIO mapping tables are hardly linked to ACPI ID and certain 193 objects, as listed in the above chapter, of the device in question. 194 195 Getting GPIO descriptor 196 ----------------------- 197 198 There 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); 201 202 We may consider two different cases here, i.e. when connection ID is 203 provided and otherwise. 204 205 Case 1: 206 desc = gpiod_get(dev, "non-null-connection-id", flags); 207 desc = gpiod_get_index(dev, "non-null-connection-id", index, flags); 208 209 Case 2: 210 desc = gpiod_get(dev, NULL, flags); 211 desc = gpiod_get_index(dev, NULL, index, flags); 212 213 Case 1 assumes that corresponding ACPI device description must have 214 defined device properties and will prevent to getting any GPIO resources 215 otherwise. 216 217 Case 2 explicitly tells GPIO core to look for resources in _CRS. 218 219 Be aware that gpiod_get_index() in cases 1 and 2, assuming that there 220 are two versions of ACPI device description provided and no mapping is 221 present in the driver, will return different resources. That's why a 222 certain driver has to handle them carefully as explained in previous 223 chapter.