Based on kernel version 4.16.1. Page generated on 2018-04-09 11:53 EST.
1 Platform Devices and Drivers 2 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 3 See <linux/platform_device.h> for the driver model interface to the 4 platform bus: platform_device, and platform_driver. This pseudo-bus 5 is used to connect devices on busses with minimal infrastructure, 6 like those used to integrate peripherals on many system-on-chip 7 processors, or some "legacy" PC interconnects; as opposed to large 8 formally specified ones like PCI or USB. 9 10 11 Platform devices 12 ~~~~~~~~~~~~~~~~ 13 Platform devices are devices that typically appear as autonomous 14 entities in the system. This includes legacy port-based devices and 15 host bridges to peripheral buses, and most controllers integrated 16 into system-on-chip platforms. What they usually have in common 17 is direct addressing from a CPU bus. Rarely, a platform_device will 18 be connected through a segment of some other kind of bus; but its 19 registers will still be directly addressable. 20 21 Platform devices are given a name, used in driver binding, and a 22 list of resources such as addresses and IRQs. 23 24 struct platform_device { 25 const char *name; 26 u32 id; 27 struct device dev; 28 u32 num_resources; 29 struct resource *resource; 30 }; 31 32 33 Platform drivers 34 ~~~~~~~~~~~~~~~~ 35 Platform drivers follow the standard driver model convention, where 36 discovery/enumeration is handled outside the drivers, and drivers 37 provide probe() and remove() methods. They support power management 38 and shutdown notifications using the standard conventions. 39 40 struct platform_driver { 41 int (*probe)(struct platform_device *); 42 int (*remove)(struct platform_device *); 43 void (*shutdown)(struct platform_device *); 44 int (*suspend)(struct platform_device *, pm_message_t state); 45 int (*suspend_late)(struct platform_device *, pm_message_t state); 46 int (*resume_early)(struct platform_device *); 47 int (*resume)(struct platform_device *); 48 struct device_driver driver; 49 }; 50 51 Note that probe() should in general verify that the specified device hardware 52 actually exists; sometimes platform setup code can't be sure. The probing 53 can use device resources, including clocks, and device platform_data. 54 55 Platform drivers register themselves the normal way: 56 57 int platform_driver_register(struct platform_driver *drv); 58 59 Or, in common situations where the device is known not to be hot-pluggable, 60 the probe() routine can live in an init section to reduce the driver's 61 runtime memory footprint: 62 63 int platform_driver_probe(struct platform_driver *drv, 64 int (*probe)(struct platform_device *)) 65 66 Kernel modules can be composed of several platform drivers. The platform core 67 provides helpers to register and unregister an array of drivers: 68 69 int __platform_register_drivers(struct platform_driver * const *drivers, 70 unsigned int count, struct module *owner); 71 void platform_unregister_drivers(struct platform_driver * const *drivers, 72 unsigned int count); 73 74 If one of the drivers fails to register, all drivers registered up to that 75 point will be unregistered in reverse order. Note that there is a convenience 76 macro that passes THIS_MODULE as owner parameter: 77 78 #define platform_register_drivers(drivers, count) 79 80 81 Device Enumeration 82 ~~~~~~~~~~~~~~~~~~ 83 As a rule, platform specific (and often board-specific) setup code will 84 register platform devices: 85 86 int platform_device_register(struct platform_device *pdev); 87 88 int platform_add_devices(struct platform_device **pdevs, int ndev); 89 90 The general rule is to register only those devices that actually exist, 91 but in some cases extra devices might be registered. For example, a kernel 92 might be configured to work with an external network adapter that might not 93 be populated on all boards, or likewise to work with an integrated controller 94 that some boards might not hook up to any peripherals. 95 96 In some cases, boot firmware will export tables describing the devices 97 that are populated on a given board. Without such tables, often the 98 only way for system setup code to set up the correct devices is to build 99 a kernel for a specific target board. Such board-specific kernels are 100 common with embedded and custom systems development. 101 102 In many cases, the memory and IRQ resources associated with the platform 103 device are not enough to let the device's driver work. Board setup code 104 will often provide additional information using the device's platform_data 105 field to hold additional information. 106 107 Embedded systems frequently need one or more clocks for platform devices, 108 which are normally kept off until they're actively needed (to save power). 109 System setup also associates those clocks with the device, so that that 110 calls to clk_get(&pdev->dev, clock_name) return them as needed. 111 112 113 Legacy Drivers: Device Probing 114 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 115 Some drivers are not fully converted to the driver model, because they take 116 on a non-driver role: the driver registers its platform device, rather than 117 leaving that for system infrastructure. Such drivers can't be hotplugged 118 or coldplugged, since those mechanisms require device creation to be in a 119 different system component than the driver. 120 121 The only "good" reason for this is to handle older system designs which, like 122 original IBM PCs, rely on error-prone "probe-the-hardware" models for hardware 123 configuration. Newer systems have largely abandoned that model, in favor of 124 bus-level support for dynamic configuration (PCI, USB), or device tables 125 provided by the boot firmware (e.g. PNPACPI on x86). There are too many 126 conflicting options about what might be where, and even educated guesses by 127 an operating system will be wrong often enough to make trouble. 128 129 This style of driver is discouraged. If you're updating such a driver, 130 please try to move the device enumeration to a more appropriate location, 131 outside the driver. This will usually be cleanup, since such drivers 132 tend to already have "normal" modes, such as ones using device nodes that 133 were created by PNP or by platform device setup. 134 135 None the less, there are some APIs to support such legacy drivers. Avoid 136 using these calls except with such hotplug-deficient drivers. 137 138 struct platform_device *platform_device_alloc( 139 const char *name, int id); 140 141 You can use platform_device_alloc() to dynamically allocate a device, which 142 you will then initialize with resources and platform_device_register(). 143 A better solution is usually: 144 145 struct platform_device *platform_device_register_simple( 146 const char *name, int id, 147 struct resource *res, unsigned int nres); 148 149 You can use platform_device_register_simple() as a one-step call to allocate 150 and register a device. 151 152 153 Device Naming and Driver Binding 154 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 155 The platform_device.dev.bus_id is the canonical name for the devices. 156 It's built from two components: 157 158 * platform_device.name ... which is also used to for driver matching. 159 160 * platform_device.id ... the device instance number, or else "-1" 161 to indicate there's only one. 162 163 These are concatenated, so name/id "serial"/0 indicates bus_id "serial.0", and 164 "serial/3" indicates bus_id "serial.3"; both would use the platform_driver 165 named "serial". While "my_rtc"/-1 would be bus_id "my_rtc" (no instance id) 166 and use the platform_driver called "my_rtc". 167 168 Driver binding is performed automatically by the driver core, invoking 169 driver probe() after finding a match between device and driver. If the 170 probe() succeeds, the driver and device are bound as usual. There are 171 three different ways to find such a match: 172 173 - Whenever a device is registered, the drivers for that bus are 174 checked for matches. Platform devices should be registered very 175 early during system boot. 176 177 - When a driver is registered using platform_driver_register(), all 178 unbound devices on that bus are checked for matches. Drivers 179 usually register later during booting, or by module loading. 180 181 - Registering a driver using platform_driver_probe() works just like 182 using platform_driver_register(), except that the driver won't 183 be probed later if another device registers. (Which is OK, since 184 this interface is only for use with non-hotpluggable devices.) 185 186 187 Early Platform Devices and Drivers 188 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 189 The early platform interfaces provide platform data to platform device 190 drivers early on during the system boot. The code is built on top of the 191 early_param() command line parsing and can be executed very early on. 192 193 Example: "earlyprintk" class early serial console in 6 steps 194 195 1. Registering early platform device data 196 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 197 The architecture code registers platform device data using the function 198 early_platform_add_devices(). In the case of early serial console this 199 should be hardware configuration for the serial port. Devices registered 200 at this point will later on be matched against early platform drivers. 201 202 2. Parsing kernel command line 203 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 204 The architecture code calls parse_early_param() to parse the kernel 205 command line. This will execute all matching early_param() callbacks. 206 User specified early platform devices will be registered at this point. 207 For the early serial console case the user can specify port on the 208 kernel command line as "earlyprintk=serial.0" where "earlyprintk" is 209 the class string, "serial" is the name of the platform driver and 210 0 is the platform device id. If the id is -1 then the dot and the 211 id can be omitted. 212 213 3. Installing early platform drivers belonging to a certain class 214 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 215 The architecture code may optionally force registration of all early 216 platform drivers belonging to a certain class using the function 217 early_platform_driver_register_all(). User specified devices from 218 step 2 have priority over these. This step is omitted by the serial 219 driver example since the early serial driver code should be disabled 220 unless the user has specified port on the kernel command line. 221 222 4. Early platform driver registration 223 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 224 Compiled-in platform drivers making use of early_platform_init() are 225 automatically registered during step 2 or 3. The serial driver example 226 should use early_platform_init("earlyprintk", &platform_driver). 227 228 5. Probing of early platform drivers belonging to a certain class 229 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 230 The architecture code calls early_platform_driver_probe() to match 231 registered early platform devices associated with a certain class with 232 registered early platform drivers. Matched devices will get probed(). 233 This step can be executed at any point during the early boot. As soon 234 as possible may be good for the serial port case. 235 236 6. Inside the early platform driver probe() 237 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 238 The driver code needs to take special care during early boot, especially 239 when it comes to memory allocation and interrupt registration. The code 240 in the probe() function can use is_early_platform_device() to check if 241 it is called at early platform device or at the regular platform device 242 time. The early serial driver performs register_console() at this point. 243 244 For further information, see <linux/platform_device.h>.