1============================ 2Platform Devices and Drivers 3============================ 4 5See <linux/platform_device.h> for the driver model interface to the 6platform bus: platform_device, and platform_driver. This pseudo-bus 7is used to connect devices on busses with minimal infrastructure, 8like those used to integrate peripherals on many system-on-chip 9processors, or some "legacy" PC interconnects; as opposed to large 10formally specified ones like PCI or USB. 11 12 13Platform devices 14~~~~~~~~~~~~~~~~ 15Platform devices are devices that typically appear as autonomous 16entities in the system. This includes legacy port-based devices and 17host bridges to peripheral buses, and most controllers integrated 18into system-on-chip platforms. What they usually have in common 19is direct addressing from a CPU bus. Rarely, a platform_device will 20be connected through a segment of some other kind of bus; but its 21registers will still be directly addressable. 22 23Platform devices are given a name, used in driver binding, and a 24list of resources such as addresses and IRQs:: 25 26 struct platform_device { 27 const char *name; 28 u32 id; 29 struct device dev; 30 u32 num_resources; 31 struct resource *resource; 32 }; 33 34 35Platform drivers 36~~~~~~~~~~~~~~~~ 37Platform drivers follow the standard driver model convention, where 38discovery/enumeration is handled outside the drivers, and drivers 39provide probe() and remove() methods. They support power management 40and shutdown notifications using the standard conventions:: 41 42 struct platform_driver { 43 int (*probe)(struct platform_device *); 44 void (*remove)(struct platform_device *); 45 void (*shutdown)(struct platform_device *); 46 int (*suspend)(struct platform_device *, pm_message_t state); 47 int (*resume)(struct platform_device *); 48 struct device_driver driver; 49 const struct platform_device_id *id_table; 50 bool prevent_deferred_probe; 51 bool driver_managed_dma; 52 }; 53 54Note that probe() should in general verify that the specified device hardware 55actually exists; sometimes platform setup code can't be sure. The probing 56can use device resources, including clocks, and device platform_data. 57 58Platform drivers register themselves the normal way:: 59 60 int platform_driver_register(struct platform_driver *drv); 61 62Or, in common situations where the device is known not to be hot-pluggable, 63the probe() routine can live in an init section to reduce the driver's 64runtime memory footprint:: 65 66 int platform_driver_probe(struct platform_driver *drv, 67 int (*probe)(struct platform_device *)) 68 69Kernel modules can be composed of several platform drivers. The platform core 70provides helpers to register and unregister an array of drivers:: 71 72 int __platform_register_drivers(struct platform_driver * const *drivers, 73 unsigned int count, struct module *owner); 74 void platform_unregister_drivers(struct platform_driver * const *drivers, 75 unsigned int count); 76 77If one of the drivers fails to register, all drivers registered up to that 78point will be unregistered in reverse order. Note that there is a convenience 79macro that passes THIS_MODULE as owner parameter:: 80 81 #define platform_register_drivers(drivers, count) 82 83 84Device Enumeration 85~~~~~~~~~~~~~~~~~~ 86As a rule, platform specific (and often board-specific) setup code will 87register platform devices:: 88 89 int platform_device_register(struct platform_device *pdev); 90 91 int platform_add_devices(struct platform_device **pdevs, int ndev); 92 93The general rule is to register only those devices that actually exist, 94but in some cases extra devices might be registered. For example, a kernel 95might be configured to work with an external network adapter that might not 96be populated on all boards, or likewise to work with an integrated controller 97that some boards might not hook up to any peripherals. 98 99In some cases, boot firmware will export tables describing the devices 100that are populated on a given board. Without such tables, often the 101only way for system setup code to set up the correct devices is to build 102a kernel for a specific target board. Such board-specific kernels are 103common with embedded and custom systems development. 104 105In many cases, the memory and IRQ resources associated with the platform 106device are not enough to let the device's driver work. Board setup code 107will often provide additional information using the device's platform_data 108field to hold additional information. 109 110Embedded systems frequently need one or more clocks for platform devices, 111which are normally kept off until they're actively needed (to save power). 112System setup also associates those clocks with the device, so that 113calls to clk_get(&pdev->dev, clock_name) return them as needed. 114 115 116Legacy Drivers: Device Probing 117~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 118Some drivers are not fully converted to the driver model, because they take 119on a non-driver role: the driver registers its platform device, rather than 120leaving that for system infrastructure. Such drivers can't be hotplugged 121or coldplugged, since those mechanisms require device creation to be in a 122different system component than the driver. 123 124The only "good" reason for this is to handle older system designs which, like 125original IBM PCs, rely on error-prone "probe-the-hardware" models for hardware 126configuration. Newer systems have largely abandoned that model, in favor of 127bus-level support for dynamic configuration (PCI, USB), or device tables 128provided by the boot firmware (e.g. PNPACPI on x86). There are too many 129conflicting options about what might be where, and even educated guesses by 130an operating system will be wrong often enough to make trouble. 131 132This style of driver is discouraged. If you're updating such a driver, 133please try to move the device enumeration to a more appropriate location, 134outside the driver. This will usually be cleanup, since such drivers 135tend to already have "normal" modes, such as ones using device nodes that 136were created by PNP or by platform device setup. 137 138None the less, there are some APIs to support such legacy drivers. Avoid 139using these calls except with such hotplug-deficient drivers:: 140 141 struct platform_device *platform_device_alloc( 142 const char *name, int id); 143 144You can use platform_device_alloc() to dynamically allocate a device, which 145you will then initialize with resources and platform_device_register(). 146A better solution is usually:: 147 148 struct platform_device *platform_device_register_simple( 149 const char *name, int id, 150 struct resource *res, unsigned int nres); 151 152You can use platform_device_register_simple() as a one-step call to allocate 153and register a device. 154 155 156Device Naming and Driver Binding 157~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 158The platform_device.dev.bus_id is the canonical name for the devices. 159It's built from two components: 160 161 * platform_device.name ... which is also used to for driver matching. 162 163 * platform_device.id ... the device instance number, or else "-1" 164 to indicate there's only one. 165 166These are concatenated, so name/id "serial"/0 indicates bus_id "serial.0", and 167"serial/3" indicates bus_id "serial.3"; both would use the platform_driver 168named "serial". While "my_rtc"/-1 would be bus_id "my_rtc" (no instance id) 169and use the platform_driver called "my_rtc". 170 171Driver binding is performed automatically by the driver core, invoking 172driver probe() after finding a match between device and driver. If the 173probe() succeeds, the driver and device are bound as usual. There are 174three different ways to find such a match: 175 176 - Whenever a device is registered, the drivers for that bus are 177 checked for matches. Platform devices should be registered very 178 early during system boot. 179 180 - When a driver is registered using platform_driver_register(), all 181 unbound devices on that bus are checked for matches. Drivers 182 usually register later during booting, or by module loading. 183 184 - Registering a driver using platform_driver_probe() works just like 185 using platform_driver_register(), except that the driver won't 186 be probed later if another device registers. (Which is OK, since 187 this interface is only for use with non-hotpluggable devices.) 188 189 190Early Platform Devices and Drivers 191~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 192The early platform interfaces provide platform data to platform device 193drivers early on during the system boot. The code is built on top of the 194early_param() command line parsing and can be executed very early on. 195 196Example: "earlyprintk" class early serial console in 6 steps 197 1981. Registering early platform device data 199~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 200The architecture code registers platform device data using the function 201early_platform_add_devices(). In the case of early serial console this 202should be hardware configuration for the serial port. Devices registered 203at this point will later on be matched against early platform drivers. 204 2052. Parsing kernel command line 206~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 207The architecture code calls parse_early_param() to parse the kernel 208command line. This will execute all matching early_param() callbacks. 209User specified early platform devices will be registered at this point. 210For the early serial console case the user can specify port on the 211kernel command line as "earlyprintk=serial.0" where "earlyprintk" is 212the class string, "serial" is the name of the platform driver and 2130 is the platform device id. If the id is -1 then the dot and the 214id can be omitted. 215 2163. Installing early platform drivers belonging to a certain class 217~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 218The architecture code may optionally force registration of all early 219platform drivers belonging to a certain class using the function 220early_platform_driver_register_all(). User specified devices from 221step 2 have priority over these. This step is omitted by the serial 222driver example since the early serial driver code should be disabled 223unless the user has specified port on the kernel command line. 224 2254. Early platform driver registration 226~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 227Compiled-in platform drivers making use of early_platform_init() are 228automatically registered during step 2 or 3. The serial driver example 229should use early_platform_init("earlyprintk", &platform_driver). 230 2315. Probing of early platform drivers belonging to a certain class 232~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 233The architecture code calls early_platform_driver_probe() to match 234registered early platform devices associated with a certain class with 235registered early platform drivers. Matched devices will get probed(). 236This step can be executed at any point during the early boot. As soon 237as possible may be good for the serial port case. 238 2396. Inside the early platform driver probe() 240~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 241The driver code needs to take special care during early boot, especially 242when it comes to memory allocation and interrupt registration. The code 243in the probe() function can use is_early_platform_device() to check if 244it is called at early platform device or at the regular platform device 245time. The early serial driver performs register_console() at this point. 246 247For further information, see <linux/platform_device.h>. 248