1Platform Devices and Drivers 2~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 3See <linux/platform_device.h> for the driver model interface to the 4platform bus: platform_device, and platform_driver. This pseudo-bus 5is used to connect devices on busses with minimal infrastructure, 6like those used to integrate peripherals on many system-on-chip 7processors, or some "legacy" PC interconnects; as opposed to large 8formally specified ones like PCI or USB. 9 10 11Platform devices 12~~~~~~~~~~~~~~~~ 13Platform devices are devices that typically appear as autonomous 14entities in the system. This includes legacy port-based devices and 15host bridges to peripheral buses, and most controllers integrated 16into system-on-chip platforms. What they usually have in common 17is direct addressing from a CPU bus. Rarely, a platform_device will 18be connected through a segment of some other kind of bus; but its 19registers will still be directly addressable. 20 21Platform devices are given a name, used in driver binding, and a 22list of resources such as addresses and IRQs. 23 24struct platform_device { 25 const char *name; 26 u32 id; 27 struct device dev; 28 u32 num_resources; 29 struct resource *resource; 30}; 31 32 33Platform drivers 34~~~~~~~~~~~~~~~~ 35Platform drivers follow the standard driver model convention, where 36discovery/enumeration is handled outside the drivers, and drivers 37provide probe() and remove() methods. They support power management 38and shutdown notifications using the standard conventions. 39 40struct 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 51Note that probe() should in general verify that the specified device hardware 52actually exists; sometimes platform setup code can't be sure. The probing 53can use device resources, including clocks, and device platform_data. 54 55Platform drivers register themselves the normal way: 56 57 int platform_driver_register(struct platform_driver *drv); 58 59Or, in common situations where the device is known not to be hot-pluggable, 60the probe() routine can live in an init section to reduce the driver's 61runtime memory footprint: 62 63 int platform_driver_probe(struct platform_driver *drv, 64 int (*probe)(struct platform_device *)) 65 66 67Device Enumeration 68~~~~~~~~~~~~~~~~~~ 69As a rule, platform specific (and often board-specific) setup code will 70register platform devices: 71 72 int platform_device_register(struct platform_device *pdev); 73 74 int platform_add_devices(struct platform_device **pdevs, int ndev); 75 76The general rule is to register only those devices that actually exist, 77but in some cases extra devices might be registered. For example, a kernel 78might be configured to work with an external network adapter that might not 79be populated on all boards, or likewise to work with an integrated controller 80that some boards might not hook up to any peripherals. 81 82In some cases, boot firmware will export tables describing the devices 83that are populated on a given board. Without such tables, often the 84only way for system setup code to set up the correct devices is to build 85a kernel for a specific target board. Such board-specific kernels are 86common with embedded and custom systems development. 87 88In many cases, the memory and IRQ resources associated with the platform 89device are not enough to let the device's driver work. Board setup code 90will often provide additional information using the device's platform_data 91field to hold additional information. 92 93Embedded systems frequently need one or more clocks for platform devices, 94which are normally kept off until they're actively needed (to save power). 95System setup also associates those clocks with the device, so that that 96calls to clk_get(&pdev->dev, clock_name) return them as needed. 97 98 99Legacy Drivers: Device Probing 100~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 101Some drivers are not fully converted to the driver model, because they take 102on a non-driver role: the driver registers its platform device, rather than 103leaving that for system infrastructure. Such drivers can't be hotplugged 104or coldplugged, since those mechanisms require device creation to be in a 105different system component than the driver. 106 107The only "good" reason for this is to handle older system designs which, like 108original IBM PCs, rely on error-prone "probe-the-hardware" models for hardware 109configuration. Newer systems have largely abandoned that model, in favor of 110bus-level support for dynamic configuration (PCI, USB), or device tables 111provided by the boot firmware (e.g. PNPACPI on x86). There are too many 112conflicting options about what might be where, and even educated guesses by 113an operating system will be wrong often enough to make trouble. 114 115This style of driver is discouraged. If you're updating such a driver, 116please try to move the device enumeration to a more appropriate location, 117outside the driver. This will usually be cleanup, since such drivers 118tend to already have "normal" modes, such as ones using device nodes that 119were created by PNP or by platform device setup. 120 121None the less, there are some APIs to support such legacy drivers. Avoid 122using these calls except with such hotplug-deficient drivers. 123 124 struct platform_device *platform_device_alloc( 125 const char *name, int id); 126 127You can use platform_device_alloc() to dynamically allocate a device, which 128you will then initialize with resources and platform_device_register(). 129A better solution is usually: 130 131 struct platform_device *platform_device_register_simple( 132 const char *name, int id, 133 struct resource *res, unsigned int nres); 134 135You can use platform_device_register_simple() as a one-step call to allocate 136and register a device. 137 138 139Device Naming and Driver Binding 140~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 141The platform_device.dev.bus_id is the canonical name for the devices. 142It's built from two components: 143 144 * platform_device.name ... which is also used to for driver matching. 145 146 * platform_device.id ... the device instance number, or else "-1" 147 to indicate there's only one. 148 149These are concatenated, so name/id "serial"/0 indicates bus_id "serial.0", and 150"serial/3" indicates bus_id "serial.3"; both would use the platform_driver 151named "serial". While "my_rtc"/-1 would be bus_id "my_rtc" (no instance id) 152and use the platform_driver called "my_rtc". 153 154Driver binding is performed automatically by the driver core, invoking 155driver probe() after finding a match between device and driver. If the 156probe() succeeds, the driver and device are bound as usual. There are 157three different ways to find such a match: 158 159 - Whenever a device is registered, the drivers for that bus are 160 checked for matches. Platform devices should be registered very 161 early during system boot. 162 163 - When a driver is registered using platform_driver_register(), all 164 unbound devices on that bus are checked for matches. Drivers 165 usually register later during booting, or by module loading. 166 167 - Registering a driver using platform_driver_probe() works just like 168 using platform_driver_register(), except that the driver won't 169 be probed later if another device registers. (Which is OK, since 170 this interface is only for use with non-hotpluggable devices.) 171 172 173Early Platform Devices and Drivers 174~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 175The early platform interfaces provide platform data to platform device 176drivers early on during the system boot. The code is built on top of the 177early_param() command line parsing and can be executed very early on. 178 179Example: "earlyprintk" class early serial console in 6 steps 180 1811. Registering early platform device data 182~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 183The architecture code registers platform device data using the function 184early_platform_add_devices(). In the case of early serial console this 185should be hardware configuration for the serial port. Devices registered 186at this point will later on be matched against early platform drivers. 187 1882. Parsing kernel command line 189~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 190The architecture code calls parse_early_param() to parse the kernel 191command line. This will execute all matching early_param() callbacks. 192User specified early platform devices will be registered at this point. 193For the early serial console case the user can specify port on the 194kernel command line as "earlyprintk=serial.0" where "earlyprintk" is 195the class string, "serial" is the name of the platform driver and 1960 is the platform device id. If the id is -1 then the dot and the 197id can be omitted. 198 1993. Installing early platform drivers belonging to a certain class 200~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 201The architecture code may optionally force registration of all early 202platform drivers belonging to a certain class using the function 203early_platform_driver_register_all(). User specified devices from 204step 2 have priority over these. This step is omitted by the serial 205driver example since the early serial driver code should be disabled 206unless the user has specified port on the kernel command line. 207 2084. Early platform driver registration 209~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 210Compiled-in platform drivers making use of early_platform_init() are 211automatically registered during step 2 or 3. The serial driver example 212should use early_platform_init("earlyprintk", &platform_driver). 213 2145. Probing of early platform drivers belonging to a certain class 215~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 216The architecture code calls early_platform_driver_probe() to match 217registered early platform devices associated with a certain class with 218registered early platform drivers. Matched devices will get probed(). 219This step can be executed at any point during the early boot. As soon 220as possible may be good for the serial port case. 221 2226. Inside the early platform driver probe() 223~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 224The driver code needs to take special care during early boot, especially 225when it comes to memory allocation and interrupt registration. The code 226in the probe() function can use is_early_platform_device() to check if 227it is called at early platform device or at the regular platform device 228time. The early serial driver performs register_console() at this point. 229 230For further information, see <linux/platform_device.h>. 231