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 int (*remove)(struct platform_device *); 45 void (*shutdown)(struct platform_device *); 46 int (*suspend)(struct platform_device *, pm_message_t state); 47 int (*suspend_late)(struct platform_device *, pm_message_t state); 48 int (*resume_early)(struct platform_device *); 49 int (*resume)(struct platform_device *); 50 struct device_driver driver; 51 }; 52 53Note that probe() should in general verify that the specified device hardware 54actually exists; sometimes platform setup code can't be sure. The probing 55can use device resources, including clocks, and device platform_data. 56 57Platform drivers register themselves the normal way:: 58 59 int platform_driver_register(struct platform_driver *drv); 60 61Or, in common situations where the device is known not to be hot-pluggable, 62the probe() routine can live in an init section to reduce the driver's 63runtime memory footprint:: 64 65 int platform_driver_probe(struct platform_driver *drv, 66 int (*probe)(struct platform_device *)) 67 68Kernel modules can be composed of several platform drivers. The platform core 69provides helpers to register and unregister an array of drivers:: 70 71 int __platform_register_drivers(struct platform_driver * const *drivers, 72 unsigned int count, struct module *owner); 73 void platform_unregister_drivers(struct platform_driver * const *drivers, 74 unsigned int count); 75 76If one of the drivers fails to register, all drivers registered up to that 77point will be unregistered in reverse order. Note that there is a convenience 78macro that passes THIS_MODULE as owner parameter:: 79 80 #define platform_register_drivers(drivers, count) 81 82 83Device Enumeration 84~~~~~~~~~~~~~~~~~~ 85As a rule, platform specific (and often board-specific) setup code will 86register platform devices:: 87 88 int platform_device_register(struct platform_device *pdev); 89 90 int platform_add_devices(struct platform_device **pdevs, int ndev); 91 92The general rule is to register only those devices that actually exist, 93but in some cases extra devices might be registered. For example, a kernel 94might be configured to work with an external network adapter that might not 95be populated on all boards, or likewise to work with an integrated controller 96that some boards might not hook up to any peripherals. 97 98In some cases, boot firmware will export tables describing the devices 99that are populated on a given board. Without such tables, often the 100only way for system setup code to set up the correct devices is to build 101a kernel for a specific target board. Such board-specific kernels are 102common with embedded and custom systems development. 103 104In many cases, the memory and IRQ resources associated with the platform 105device are not enough to let the device's driver work. Board setup code 106will often provide additional information using the device's platform_data 107field to hold additional information. 108 109Embedded systems frequently need one or more clocks for platform devices, 110which are normally kept off until they're actively needed (to save power). 111System setup also associates those clocks with the device, so that that 112calls to clk_get(&pdev->dev, clock_name) return them as needed. 113 114 115Legacy Drivers: Device Probing 116~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 117Some drivers are not fully converted to the driver model, because they take 118on a non-driver role: the driver registers its platform device, rather than 119leaving that for system infrastructure. Such drivers can't be hotplugged 120or coldplugged, since those mechanisms require device creation to be in a 121different system component than the driver. 122 123The only "good" reason for this is to handle older system designs which, like 124original IBM PCs, rely on error-prone "probe-the-hardware" models for hardware 125configuration. Newer systems have largely abandoned that model, in favor of 126bus-level support for dynamic configuration (PCI, USB), or device tables 127provided by the boot firmware (e.g. PNPACPI on x86). There are too many 128conflicting options about what might be where, and even educated guesses by 129an operating system will be wrong often enough to make trouble. 130 131This style of driver is discouraged. If you're updating such a driver, 132please try to move the device enumeration to a more appropriate location, 133outside the driver. This will usually be cleanup, since such drivers 134tend to already have "normal" modes, such as ones using device nodes that 135were created by PNP or by platform device setup. 136 137None the less, there are some APIs to support such legacy drivers. Avoid 138using these calls except with such hotplug-deficient drivers:: 139 140 struct platform_device *platform_device_alloc( 141 const char *name, int id); 142 143You can use platform_device_alloc() to dynamically allocate a device, which 144you will then initialize with resources and platform_device_register(). 145A better solution is usually:: 146 147 struct platform_device *platform_device_register_simple( 148 const char *name, int id, 149 struct resource *res, unsigned int nres); 150 151You can use platform_device_register_simple() as a one-step call to allocate 152and register a device. 153 154 155Device Naming and Driver Binding 156~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 157The platform_device.dev.bus_id is the canonical name for the devices. 158It's built from two components: 159 160 * platform_device.name ... which is also used to for driver matching. 161 162 * platform_device.id ... the device instance number, or else "-1" 163 to indicate there's only one. 164 165These are concatenated, so name/id "serial"/0 indicates bus_id "serial.0", and 166"serial/3" indicates bus_id "serial.3"; both would use the platform_driver 167named "serial". While "my_rtc"/-1 would be bus_id "my_rtc" (no instance id) 168and use the platform_driver called "my_rtc". 169 170Driver binding is performed automatically by the driver core, invoking 171driver probe() after finding a match between device and driver. If the 172probe() succeeds, the driver and device are bound as usual. There are 173three different ways to find such a match: 174 175 - Whenever a device is registered, the drivers for that bus are 176 checked for matches. Platform devices should be registered very 177 early during system boot. 178 179 - When a driver is registered using platform_driver_register(), all 180 unbound devices on that bus are checked for matches. Drivers 181 usually register later during booting, or by module loading. 182 183 - Registering a driver using platform_driver_probe() works just like 184 using platform_driver_register(), except that the driver won't 185 be probed later if another device registers. (Which is OK, since 186 this interface is only for use with non-hotpluggable devices.) 187 188 189Early Platform Devices and Drivers 190~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 191The early platform interfaces provide platform data to platform device 192drivers early on during the system boot. The code is built on top of the 193early_param() command line parsing and can be executed very early on. 194 195Example: "earlyprintk" class early serial console in 6 steps 196 1971. Registering early platform device data 198~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 199The architecture code registers platform device data using the function 200early_platform_add_devices(). In the case of early serial console this 201should be hardware configuration for the serial port. Devices registered 202at this point will later on be matched against early platform drivers. 203 2042. Parsing kernel command line 205~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 206The architecture code calls parse_early_param() to parse the kernel 207command line. This will execute all matching early_param() callbacks. 208User specified early platform devices will be registered at this point. 209For the early serial console case the user can specify port on the 210kernel command line as "earlyprintk=serial.0" where "earlyprintk" is 211the class string, "serial" is the name of the platform driver and 2120 is the platform device id. If the id is -1 then the dot and the 213id can be omitted. 214 2153. Installing early platform drivers belonging to a certain class 216~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 217The architecture code may optionally force registration of all early 218platform drivers belonging to a certain class using the function 219early_platform_driver_register_all(). User specified devices from 220step 2 have priority over these. This step is omitted by the serial 221driver example since the early serial driver code should be disabled 222unless the user has specified port on the kernel command line. 223 2244. Early platform driver registration 225~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 226Compiled-in platform drivers making use of early_platform_init() are 227automatically registered during step 2 or 3. The serial driver example 228should use early_platform_init("earlyprintk", &platform_driver). 229 2305. Probing of early platform drivers belonging to a certain class 231~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 232The architecture code calls early_platform_driver_probe() to match 233registered early platform devices associated with a certain class with 234registered early platform drivers. Matched devices will get probed(). 235This step can be executed at any point during the early boot. As soon 236as possible may be good for the serial port case. 237 2386. Inside the early platform driver probe() 239~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 240The driver code needs to take special care during early boot, especially 241when it comes to memory allocation and interrupt registration. The code 242in the probe() function can use is_early_platform_device() to check if 243it is called at early platform device or at the regular platform device 244time. The early serial driver performs register_console() at this point. 245 246For further information, see <linux/platform_device.h>. 247