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Diffstat (limited to 'Documentation/driver-model/platform.txt')
| -rw-r--r-- | Documentation/driver-model/platform.txt | 301 |
1 files changed, 216 insertions, 85 deletions
diff --git a/Documentation/driver-model/platform.txt b/Documentation/driver-model/platform.txt index 5eee3e0bfc4..07795ec51cd 100644 --- a/Documentation/driver-model/platform.txt +++ b/Documentation/driver-model/platform.txt @@ -1,99 +1,230 @@ Platform Devices and Drivers ~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +See <linux/platform_device.h> for the driver model interface to the +platform bus: platform_device, and platform_driver. This pseudo-bus +is used to connect devices on busses with minimal infrastructure, +like those used to integrate peripherals on many system-on-chip +processors, or some "legacy" PC interconnects; as opposed to large +formally specified ones like PCI or USB. + Platform devices ~~~~~~~~~~~~~~~~ Platform devices are devices that typically appear as autonomous entities in the system. This includes legacy port-based devices and -host bridges to peripheral buses. - - -Platform drivers -~~~~~~~~~~~~~~~~ -Drivers for platform devices are typically very simple and -unstructured. Either the device was present at a particular I/O port -and the driver was loaded, or it was not. There was no possibility -of hotplugging or alternative discovery besides probing at a specific -I/O address and expecting a specific response. +host bridges to peripheral buses, and most controllers integrated +into system-on-chip platforms. What they usually have in common +is direct addressing from a CPU bus. Rarely, a platform_device will +be connected through a segment of some other kind of bus; but its +registers will still be directly addressable. +Platform devices are given a name, used in driver binding, and a +list of resources such as addresses and IRQs. -Other Architectures, Modern Firmware, and new Platforms -~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ -These devices are not always at the legacy I/O ports. This is true on -other architectures and on some modern architectures. In most cases, -the drivers are modified to discover the devices at other well-known -ports for the given platform. However, the firmware in these systems -does usually know where exactly these devices reside, and in some -cases, it's the only way of discovering them. +struct platform_device { + const char *name; + u32 id; + struct device dev; + u32 num_resources; + struct resource *resource; +}; -The Platform Bus +Platform drivers ~~~~~~~~~~~~~~~~ -A platform bus has been created to deal with these issues. First and -foremost, it groups all the legacy devices under a common bus, and -gives them a common parent if they don't already have one. - -But, besides the organizational benefits, the platform bus can also -accommodate firmware-based enumeration. +Platform drivers follow the standard driver model convention, where +discovery/enumeration is handled outside the drivers, and drivers +provide probe() and remove() methods. They support power management +and shutdown notifications using the standard conventions. + +struct platform_driver { + int (*probe)(struct platform_device *); + int (*remove)(struct platform_device *); + void (*shutdown)(struct platform_device *); + int (*suspend)(struct platform_device *, pm_message_t state); + int (*suspend_late)(struct platform_device *, pm_message_t state); + int (*resume_early)(struct platform_device *); + int (*resume)(struct platform_device *); + struct device_driver driver; +}; + +Note that probe() should in general verify that the specified device hardware +actually exists; sometimes platform setup code can't be sure. The probing +can use device resources, including clocks, and device platform_data. + +Platform drivers register themselves the normal way: + + int platform_driver_register(struct platform_driver *drv); + +Or, in common situations where the device is known not to be hot-pluggable, +the probe() routine can live in an init section to reduce the driver's +runtime memory footprint: + + int platform_driver_probe(struct platform_driver *drv, + int (*probe)(struct platform_device *)) + + +Device Enumeration +~~~~~~~~~~~~~~~~~~ +As a rule, platform specific (and often board-specific) setup code will +register platform devices: + + int platform_device_register(struct platform_device *pdev); + int platform_add_devices(struct platform_device **pdevs, int ndev); + +The general rule is to register only those devices that actually exist, +but in some cases extra devices might be registered. For example, a kernel +might be configured to work with an external network adapter that might not +be populated on all boards, or likewise to work with an integrated controller +that some boards might not hook up to any peripherals. -Device Discovery -~~~~~~~~~~~~~~~~ -The platform bus has no concept of probing for devices. Devices -discovery is left up to either the legacy drivers or the -firmware. These entities are expected to notify the platform of -devices that it discovers via the bus's add() callback: - - platform_bus.add(parent,bus_id). - - -Bus IDs -~~~~~~~ -Bus IDs are the canonical names for the devices. There is no globally -standard addressing mechanism for legacy devices. In the IA-32 world, -we have Pnp IDs to use, as well as the legacy I/O ports. However, -neither tell what the device really is or have any meaning on other -platforms. - -Since both PnP IDs and the legacy I/O ports (and other standard I/O -ports for specific devices) have a 1:1 mapping, we map the -platform-specific name or identifier to a generic name (at least -within the scope of the kernel). - -For example, a serial driver might find a device at I/O 0x3f8. The -ACPI firmware might also discover a device with PnP ID (_HID) -PNP0501. Both correspond to the same device and should be mapped to the -canonical name 'serial'. - -The bus_id field should be a concatenation of the canonical name and -the instance of that type of device. For example, the device at I/O -port 0x3f8 should have a bus_id of "serial0". This places the -responsibility of enumerating devices of a particular type up to the -discovery mechanism. But, they are the entity that should know best -(as opposed to the platform bus driver). - - -Drivers -~~~~~~~ -Drivers for platform devices should have a name that is the same as -the canonical name of the devices they support. This allows the -platform bus driver to do simple matching with the basic data -structures to determine if a driver supports a certain device. - -For example, a legacy serial driver should have a name of 'serial' and -register itself with the platform bus. - - -Driver Binding -~~~~~~~~~~~~~~ -Legacy drivers assume they are bound to the device once they start up -and probe an I/O port. Divorcing them from this will be a difficult -process. However, that shouldn't prevent us from implementing -firmware-based enumeration. - -The firmware should notify the platform bus about devices before the -legacy drivers have had a chance to load. Once the drivers are loaded, -they driver model core will attempt to bind the driver to any -previously-discovered devices. Once that has happened, it will be free -to discover any other devices it pleases. - +In some cases, boot firmware will export tables describing the devices +that are populated on a given board. Without such tables, often the +only way for system setup code to set up the correct devices is to build +a kernel for a specific target board. Such board-specific kernels are +common with embedded and custom systems development. + +In many cases, the memory and IRQ resources associated with the platform +device are not enough to let the device's driver work. Board setup code +will often provide additional information using the device's platform_data +field to hold additional information. + +Embedded systems frequently need one or more clocks for platform devices, +which are normally kept off until they're actively needed (to save power). +System setup also associates those clocks with the device, so that that +calls to clk_get(&pdev->dev, clock_name) return them as needed. + + +Legacy Drivers: Device Probing +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +Some drivers are not fully converted to the driver model, because they take +on a non-driver role: the driver registers its platform device, rather than +leaving that for system infrastructure. Such drivers can't be hotplugged +or coldplugged, since those mechanisms require device creation to be in a +different system component than the driver. + +The only "good" reason for this is to handle older system designs which, like +original IBM PCs, rely on error-prone "probe-the-hardware" models for hardware +configuration. Newer systems have largely abandoned that model, in favor of +bus-level support for dynamic configuration (PCI, USB), or device tables +provided by the boot firmware (e.g. PNPACPI on x86). There are too many +conflicting options about what might be where, and even educated guesses by +an operating system will be wrong often enough to make trouble. + +This style of driver is discouraged. If you're updating such a driver, +please try to move the device enumeration to a more appropriate location, +outside the driver. This will usually be cleanup, since such drivers +tend to already have "normal" modes, such as ones using device nodes that +were created by PNP or by platform device setup. + +None the less, there are some APIs to support such legacy drivers. Avoid +using these calls except with such hotplug-deficient drivers. + + struct platform_device *platform_device_alloc( + const char *name, int id); + +You can use platform_device_alloc() to dynamically allocate a device, which +you will then initialize with resources and platform_device_register(). +A better solution is usually: + + struct platform_device *platform_device_register_simple( + const char *name, int id, + struct resource *res, unsigned int nres); + +You can use platform_device_register_simple() as a one-step call to allocate +and register a device. + + +Device Naming and Driver Binding +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +The platform_device.dev.bus_id is the canonical name for the devices. +It's built from two components: + + * platform_device.name ... which is also used to for driver matching. + + * platform_device.id ... the device instance number, or else "-1" + to indicate there's only one. + +These are concatenated, so name/id "serial"/0 indicates bus_id "serial.0", and +"serial/3" indicates bus_id "serial.3"; both would use the platform_driver +named "serial". While "my_rtc"/-1 would be bus_id "my_rtc" (no instance id) +and use the platform_driver called "my_rtc". + +Driver binding is performed automatically by the driver core, invoking +driver probe() after finding a match between device and driver. If the +probe() succeeds, the driver and device are bound as usual. There are +three different ways to find such a match: + + - Whenever a device is registered, the drivers for that bus are + checked for matches. Platform devices should be registered very + early during system boot. + + - When a driver is registered using platform_driver_register(), all + unbound devices on that bus are checked for matches. Drivers + usually register later during booting, or by module loading. + + - Registering a driver using platform_driver_probe() works just like + using platform_driver_register(), except that the driver won't + be probed later if another device registers. (Which is OK, since + this interface is only for use with non-hotpluggable devices.) + + +Early Platform Devices and Drivers +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +The early platform interfaces provide platform data to platform device +drivers early on during the system boot. The code is built on top of the +early_param() command line parsing and can be executed very early on. + +Example: "earlyprintk" class early serial console in 6 steps + +1. Registering early platform device data +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +The architecture code registers platform device data using the function +early_platform_add_devices(). In the case of early serial console this +should be hardware configuration for the serial port. Devices registered +at this point will later on be matched against early platform drivers. + +2. Parsing kernel command line +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +The architecture code calls parse_early_param() to parse the kernel +command line. This will execute all matching early_param() callbacks. +User specified early platform devices will be registered at this point. +For the early serial console case the user can specify port on the +kernel command line as "earlyprintk=serial.0" where "earlyprintk" is +the class string, "serial" is the name of the platform driver and +0 is the platform device id. If the id is -1 then the dot and the +id can be omitted. + +3. Installing early platform drivers belonging to a certain class +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +The architecture code may optionally force registration of all early +platform drivers belonging to a certain class using the function +early_platform_driver_register_all(). User specified devices from +step 2 have priority over these. This step is omitted by the serial +driver example since the early serial driver code should be disabled +unless the user has specified port on the kernel command line. + +4. Early platform driver registration +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +Compiled-in platform drivers making use of early_platform_init() are +automatically registered during step 2 or 3. The serial driver example +should use early_platform_init("earlyprintk", &platform_driver). + +5. Probing of early platform drivers belonging to a certain class +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +The architecture code calls early_platform_driver_probe() to match +registered early platform devices associated with a certain class with +registered early platform drivers. Matched devices will get probed(). +This step can be executed at any point during the early boot. As soon +as possible may be good for the serial port case. + +6. Inside the early platform driver probe() +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +The driver code needs to take special care during early boot, especially +when it comes to memory allocation and interrupt registration. The code +in the probe() function can use is_early_platform_device() to check if +it is called at early platform device or at the regular platform device +time. The early serial driver performs register_console() at this point. + +For further information, see <linux/platform_device.h>. |