231 lines
10 KiB
Plaintext
231 lines
10 KiB
Plaintext
|
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, 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.
|
||
|
|
|
||
|
|
struct platform_device {
|
||
|
|
const char *name;
|
||
|
|
u32 id;
|
||
|
|
struct device dev;
|
||
|
|
u32 num_resources;
|
||
|
|
struct resource *resource;
|
||
|
|
};
|
||
|
|
|
||
|
|
|
||
|
|
Platform drivers
|
||
|
|
~~~~~~~~~~~~~~~~
|
||
|
|
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 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.
|
||
|
|
|
||
|
|
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 platfrom 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>.
|