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Documentation/driver-model/platform.txt update/rewrite

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This is almost a rewrite of the driver-model/platform.txt documentation;
the previous text was obsolete (for several years), evidently it never
got updated to match the change from being a PC "legacy_bus" to the more
widely used core bus for most embedded systems.

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>
This commit is contained in:
David Brownell 2006-11-16 23:30:14 -08:00 committed by Greg Kroah-Hartman
parent c67334fbdf
commit c957b32406
1 changed files with 108 additions and 76 deletions
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184
Documentation/driver-model/platform.txt
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@ -1,99 +1,131 @@
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.
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 addressible.
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
~~~~~~~~~~~~~~~~
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.
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 *))
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.
Device Enumeration
~~~~~~~~~~~~~~~~~~
As a rule, platform specific (and often board-specific) setup code wil
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.
The Platform Bus
~~~~~~~~~~~~~~~~
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.
Device Naming and Driver Binding
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
The platform_device.dev.bus_id is the canonical name for the devices.
It's built from two components:
But, besides the organizational benefits, the platform bus can also
accommodate firmware-based enumeration.
* 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.
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:
These are catenated, 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".
platform_bus.add(parent,bus_id).
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.
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.
- 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.
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.
- Registering a driver using platform_driver_probe() works just like
using platform_driver_register(), except that the 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.)