287 lines
11 KiB
ReStructuredText
287 lines
11 KiB
ReStructuredText
==============
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Device Drivers
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==============
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See the kerneldoc for the struct device_driver.
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Allocation
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~~~~~~~~~~
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Device drivers are statically allocated structures. Though there may
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be multiple devices in a system that a driver supports, struct
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device_driver represents the driver as a whole (not a particular
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device instance).
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Initialization
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~~~~~~~~~~~~~~
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The driver must initialize at least the name and bus fields. It should
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also initialize the devclass field (when it arrives), so it may obtain
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the proper linkage internally. It should also initialize as many of
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the callbacks as possible, though each is optional.
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Declaration
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~~~~~~~~~~~
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As stated above, struct device_driver objects are statically
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allocated. Below is an example declaration of the eepro100
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driver. This declaration is hypothetical only; it relies on the driver
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being converted completely to the new model::
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static struct device_driver eepro100_driver = {
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.name = "eepro100",
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.bus = &pci_bus_type,
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.probe = eepro100_probe,
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.remove = eepro100_remove,
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.suspend = eepro100_suspend,
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.resume = eepro100_resume,
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};
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Most drivers will not be able to be converted completely to the new
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model because the bus they belong to has a bus-specific structure with
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bus-specific fields that cannot be generalized.
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The most common example of this are device ID structures. A driver
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typically defines an array of device IDs that it supports. The format
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of these structures and the semantics for comparing device IDs are
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completely bus-specific. Defining them as bus-specific entities would
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sacrifice type-safety, so we keep bus-specific structures around.
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Bus-specific drivers should include a generic struct device_driver in
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the definition of the bus-specific driver. Like this::
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struct pci_driver {
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const struct pci_device_id *id_table;
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struct device_driver driver;
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};
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A definition that included bus-specific fields would look like
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(using the eepro100 driver again)::
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static struct pci_driver eepro100_driver = {
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.id_table = eepro100_pci_tbl,
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.driver = {
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.name = "eepro100",
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.bus = &pci_bus_type,
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.probe = eepro100_probe,
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.remove = eepro100_remove,
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.suspend = eepro100_suspend,
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.resume = eepro100_resume,
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},
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};
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Some may find the syntax of embedded struct initialization awkward or
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even a bit ugly. So far, it's the best way we've found to do what we want...
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Registration
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~~~~~~~~~~~~
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::
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int driver_register(struct device_driver *drv);
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The driver registers the structure on startup. For drivers that have
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no bus-specific fields (i.e. don't have a bus-specific driver
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structure), they would use driver_register and pass a pointer to their
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struct device_driver object.
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Most drivers, however, will have a bus-specific structure and will
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need to register with the bus using something like pci_driver_register.
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It is important that drivers register their driver structure as early as
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possible. Registration with the core initializes several fields in the
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struct device_driver object, including the reference count and the
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lock. These fields are assumed to be valid at all times and may be
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used by the device model core or the bus driver.
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Transition Bus Drivers
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~~~~~~~~~~~~~~~~~~~~~~
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By defining wrapper functions, the transition to the new model can be
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made easier. Drivers can ignore the generic structure altogether and
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let the bus wrapper fill in the fields. For the callbacks, the bus can
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define generic callbacks that forward the call to the bus-specific
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callbacks of the drivers.
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This solution is intended to be only temporary. In order to get class
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information in the driver, the drivers must be modified anyway. Since
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converting drivers to the new model should reduce some infrastructural
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complexity and code size, it is recommended that they are converted as
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class information is added.
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Access
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~~~~~~
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Once the object has been registered, it may access the common fields of
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the object, like the lock and the list of devices::
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int driver_for_each_dev(struct device_driver *drv, void *data,
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int (*callback)(struct device *dev, void *data));
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The devices field is a list of all the devices that have been bound to
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the driver. The LDM core provides a helper function to operate on all
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the devices a driver controls. This helper locks the driver on each
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node access, and does proper reference counting on each device as it
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accesses it.
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sysfs
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~~~~~
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When a driver is registered, a sysfs directory is created in its
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bus's directory. In this directory, the driver can export an interface
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to userspace to control operation of the driver on a global basis;
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e.g. toggling debugging output in the driver.
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A future feature of this directory will be a 'devices' directory. This
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directory will contain symlinks to the directories of devices it
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supports.
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Callbacks
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~~~~~~~~~
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::
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int (*probe) (struct device *dev);
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The probe() entry is called in task context, with the bus's rwsem locked
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and the driver partially bound to the device. Drivers commonly use
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container_of() to convert "dev" to a bus-specific type, both in probe()
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and other routines. That type often provides device resource data, such
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as pci_dev.resource[] or platform_device.resources, which is used in
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addition to dev->platform_data to initialize the driver.
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This callback holds the driver-specific logic to bind the driver to a
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given device. That includes verifying that the device is present, that
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it's a version the driver can handle, that driver data structures can
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be allocated and initialized, and that any hardware can be initialized.
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Drivers often store a pointer to their state with dev_set_drvdata().
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When the driver has successfully bound itself to that device, then probe()
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returns zero and the driver model code will finish its part of binding
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the driver to that device.
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A driver's probe() may return a negative errno value to indicate that
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the driver did not bind to this device, in which case it should have
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released all resources it allocated.
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Optionally, probe() may return -EPROBE_DEFER if the driver depends on
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resources that are not yet available (e.g., supplied by a driver that
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hasn't initialized yet). The driver core will put the device onto the
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deferred probe list and will try to call it again later. If a driver
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must defer, it should return -EPROBE_DEFER as early as possible to
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reduce the amount of time spent on setup work that will need to be
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unwound and reexecuted at a later time.
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.. warning::
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-EPROBE_DEFER must not be returned if probe() has already created
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child devices, even if those child devices are removed again
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in a cleanup path. If -EPROBE_DEFER is returned after a child
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device has been registered, it may result in an infinite loop of
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.probe() calls to the same driver.
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::
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void (*sync_state) (struct device *dev);
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sync_state is called only once for a device. It's called when all the consumer
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devices of the device have successfully probed. The list of consumers of the
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device is obtained by looking at the device links connecting that device to its
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consumer devices.
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The first attempt to call sync_state() is made during late_initcall_sync() to
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give firmware and drivers time to link devices to each other. During the first
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attempt at calling sync_state(), if all the consumers of the device at that
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point in time have already probed successfully, sync_state() is called right
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away. If there are no consumers of the device during the first attempt, that
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too is considered as "all consumers of the device have probed" and sync_state()
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is called right away.
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If during the first attempt at calling sync_state() for a device, there are
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still consumers that haven't probed successfully, the sync_state() call is
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postponed and reattempted in the future only when one or more consumers of the
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device probe successfully. If during the reattempt, the driver core finds that
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there are one or more consumers of the device that haven't probed yet, then
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sync_state() call is postponed again.
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A typical use case for sync_state() is to have the kernel cleanly take over
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management of devices from the bootloader. For example, if a device is left on
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and at a particular hardware configuration by the bootloader, the device's
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driver might need to keep the device in the boot configuration until all the
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consumers of the device have probed. Once all the consumers of the device have
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probed, the device's driver can synchronize the hardware state of the device to
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match the aggregated software state requested by all the consumers. Hence the
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name sync_state().
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While obvious examples of resources that can benefit from sync_state() include
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resources such as regulator, sync_state() can also be useful for complex
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resources like IOMMUs. For example, IOMMUs with multiple consumers (devices
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whose addresses are remapped by the IOMMU) might need to keep their mappings
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fixed at (or additive to) the boot configuration until all its consumers have
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probed.
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While the typical use case for sync_state() is to have the kernel cleanly take
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over management of devices from the bootloader, the usage of sync_state() is
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not restricted to that. Use it whenever it makes sense to take an action after
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all the consumers of a device have probed::
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int (*remove) (struct device *dev);
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remove is called to unbind a driver from a device. This may be
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called if a device is physically removed from the system, if the
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driver module is being unloaded, during a reboot sequence, or
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in other cases.
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It is up to the driver to determine if the device is present or
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not. It should free any resources allocated specifically for the
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device; i.e. anything in the device's driver_data field.
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If the device is still present, it should quiesce the device and place
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it into a supported low-power state.
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::
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int (*suspend) (struct device *dev, pm_message_t state);
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suspend is called to put the device in a low power state.
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::
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int (*resume) (struct device *dev);
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Resume is used to bring a device back from a low power state.
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Attributes
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~~~~~~~~~~
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::
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struct driver_attribute {
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struct attribute attr;
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ssize_t (*show)(struct device_driver *driver, char *buf);
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ssize_t (*store)(struct device_driver *, const char *buf, size_t count);
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};
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Device drivers can export attributes via their sysfs directories.
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Drivers can declare attributes using a DRIVER_ATTR_RW and DRIVER_ATTR_RO
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macro that works identically to the DEVICE_ATTR_RW and DEVICE_ATTR_RO
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macros.
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Example::
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DRIVER_ATTR_RW(debug);
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This is equivalent to declaring::
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struct driver_attribute driver_attr_debug;
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This can then be used to add and remove the attribute from the
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driver's directory using::
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int driver_create_file(struct device_driver *, const struct driver_attribute *);
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void driver_remove_file(struct device_driver *, const struct driver_attribute *);
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