android_kernel_xiaomi_sm8350/Documentation/driver-model/driver.txt

Device Drivers

struct device_driver {
        char                    * name;
        struct bus_type         * bus;

        struct completion	unloaded;
        struct kobject		kobj;
        list_t                  devices;

        struct module		*owner;

        int     (*probe)        (struct device * dev);
        int     (*remove)       (struct device * dev);

        int     (*suspend)      (struct device * dev, pm_message_t state, u32 level);
        int     (*resume)       (struct device * dev, u32 level);
};



Allocation
~~~~~~~~~~

Device drivers are statically allocated structures. Though there may
be multiple devices in a system that a driver supports, struct
device_driver represents the driver as a whole (not a particular
device instance).

Initialization
~~~~~~~~~~~~~~

The driver must initialize at least the name and bus fields. It should
also initialize the devclass field (when it arrives), so it may obtain
the proper linkage internally. It should also initialize as many of
the callbacks as possible, though each is optional.

Declaration
~~~~~~~~~~~

As stated above, struct device_driver objects are statically
allocated. Below is an example declaration of the eepro100
driver. This declaration is hypothetical only; it relies on the driver
being converted completely to the new model. 

static struct device_driver eepro100_driver = {
       .name		= "eepro100",
       .bus		= &pci_bus_type,
       
       .probe		= eepro100_probe,
       .remove		= eepro100_remove,
       .suspend		= eepro100_suspend,
       .resume		= eepro100_resume,
};

Most drivers will not be able to be converted completely to the new
model because the bus they belong to has a bus-specific structure with
bus-specific fields that cannot be generalized. 

The most common example of this are device ID structures. A driver
typically defines an array of device IDs that it supports. The format
of these structures and the semantics for comparing device IDs are
completely bus-specific. Defining them as bus-specific entities would
sacrifice type-safety, so we keep bus-specific structures around. 

Bus-specific drivers should include a generic struct device_driver in
the definition of the bus-specific driver. Like this:

struct pci_driver {
       const struct pci_device_id *id_table;
       struct device_driver	  driver;
};

A definition that included bus-specific fields would look like
(using the eepro100 driver again):

static struct pci_driver eepro100_driver = {
       .id_table       = eepro100_pci_tbl,
       .driver	       = {
		.name		= "eepro100",
		.bus		= &pci_bus_type,
		.probe		= eepro100_probe,
		.remove		= eepro100_remove,
		.suspend	= eepro100_suspend,
		.resume		= eepro100_resume,
       },
};

Some may find the syntax of embedded struct initialization awkward or
even a bit ugly. So far, it's the best way we've found to do what we want...

Registration
~~~~~~~~~~~~

int driver_register(struct device_driver * drv);

The driver registers the structure on startup. For drivers that have
no bus-specific fields (i.e. don't have a bus-specific driver
structure), they would use driver_register and pass a pointer to their
struct device_driver object. 

Most drivers, however, will have a bus-specific structure and will
need to register with the bus using something like pci_driver_register.

It is important that drivers register their driver structure as early as
possible. Registration with the core initializes several fields in the
struct device_driver object, including the reference count and the
lock. These fields are assumed to be valid at all times and may be
used by the device model core or the bus driver.


Transition Bus Drivers
~~~~~~~~~~~~~~~~~~~~~~

By defining wrapper functions, the transition to the new model can be
made easier. Drivers can ignore the generic structure altogether and
let the bus wrapper fill in the fields. For the callbacks, the bus can
define generic callbacks that forward the call to the bus-specific
callbacks of the drivers. 

This solution is intended to be only temporary. In order to get class
information in the driver, the drivers must be modified anyway. Since
converting drivers to the new model should reduce some infrastructural
complexity and code size, it is recommended that they are converted as
class information is added.

Access
~~~~~~

Once the object has been registered, it may access the common fields of
the object, like the lock and the list of devices. 

int driver_for_each_dev(struct device_driver * drv, void * data, 
		        int (*callback)(struct device * dev, void * data));

The devices field is a list of all the devices that have been bound to
the driver. The LDM core provides a helper function to operate on all
the devices a driver controls. This helper locks the driver on each
node access, and does proper reference counting on each device as it
accesses it. 


sysfs
~~~~~

When a driver is registered, a sysfs directory is created in its
bus's directory. In this directory, the driver can export an interface
to userspace to control operation of the driver on a global basis;
e.g. toggling debugging output in the driver.

A future feature of this directory will be a 'devices' directory. This
directory will contain symlinks to the directories of devices it
supports.



Callbacks
~~~~~~~~~

	int	(*probe)	(struct device * dev);

The probe() entry is called in task context, with the bus's rwsem locked
and the driver partially bound to the device.  Drivers commonly use
container_of() to convert "dev" to a bus-specific type, both in probe()
and other routines.  That type often provides device resource data, such
as pci_dev.resource[] or platform_device.resources, which is used in
addition to dev->platform_data to initialize the driver.

This callback holds the driver-specific logic to bind the driver to a
given device.  That includes verifying that the device is present, that
it's a version the driver can handle, that driver data structures can
be allocated and initialized, and that any hardware can be initialized.
Drivers often store a pointer to their state with dev_set_drvdata().
When the driver has successfully bound itself to that device, then probe()
returns zero and the driver model code will finish its part of binding
the driver to that device.

A driver's probe() may return a negative errno value to indicate that
the driver did not bind to this device, in which case it should have
released all reasources it allocated.

	int 	(*remove)	(struct device * dev);

remove is called to unbind a driver from a device. This may be
called if a device is physically removed from the system, if the
driver module is being unloaded, during a reboot sequence, or
in other cases.

It is up to the driver to determine if the device is present or
not. It should free any resources allocated specifically for the
device; i.e. anything in the device's driver_data field. 

If the device is still present, it should quiesce the device and place
it into a supported low-power state.

	int	(*suspend)	(struct device * dev, pm_message_t state, u32 level);

suspend is called to put the device in a low power state. There are
several stages to successfully suspending a device, which is denoted in
the @level parameter. Breaking the suspend transition into several
stages affords the platform flexibility in performing device power
management based on the requirements of the system and the
user-defined policy.

SUSPEND_NOTIFY notifies the device that a suspend transition is about
to happen. This happens on system power state transitions to verify
that all devices can successfully suspend.

A driver may choose to fail on this call, which should cause the
entire suspend transition to fail. A driver should fail only if it
knows that the device will not be able to be resumed properly when the
system wakes up again. It could also fail if it somehow determines it
is in the middle of an operation too important to stop.

SUSPEND_DISABLE tells the device to stop I/O transactions. When it
stops transactions, or what it should do with unfinished transactions
is a policy of the driver. After this call, the driver should not
accept any other I/O requests.

SUSPEND_SAVE_STATE tells the device to save the context of the
hardware. This includes any bus-specific hardware state and
device-specific hardware state. A pointer to this saved state can be
stored in the device's saved_state field.

SUSPEND_POWER_DOWN tells the driver to place the device in the low
power state requested. 

Whether suspend is called with a given level is a policy of the
platform. Some levels may be omitted; drivers must not assume the
reception of any level. However, all levels must be called in the
order above; i.e. notification will always come before disabling;
disabling the device will come before suspending the device.

All calls are made with interrupts enabled, except for the
SUSPEND_POWER_DOWN level.

	int	(*resume)	(struct device * dev, u32 level);

Resume is used to bring a device back from a low power state. Like the
suspend transition, it happens in several stages. 

RESUME_POWER_ON tells the driver to set the power state to the state
before the suspend call (The device could have already been in a low
power state before the suspend call to put in a lower power state). 

RESUME_RESTORE_STATE tells the driver to restore the state saved by
the SUSPEND_SAVE_STATE suspend call. 

RESUME_ENABLE tells the driver to start accepting I/O transactions
again. Depending on driver policy, the device may already have pending
I/O requests. 

RESUME_POWER_ON is called with interrupts disabled. The other resume
levels are called with interrupts enabled. 

As with the various suspend stages, the driver must not assume that
any other resume calls have been or will be made. Each call should be
self-contained and not dependent on any external state.


Attributes
~~~~~~~~~~
struct driver_attribute {
        struct attribute        attr;
        ssize_t (*show)(struct device_driver *, char * buf, size_t count, loff_t off);
        ssize_t (*store)(struct device_driver *, const char * buf, size_t count, loff_t off);
};

Device drivers can export attributes via their sysfs directories. 
Drivers can declare attributes using a DRIVER_ATTR macro that works
identically to the DEVICE_ATTR macro. 

Example:

DRIVER_ATTR(debug,0644,show_debug,store_debug);

This is equivalent to declaring:

struct driver_attribute driver_attr_debug;

This can then be used to add and remove the attribute from the
driver's directory using:

int driver_create_file(struct device_driver *, struct driver_attribute *);
void driver_remove_file(struct device_driver *, struct driver_attribute *);