1.7. V4L2 sub-devices¶
Many drivers need to communicate with sub-devices. These devices can do all sort of tasks, but most commonly they handle audio and/or video muxing, encoding or decoding. For webcams common sub-devices are sensors and camera controllers.
Usually these are I2C devices, but not necessarily. In order to provide the
driver with a consistent interface to these sub-devices the
v4l2_subdev
struct (v4l2-subdev.h) was created.
Each sub-device driver must have a v4l2_subdev
struct. This struct
can be stand-alone for simple sub-devices or it might be embedded in a larger
struct if more state information needs to be stored. Usually there is a
low-level device struct (e.g. i2c_client
) that contains the device data as
setup by the kernel. It is recommended to store that pointer in the private
data of v4l2_subdev
using v4l2_set_subdevdata()
. That makes
it easy to go from a v4l2_subdev
to the actual low-level bus-specific
device data.
You also need a way to go from the low-level struct to v4l2_subdev
.
For the common i2c_client struct the i2c_set_clientdata() call is used to store
a v4l2_subdev
pointer, for other buses you may have to use other
methods.
Bridges might also need to store per-subdev private data, such as a pointer to
bridge-specific per-subdev private data. The v4l2_subdev
structure
provides host private data for that purpose that can be accessed with
v4l2_get_subdev_hostdata()
and v4l2_set_subdev_hostdata()
.
From the bridge driver perspective, you load the sub-device module and somehow
obtain the v4l2_subdev
pointer. For i2c devices this is easy: you call
i2c_get_clientdata()
. For other buses something similar needs to be done.
Helper functions exist for sub-devices on an I2C bus that do most of this
tricky work for you.
Each v4l2_subdev
contains function pointers that sub-device drivers
can implement (or leave NULL
if it is not applicable). Since sub-devices can
do so many different things and you do not want to end up with a huge ops struct
of which only a handful of ops are commonly implemented, the function pointers
are sorted according to category and each category has its own ops struct.
The top-level ops struct contains pointers to the category ops structs, which may be NULL if the subdev driver does not support anything from that category.
It looks like this:
struct v4l2_subdev_core_ops {
int (*log_status)(struct v4l2_subdev *sd);
int (*init)(struct v4l2_subdev *sd, u32 val);
...
};
struct v4l2_subdev_tuner_ops {
...
};
struct v4l2_subdev_audio_ops {
...
};
struct v4l2_subdev_video_ops {
...
};
struct v4l2_subdev_pad_ops {
...
};
struct v4l2_subdev_ops {
const struct v4l2_subdev_core_ops *core;
const struct v4l2_subdev_tuner_ops *tuner;
const struct v4l2_subdev_audio_ops *audio;
const struct v4l2_subdev_video_ops *video;
const struct v4l2_subdev_pad_ops *video;
};
The core ops are common to all subdevs, the other categories are implemented depending on the sub-device. E.g. a video device is unlikely to support the audio ops and vice versa.
This setup limits the number of function pointers while still making it easy to add new ops and categories.
A sub-device driver initializes the v4l2_subdev
struct using:
v4l2_subdev_init
(sd
, &ops
).
Afterwards you need to initialize sd
->name with a
unique name and set the module owner. This is done for you if you use the
i2c helper functions.
If integration with the media framework is needed, you must initialize the
media_entity
struct embedded in the v4l2_subdev
struct
(entity field) by calling media_entity_pads_init()
, if the entity has
pads:
struct media_pad *pads = &my_sd->pads;
int err;
err = media_entity_pads_init(&sd->entity, npads, pads);
The pads array must have been previously initialized. There is no need to manually set the struct media_entity function and name fields, but the revision field must be initialized if needed.
A reference to the entity will be automatically acquired/released when the subdev device node (if any) is opened/closed.
Don't forget to cleanup the media entity before the sub-device is destroyed:
media_entity_cleanup(&sd->entity);
If the subdev driver intends to process video and integrate with the media
framework, it must implement format related functionality using
v4l2_subdev_pad_ops
instead of v4l2_subdev_video_ops
.
In that case, the subdev driver may set the link_validate field to provide its own link validation function. The link validation function is called for every link in the pipeline where both of the ends of the links are V4L2 sub-devices. The driver is still responsible for validating the correctness of the format configuration between sub-devices and video nodes.
If link_validate op is not set, the default function
v4l2_subdev_link_validate_default()
is used instead. This function
ensures that width, height and the media bus pixel code are equal on both source
and sink of the link. Subdev drivers are also free to use this function to
perform the checks mentioned above in addition to their own checks.
1.7.1. Subdev registration¶
There are currently two ways to register subdevices with the V4L2 core. The first (traditional) possibility is to have subdevices registered by bridge drivers. This can be done when the bridge driver has the complete information about subdevices connected to it and knows exactly when to register them. This is typically the case for internal subdevices, like video data processing units within SoCs or complex PCI(e) boards, camera sensors in USB cameras or connected to SoCs, which pass information about them to bridge drivers, usually in their platform data.
There are however also situations where subdevices have to be registered asynchronously to bridge devices. An example of such a configuration is a Device Tree based system where information about subdevices is made available to the system independently from the bridge devices, e.g. when subdevices are defined in DT as I2C device nodes. The API used in this second case is described further below.
Using one or the other registration method only affects the probing process, the run-time bridge-subdevice interaction is in both cases the same.
In the synchronous case a device (bridge) driver needs to register the
v4l2_subdev
with the v4l2_device:
v4l2_device_register_subdev
(v4l2_dev
,sd
).
This can fail if the subdev module disappeared before it could be registered.
After this function was called successfully the subdev->dev field points to
the v4l2_device
.
If the v4l2_device parent device has a non-NULL mdev field, the sub-device entity will be automatically registered with the media device.
You can unregister a sub-device using:
v4l2_device_unregister_subdev
(sd
).
Afterwards the subdev module can be unloaded and
sd
->dev == NULL
.
In the asynchronous case subdevice probing can be invoked independently of
the bridge driver availability. The subdevice driver then has to verify whether
all the requirements for a successful probing are satisfied. This can include a
check for a master clock availability. If any of the conditions aren't satisfied
the driver might decide to return -EPROBE_DEFER
to request further reprobing
attempts. Once all conditions are met the subdevice shall be registered using
the v4l2_async_register_subdev()
function. Unregistration is
performed using the v4l2_async_unregister_subdev()
call. Subdevices
registered this way are stored in a global list of subdevices, ready to be
picked up by bridge drivers.
Bridge drivers in turn have to register a notifier object. This is
performed using the v4l2_async_notifier_register()
call. To
unregister the notifier the driver has to call
v4l2_async_notifier_unregister()
. The former of the two functions
takes two arguments: a pointer to struct v4l2_device
and a
pointer to struct v4l2_async_notifier
.
Before registering the notifier, bridge drivers must do two things:
first, the notifier must be initialized using the
v4l2_async_notifier_init()
. Second, bridge drivers can then
begin to form a list of subdevice descriptors that the bridge device
needs for its operation. Subdevice descriptors are added to the notifier
using the v4l2_async_notifier_add_subdev()
call. This function
takes two arguments: a pointer to struct v4l2_async_notifier
,
and a pointer to the subdevice descripter, which is of type struct
v4l2_async_subdev
.
The V4L2 core will then use these descriptors to match asynchronously
registered subdevices to them. If a match is detected the .bound()
notifier callback is called. After all subdevices have been located the
.complete() callback is called. When a subdevice is removed from the
system the .unbind() method is called. All three callbacks are optional.
1.7.2. Calling subdev operations¶
The advantage of using v4l2_subdev
is that it is a generic struct and
does not contain any knowledge about the underlying hardware. So a driver might
contain several subdevs that use an I2C bus, but also a subdev that is
controlled through GPIO pins. This distinction is only relevant when setting
up the device, but once the subdev is registered it is completely transparent.
Once te subdev has been registered you can call an ops function either directly:
err = sd->ops->core->g_std(sd, &norm);
but it is better and easier to use this macro:
err = v4l2_subdev_call(sd, core, g_std, &norm);
The macro will do the right NULL
pointer checks and returns -ENODEV
if sd
is NULL
, -ENOIOCTLCMD
if either
sd
->core or sd
->core->g_std is NULL
, or the actual result of the
sd
->ops->core->g_std ops.
It is also possible to call all or a subset of the sub-devices:
v4l2_device_call_all(v4l2_dev, 0, core, g_std, &norm);
Any subdev that does not support this ops is skipped and error results are ignored. If you want to check for errors use this:
err = v4l2_device_call_until_err(v4l2_dev, 0, core, g_std, &norm);
Any error except -ENOIOCTLCMD
will exit the loop with that error. If no
errors (except -ENOIOCTLCMD
) occurred, then 0 is returned.
The second argument to both calls is a group ID. If 0, then all subdevs are
called. If non-zero, then only those whose group ID match that value will
be called. Before a bridge driver registers a subdev it can set
sd
->grp_id to whatever value it wants (it's 0 by
default). This value is owned by the bridge driver and the sub-device driver
will never modify or use it.
The group ID gives the bridge driver more control how callbacks are called.
For example, there may be multiple audio chips on a board, each capable of
changing the volume. But usually only one will actually be used when the
user want to change the volume. You can set the group ID for that subdev to
e.g. AUDIO_CONTROLLER and specify that as the group ID value when calling
v4l2_device_call_all()
. That ensures that it will only go to the subdev
that needs it.
If the sub-device needs to notify its v4l2_device parent of an event, then
it can call v4l2_subdev_notify(sd, notification, arg)
. This macro checks
whether there is a notify()
callback defined and returns -ENODEV
if not.
Otherwise the result of the notify()
call is returned.
1.8. V4L2 sub-device userspace API¶
Bridge drivers traditionally expose one or multiple video nodes to userspace,
and control subdevices through the v4l2_subdev_ops
operations in
response to video node operations. This hides the complexity of the underlying
hardware from applications. For complex devices, finer-grained control of the
device than what the video nodes offer may be required. In those cases, bridge
drivers that implement the media controller API may
opt for making the subdevice operations directly accessible from userpace.
Device nodes named v4l-subdev
X can be created in /dev
to access
sub-devices directly. If a sub-device supports direct userspace configuration
it must set the V4L2_SUBDEV_FL_HAS_DEVNODE
flag before being registered.
After registering sub-devices, the v4l2_device
driver can create
device nodes for all registered sub-devices marked with
V4L2_SUBDEV_FL_HAS_DEVNODE
by calling
v4l2_device_register_subdev_nodes()
. Those device nodes will be
automatically removed when sub-devices are unregistered.
The device node handles a subset of the V4L2 API.
VIDIOC_QUERYCTRL
,
VIDIOC_QUERYMENU
,
VIDIOC_G_CTRL
,
VIDIOC_S_CTRL
,
VIDIOC_G_EXT_CTRLS
,
VIDIOC_S_EXT_CTRLS
and
VIDIOC_TRY_EXT_CTRLS
:
The controls ioctls are identical to the ones defined in V4L2. They behave identically, with the only exception that they deal only with controls implemented in the sub-device. Depending on the driver, those controls can be also be accessed through one (or several) V4L2 device nodes.
VIDIOC_DQEVENT
,
VIDIOC_SUBSCRIBE_EVENT
and
VIDIOC_UNSUBSCRIBE_EVENT
The events ioctls are identical to the ones defined in V4L2. They behave identically, with the only exception that they deal only with events generated by the sub-device. Depending on the driver, those events can also be reported by one (or several) V4L2 device nodes.
Sub-device drivers that want to use events need to set the
V4L2_SUBDEV_FL_HAS_EVENTS
v4l2_subdev
.flags before registering the sub-device. After registration events can be queued as usual on thev4l2_subdev
.devnode device node.To properly support events, the
poll()
file operation is also implemented.
Private ioctls
All ioctls not in the above list are passed directly to the sub-device driver through the core::ioctl operation.
1.9. Read-only sub-device userspace API¶
Bridge drivers that control their connected subdevices through direct calls to
the kernel API realized by v4l2_subdev_ops
structure do not usually
want userspace to be able to change the same parameters through the subdevice
device node and thus do not usually register any.
It is sometimes useful to report to userspace the current subdevice configuration through a read-only API, that does not permit applications to change to the device parameters but allows interfacing to the subdevice device node to inspect them.
For instance, to implement cameras based on computational photography, userspace needs to know the detailed camera sensor configuration (in terms of skipping, binning, cropping and scaling) for each supported output resolution. To support such use cases, bridge drivers may expose the subdevice operations to userspace through a read-only API.
To create a read-only device node for all the subdevices registered with the
V4L2_SUBDEV_FL_HAS_DEVNODE
set, the v4l2_device
driver should call
v4l2_device_register_ro_subdev_nodes()
.
Access to the following ioctls for userspace applications is restricted on
sub-device device nodes registered with
v4l2_device_register_ro_subdev_nodes()
.
VIDIOC_SUBDEV_S_FMT
,
VIDIOC_SUBDEV_S_CROP
,
VIDIOC_SUBDEV_S_SELECTION
:
These ioctls are only allowed on a read-only subdevice device node for the V4L2_SUBDEV_FORMAT_TRY formats and selection rectangles.
VIDIOC_SUBDEV_S_FRAME_INTERVAL
,
VIDIOC_SUBDEV_S_DV_TIMINGS
,
VIDIOC_SUBDEV_S_STD
:
These ioctls are not allowed on a read-only subdevice node.
In case the ioctl is not allowed, or the format to modify is set to
V4L2_SUBDEV_FORMAT_ACTIVE
, the core returns a negative error code and
the errno variable is set to -EPERM
.
1.10. I2C sub-device drivers¶
Since these drivers are so common, special helper functions are available to
ease the use of these drivers (v4l2-common.h
).
The recommended method of adding v4l2_subdev
support to an I2C driver
is to embed the v4l2_subdev
struct into the state struct that is
created for each I2C device instance. Very simple devices have no state
struct and in that case you can just create a v4l2_subdev
directly.
A typical state struct would look like this (where 'chipname' is replaced by the name of the chip):
struct chipname_state {
struct v4l2_subdev sd;
... /* additional state fields */
};
Initialize the v4l2_subdev
struct as follows:
v4l2_i2c_subdev_init(&state->sd, client, subdev_ops);
This function will fill in all the fields of v4l2_subdev
ensure that
the v4l2_subdev
and i2c_client both point to one another.
You should also add a helper inline function to go from a v4l2_subdev
pointer to a chipname_state struct:
static inline struct chipname_state *to_state(struct v4l2_subdev *sd)
{
return container_of(sd, struct chipname_state, sd);
}
Use this to go from the v4l2_subdev
struct to the i2c_client
struct:
struct i2c_client *client = v4l2_get_subdevdata(sd);
And this to go from an i2c_client
to a v4l2_subdev
struct:
struct v4l2_subdev *sd = i2c_get_clientdata(client);
Make sure to call
v4l2_device_unregister_subdev()
(sd
)
when the remove()
callback is called. This will unregister the sub-device
from the bridge driver. It is safe to call this even if the sub-device was
never registered.
You need to do this because when the bridge driver destroys the i2c adapter
the remove()
callbacks are called of the i2c devices on that adapter.
After that the corresponding v4l2_subdev structures are invalid, so they
have to be unregistered first. Calling
v4l2_device_unregister_subdev()
(sd
)
from the remove()
callback ensures that this is always done correctly.
The bridge driver also has some helper functions it can use:
struct v4l2_subdev *sd = v4l2_i2c_new_subdev(v4l2_dev, adapter,
"module_foo", "chipid", 0x36, NULL);
This loads the given module (can be NULL
if no module needs to be loaded)
and calls i2c_new_client_device()
with the given i2c_adapter
and
chip/address arguments. If all goes well, then it registers the subdev with
the v4l2_device.
You can also use the last argument of v4l2_i2c_new_subdev()
to pass
an array of possible I2C addresses that it should probe. These probe addresses
are only used if the previous argument is 0. A non-zero argument means that you
know the exact i2c address so in that case no probing will take place.
Both functions return NULL
if something went wrong.
Note that the chipid you pass to v4l2_i2c_new_subdev()
is usually
the same as the module name. It allows you to specify a chip variant, e.g.
"saa7114" or "saa7115". In general though the i2c driver autodetects this.
The use of chipid is something that needs to be looked at more closely at a
later date. It differs between i2c drivers and as such can be confusing.
To see which chip variants are supported you can look in the i2c driver code
for the i2c_device_id table. This lists all the possibilities.
There are one more helper function:
v4l2_i2c_new_subdev_board()
uses an i2c_board_info
struct
which is passed to the i2c driver and replaces the irq, platform_data and addr
arguments.
If the subdev supports the s_config core ops, then that op is called with the irq and platform_data arguments after the subdev was setup.
The v4l2_i2c_new_subdev()
function will call
v4l2_i2c_new_subdev_board()
, internally filling a
i2c_board_info
structure using the client_type
and the
addr
to fill it.
1.11. V4L2 sub-device functions and data structures¶
Error
kernel-doc missing