--- zzzz-none-000/linux-3.10.107/Documentation/video4linux/v4l2-framework.txt 2017-06-27 09:49:32.000000000 +0000 +++ scorpion-7490-727/linux-3.10.107/Documentation/video4linux/v4l2-framework.txt 2021-02-04 17:41:59.000000000 +0000 @@ -34,6 +34,10 @@ need and this same framework should make it much easier to refactor common code into utility functions shared by all drivers. +A good example to look at as a reference is the v4l2-pci-skeleton.c +source that is available in this directory. It is a skeleton driver for +a PCI capture card, and demonstrates how to use the V4L2 driver +framework. It can be used as a template for real PCI video capture driver. Structure of a driver --------------------- @@ -246,7 +250,6 @@ It looks like this: struct v4l2_subdev_core_ops { - int (*g_chip_ident)(struct v4l2_subdev *sd, struct v4l2_dbg_chip_ident *chip); int (*log_status)(struct v4l2_subdev *sd); int (*init)(struct v4l2_subdev *sd, u32 val); ... @@ -326,8 +329,27 @@ 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. -A device (bridge) driver needs to register the v4l2_subdev with the -v4l2_device: +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: int err = v4l2_device_register_subdev(v4l2_dev, sd); @@ -346,24 +368,24 @@ You can call an ops function either directly: - err = sd->ops->core->g_chip_ident(sd, &chip); + 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_chip_ident, &chip); + err = v4l2_subdev_call(sd, core, g_std, &norm); The macro will to the right NULL pointer checks and returns -ENODEV if subdev -is NULL, -ENOIOCTLCMD if either subdev->core or subdev->core->g_chip_ident is -NULL, or the actual result of the subdev->ops->core->g_chip_ident ops. +is NULL, -ENOIOCTLCMD if either subdev->core or subdev->core->g_std is +NULL, or the actual result of the subdev->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_chip_ident, &chip); + 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_chip_ident, &chip); + 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. @@ -394,6 +416,30 @@ up the device, but once the subdev is registered it is completely transparent. +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 with an array of +subdevice descriptors that the bridge device needs for its operation. 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. The latter contains a pointer to an array +of pointers to subdevice descriptors of type struct v4l2_async_subdev type. 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. + + V4L2 sub-device userspace API ----------------------------- @@ -575,9 +621,13 @@ The default video_device_release() callback just calls kfree to free the allocated memory. +There is also a video_device_release_empty() function that does nothing +(is empty) and can be used if the struct is embedded and there is nothing +to do when it is released. + You should also set these fields: -- v4l2_dev: set to the v4l2_device parent device. +- v4l2_dev: must be set to the v4l2_device parent device. - name: set to something descriptive and unique. @@ -614,20 +664,16 @@ If you want to have a separate priority state per (group of) device node(s), then you can point it to your own struct v4l2_prio_state. -- parent: you only set this if v4l2_device was registered with NULL as +- dev_parent: you only set this if v4l2_device was registered with NULL as the parent device struct. This only happens in cases where one hardware device has multiple PCI devices that all share the same v4l2_device core. The cx88 driver is an example of this: one core v4l2_device struct, but - it is used by both an raw video PCI device (cx8800) and a MPEG PCI device - (cx8802). Since the v4l2_device cannot be associated with a particular - PCI device it is setup without a parent device. But when the struct - video_device is setup you do know which parent PCI device to use. - -- flags: optional. Set to V4L2_FL_USE_FH_PRIO if you want to let the framework - handle the VIDIOC_G/S_PRIORITY ioctls. This requires that you use struct - v4l2_fh. Eventually this flag will disappear once all drivers use the core - priority handling. But for now it has to be set explicitly. + it is used by both a raw video PCI device (cx8800) and a MPEG PCI device + (cx8802). Since the v4l2_device cannot be associated with two PCI devices + at the same time it is setup without a parent device. But when the struct + video_device is initialized you *do* know which parent PCI device to use and + so you set dev_device to the correct PCI device. If you use v4l2_ioctl_ops, then you should set .unlocked_ioctl to video_ioctl2 in your v4l2_file_operations struct. @@ -721,6 +767,7 @@ VFL_TYPE_GRABBER: videoX for video input/output devices VFL_TYPE_VBI: vbiX for vertical blank data (i.e. closed captions, teletext) VFL_TYPE_RADIO: radioX for radio tuners +VFL_TYPE_SDR: swradioX for Software Defined Radio tuners The last argument gives you a certain amount of control over the device device node number used (i.e. the X in videoX). Normally you will pass -1 @@ -746,8 +793,10 @@ Whenever a device node is created some attributes are also created for you. If you look in /sys/class/video4linux you see the devices. Go into e.g. -video0 and you will see 'name' and 'index' attributes. The 'name' attribute -is the 'name' field of the video_device struct. +video0 and you will see 'name', 'dev_debug' and 'index' attributes. The 'name' +attribute is the 'name' field of the video_device struct. The 'dev_debug' attribute +can be used to enable core debugging. See the next section for more detailed +information on this. The 'index' attribute is the index of the device node: for each call to video_register_device() the index is just increased by 1. The first video @@ -769,6 +818,25 @@ be called if the registration failed, nor should you ever attempt to unregister the device if the registration failed. +video device debugging +---------------------- + +The 'dev_debug' attribute that is created for each video, vbi, radio or swradio +device in /sys/class/video4linux// allows you to enable logging of +file operations. + +It is a bitmask and the following bits can be set: + +0x01: Log the ioctl name and error code. VIDIOC_(D)QBUF ioctls are only logged + if bit 0x08 is also set. +0x02: Log the ioctl name arguments and error code. VIDIOC_(D)QBUF ioctls are + only logged if bit 0x08 is also set. +0x04: Log the file operations open, release, read, write, mmap and + get_unmapped_area. The read and write operations are only logged if + bit 0x08 is also set. +0x08: Log the read and write file operations and the VIDIOC_QBUF and + VIDIOC_DQBUF ioctls. +0x10: Log the poll file operation. video_device cleanup -------------------- @@ -857,8 +925,7 @@ struct v4l2_fh provides a way to easily keep file handle specific data that is used by the V4L2 framework. New drivers must use struct v4l2_fh -since it is also used to implement priority handling (VIDIOC_G/S_PRIORITY) -if the video_device flag V4L2_FL_USE_FH_PRIO is also set. +since it is also used to implement priority handling (VIDIOC_G/S_PRIORITY). The users of v4l2_fh (in the V4L2 framework, not the driver) know whether a driver uses v4l2_fh as its file->private_data pointer by @@ -1061,3 +1128,33 @@ An example on how the V4L2 events may be used can be found in the OMAP 3 ISP driver (drivers/media/platform/omap3isp). + +A subdev can directly send an event to the v4l2_device notify function with +V4L2_DEVICE_NOTIFY_EVENT. This allows the bridge to map the subdev that sends +the event to the video node(s) associated with the subdev that need to be +informed about such an event. + +V4L2 clocks +----------- + +Many subdevices, like camera sensors, TV decoders and encoders, need a clock +signal to be supplied by the system. Often this clock is supplied by the +respective bridge device. The Linux kernel provides a Common Clock Framework for +this purpose. However, it is not (yet) available on all architectures. Besides, +the nature of the multi-functional (clock, data + synchronisation, I2C control) +connection of subdevices to the system might impose special requirements on the +clock API usage. E.g. V4L2 has to support clock provider driver unregistration +while a subdevice driver is holding a reference to the clock. For these reasons +a V4L2 clock helper API has been developed and is provided to bridge and +subdevice drivers. + +The API consists of two parts: two functions to register and unregister a V4L2 +clock source: v4l2_clk_register() and v4l2_clk_unregister() and calls to control +a clock object, similar to the respective generic clock API calls: +v4l2_clk_get(), v4l2_clk_put(), v4l2_clk_enable(), v4l2_clk_disable(), +v4l2_clk_get_rate(), and v4l2_clk_set_rate(). Clock suppliers have to provide +clock operations that will be called when clock users invoke respective API +methods. + +It is expected that once the CCF becomes available on all relevant +architectures this API will be removed.