本文主要是介绍V4L2调试之(五），希望对大家解决编程问题提供一定的参考价值，需要的开发者们随着小编来一起学习吧！
1.关于/dev/mediaX设备的注册，有几个V4L2设备，注册几个/dev/meidaX节点。
比如，插入两个UVC USB Camera，就会出现如下图所示的两个节点，分别用于描述各自的V4L2拓扑。下面是linux上的callstack.
[ 86.241290] v4l2_device_register line 21, kworker/2:3 register device.
[ 86.241295] CPU: 2 PID: 1920 Comm: kworker/2:3 Tainted: G W 5.4.128+ #1
[ 86.241297] Hardware name: TIMI RedmiBook 14/TM1814, BIOS RMRWL400P0503 11/13/2019
[ 86.241307] Workqueue: usb_hub_wq hub_event
[ 86.241310] Call Trace:
[ 86.241322] dump_stack+0x6d/0x8b
[ 86.241345] v4l2_device_register+0x4a/0xe0 [videodev]
[ 86.241354] uvc_probe+0x4eb/0x2a10 [uvcvideo]
[ 86.241363] usb_probe_interface+0x149/0x300
[ 86.241370] ? uvc_register_video_device+0x150/0x150 [uvcvideo]
[ 86.241374] ? usb_probe_interface+0x149/0x300
[ 86.241379] really_probe+0xf5/0x440
[ 86.241384] driver_probe_device+0x11b/0x130
[ 86.241388] __device_attach_driver+0x7b/0xe0
[ 86.241392] ? driver_allows_async_probing+0x60/0x60
[ 86.241399] bus_for_each_drv+0x6e/0xb0
[ 86.241403] __device_attach+0xe4/0x160
[ 86.241408] device_initial_probe+0x13/0x20
[ 86.241411] bus_probe_device+0x92/0xa0
[ 86.241416] device_add+0x402/0x690
[ 86.241422] ? _cond_resched+0x19/0x40
[ 86.241426] usb_set_configuration+0x3fd/0x8f0
[ 86.241432] ? kernfs_activate+0x78/0x80
[ 86.241440] generic_probe+0x2e/0x80
[ 86.241443] usb_probe_device+0x31/0x70
[ 86.241447] really_probe+0xf5/0x440
[ 86.241451] driver_probe_device+0x11b/0x130
[ 86.241455] __device_attach_driver+0x7b/0xe0
[ 86.241459] ? driver_allows_async_probing+0x60/0x60
[ 86.241465] bus_for_each_drv+0x6e/0xb0
[ 86.241468] __device_attach+0xe4/0x160
[ 86.241473] device_initial_probe+0x13/0x20
[ 86.241476] bus_probe_device+0x92/0xa0
[ 86.241480] device_add+0x402/0x690
[ 86.241487] ? add_device_randomness+0x9d/0x1c0
[ 86.241492] usb_new_device+0x218/0x4a0
[ 86.241498] hub_event+0x11b1/0x1760
[ 86.241508] process_one_work+0x20f/0x400
[ 86.241514] worker_thread+0x34/0x410
[ 86.241518] kthread+0x121/0x140
[ 86.241523] ? process_one_work+0x400/0x400
[ 86.241527] ? kthread_park+0x90/0x90
[ 86.241532] ret_from_fork+0x35/0x40
[ 86.263776] input: Integrated Camera: Integrated C as /devices/pci0000:00/0000:00:14.0/usb1/1-2/1-2:1.0/input/input25
[ 96.375646] usb 1-4: new high-speed USB device number 7 using xhci_hcd
[ 96.524824] usb 1-4: config 1 interface 0 altsetting 0 endpoint 0x83 has an invalid bInterval 32, changing to 9
[ 96.525345] usb 1-4: New USB device found, idVendor=1b3f, idProduct=2247, bcdDevice= 1.00
[ 96.525350] usb 1-4: New USB device strings: Mfr=1, Product=2, SerialNumber=0
[ 96.525353] usb 1-4: Product: GENERAL WEBCAM
[ 96.525356] usb 1-4: Manufacturer: GENERAL
[ 96.526771] uvcvideo: Found UVC 1.00 device GENERAL WEBCAM (1b3f:2247)
[ 96.526777] v4l2_device_register line 21, kworker/2:3 register device.
[ 96.526784] CPU: 2 PID: 1920 Comm: kworker/2:3 Tainted: G W 5.4.128+ #1
[ 96.526788] Hardware name: TIMI RedmiBook 14/TM1814, BIOS RMRWL400P0503 11/13/2019
[ 96.526799] Workqueue: usb_hub_wq hub_event
[ 96.526804] Call Trace:
[ 96.526819] dump_stack+0x6d/0x8b
[ 96.526853] v4l2_device_register+0x4a/0xe0 [videodev]
[ 96.526867] uvc_probe+0x4eb/0x2a10 [uvcvideo]
[ 96.526878] usb_probe_interface+0x149/0x300
[ 96.526891] ? uvc_register_video_device+0x150/0x150 [uvcvideo]
[ 96.526896] ? usb_probe_interface+0x149/0x300
[ 96.526905] really_probe+0xf5/0x440
[ 96.526911] driver_probe_device+0x11b/0x130
[ 96.526918] __device_attach_driver+0x7b/0xe0
[ 96.526925] ? driver_allows_async_probing+0x60/0x60
[ 96.526933] bus_for_each_drv+0x6e/0xb0
[ 96.526939] __device_attach+0xe4/0x160
[ 96.526945] device_initial_probe+0x13/0x20
[ 96.526951] bus_probe_device+0x92/0xa0
[ 96.526964] device_add+0x402/0x690
[ 96.526972] ? _cond_resched+0x19/0x40
[ 96.526977] usb_set_configuration+0x3fd/0x8f0
[ 96.526985] ? kernfs_activate+0x78/0x80
[ 96.526997] generic_probe+0x2e/0x80
[ 96.527008] usb_probe_device+0x31/0x70
[ 96.527014] really_probe+0xf5/0x440
[ 96.527020] driver_probe_device+0x11b/0x130
[ 96.527025] __device_attach_driver+0x7b/0xe0
[ 96.527030] ? driver_allows_async_probing+0x60/0x60
[ 96.527038] bus_for_each_drv+0x6e/0xb0
[ 96.527045] __device_attach+0xe4/0x160
[ 96.527052] device_initial_probe+0x13/0x20
[ 96.527056] bus_probe_device+0x92/0xa0
[ 96.527063] device_add+0x402/0x690
[ 96.527072] ? add_device_randomness+0x9d/0x1c0
[ 96.527079] usb_new_device+0x218/0x4a0
[ 96.527087] hub_event+0x11b1/0x1760
[ 96.527108] process_one_work+0x20f/0x400
[ 96.527123] worker_thread+0x34/0x410
[ 96.527130] kthread+0x121/0x140
[ 96.527137] ? process_one_work+0x400/0x400
[ 96.527142] ? kthread_park+0x90/0x90
[ 96.527149] ret_from_fork+0x35/0x40
struct v4l2_device的注册也是这样，它可以看成v4l2 框架的 rootdevice，每个V4L2的采集设备，都对应一个v4l2_device,也对应如上的/dev/mediaX., 所有的subdev都链接在同一个v4l2_device下. 通过v4l2_device, 我们可以遍历所有的sbudev. v4l2_device此时相当于所以subdev的父设备. 从如下调用堆栈也可以看出通用USB设备的发现流程。由于USB驱动已经事先注册进系统，这里是通过HUB的设备发现去发现USBS设备的插入，之后进行USB设备级的device_add,和设备驱动匹配上后，进行USB设备级的probe函数usb_probe_device.然后进步步，读取USB配置后，进行接口级的USB设备注册，再次调用device_add在USB总线上MATCH USB INTERFACE驱动，注册真正的video device uvc驱动。在这个层面，实际上最后一级的probe已经进入到 struct usb_driver的定义了。本质上，usb_driver是对USB INTERFACE struct device_driver的封装。
USB设备的注册接口和INTERFACE的注册接口是不同的,通过for_device变量区分。设别的变量为1.usb_register_device_driver
而接口的变量为0，usb_register_driver：
[ 58.718847] media_devnode_register line 248.comm kworker/1:2.
[ 58.718854] CPU: 1 PID: 327 Comm: kworker/1:2 Tainted: G W 5.4.128+ #1
[ 58.718856] Hardware name: TIMI RedmiBook 14/TM1814, BIOS RMRWL400P0503 11/13/2019
[ 58.718866] Workqueue: usb_hub_wq hub_event
[ 58.718869] Call Trace:
[ 58.718881] dump_stack+0x6d/0x8b
[ 58.718893] media_devnode_register+0x11b/0x1c0 [mc]
[ 58.718902] __media_device_register+0x77/0x110 [mc]
[ 58.718910] uvc_probe+0x2638/0x2a10 [uvcvideo]
[ 58.718919] usb_probe_interface+0x149/0x300
[ 58.718926] ? uvc_register_video_device+0x150/0x150 [uvcvideo]
[ 58.718929] ? usb_probe_interface+0x149/0x300
[ 58.718935] really_probe+0xf5/0x440
[ 58.718939] driver_probe_device+0x11b/0x130
[ 58.718944] __device_attach_driver+0x7b/0xe0
[ 58.718948] ? driver_allows_async_probing+0x60/0x60
[ 58.718955] bus_for_each_drv+0x6e/0xb0
[ 58.718959] __device_attach+0xe4/0x160
[ 58.718963] device_initial_probe+0x13/0x20
[ 58.718966] bus_probe_device+0x92/0xa0
[ 58.718972] device_add+0x402/0x690
[ 58.718978] ? _cond_resched+0x19/0x40
[ 58.718981] usb_set_configuration+0x3fd/0x8f0
[ 58.718988] ? kernfs_activate+0x78/0x80
[ 58.718996] generic_probe+0x2e/0x80
[ 58.718999] usb_probe_device+0x31/0x70
[ 58.719003] really_probe+0xf5/0x440
[ 58.719007] driver_probe_device+0x11b/0x130
[ 58.719011] __device_attach_driver+0x7b/0xe0
[ 58.719015] ? driver_allows_async_probing+0x60/0x60
[ 58.719020] bus_for_each_drv+0x6e/0xb0
[ 58.719024] __device_attach+0xe4/0x160
[ 58.719028] device_initial_probe+0x13/0x20
[ 58.719031] bus_probe_device+0x92/0xa0
[ 58.719036] device_add+0x402/0x690
[ 58.719043] ? add_device_randomness+0x9d/0x1c0
[ 58.719047] usb_new_device+0x218/0x4a0
[ 58.719053] hub_event+0x11b1/0x1760
[ 58.719063] process_one_work+0x20f/0x400
[ 58.719068] worker_thread+0x34/0x410
[ 58.719073] kthread+0x121/0x140
[ 58.719078] ? process_one_work+0x400/0x400
[ 58.719082] ? kthread_park+0x90/0x90
[ 58.719089] ret_from_fork+0x35/0x40
[ 58.719382] input: Integrated Camera: Integrated C as /devices/pci0000:00/0000:00:14.0/usb1/1-2/1-2:1.0/input/input24
[ 152.269448] usb 1-4: new high-speed USB device number 6 using xhci_hcd
[ 152.418887] usb 1-4: config 1 interface 0 altsetting 0 endpoint 0x83 has an invalid bInterval 32, changing to 9
[ 152.419610] usb 1-4: New USB device found, idVendor=1b3f, idProduct=2247, bcdDevice= 1.00
[ 152.419616] usb 1-4: New USB device strings: Mfr=1, Product=2, SerialNumber=0
[ 152.419619] usb 1-4: Product: GENERAL WEBCAM
[ 152.419622] usb 1-4: Manufacturer: GENERAL
[ 152.421232] uvcvideo: Found UVC 1.00 device GENERAL WEBCAM (1b3f:2247)
[ 152.421461] uvcvideo: Failed to query (GET_INFO) UVC control 2 on unit 1: -32 (exp. 1).
[ 152.424449] uvcvideo: UVC non compliance - GET_DEF(PROBE) not supported. Enabling workaround.
[ 152.424987] media_devnode_register line 248.comm kworker/1:2.
[ 152.424996] CPU: 1 PID: 327 Comm: kworker/1:2 Tainted: G W 5.4.128+ #1
[ 152.424998] Hardware name: TIMI RedmiBook 14/TM1814, BIOS RMRWL400P0503 11/13/2019
[ 152.425010] Workqueue: usb_hub_wq hub_event
[ 152.425014] Call Trace:
[ 152.425026] dump_stack+0x6d/0x8b
[ 152.425039] media_devnode_register+0x11b/0x1c0 [mc]
[ 152.425048] __media_device_register+0x77/0x110 [mc]
[ 152.425055] uvc_probe+0x2638/0x2a10 [uvcvideo]
[ 152.425063] usb_probe_interface+0x149/0x300
[ 152.425070] ? uvc_register_video_device+0x150/0x150 [uvcvideo]
[ 152.425077] ? usb_probe_interface+0x149/0x300
[ 152.425082] really_probe+0xf5/0x440
[ 152.425086] driver_probe_device+0x11b/0x130
[ 152.425089] __device_attach_driver+0x7b/0xe0
[ 152.425093] ? driver_allows_async_probing+0x60/0x60
[ 152.425099] bus_for_each_drv+0x6e/0xb0
[ 152.425103] __device_attach+0xe4/0x160
[ 152.425107] device_initial_probe+0x13/0x20
[ 152.425109] bus_probe_device+0x92/0xa0
[ 152.425114] device_add+0x402/0x690
[ 152.425119] ? _cond_resched+0x19/0x40
[ 152.425122] usb_set_configuration+0x3fd/0x8f0
[ 152.425128] ? kernfs_activate+0x78/0x80
[ 152.425135] generic_probe+0x2e/0x80
[ 152.425138] usb_probe_device+0x31/0x70
[ 152.425141] really_probe+0xf5/0x440
[ 152.425145] driver_probe_device+0x11b/0x130
[ 152.425148] __device_attach_driver+0x7b/0xe0
[ 152.425151] ? driver_allows_async_probing+0x60/0x60
[ 152.425156] bus_for_each_drv+0x6e/0xb0
[ 152.425160] __device_attach+0xe4/0x160
[ 152.425163] device_initial_probe+0x13/0x20
[ 152.425165] bus_probe_device+0x92/0xa0
[ 152.425170] device_add+0x402/0x690
[ 152.425176] ? add_device_randomness+0x9d/0x1c0
[ 152.425180] usb_new_device+0x218/0x4a0
[ 152.425185] hub_event+0x11b1/0x1760
[ 152.425195] process_one_work+0x20f/0x400
[ 152.425203] worker_thread+0x34/0x410
[ 152.425208] kthread+0x121/0x140
[ 152.425212] ? process_one_work+0x400/0x400
[ 152.425216] ? kthread_park+0x90/0x90
[ 152.425220] ret_from_fork+0x35/0x40
[ 152.425547] input: GENERAL WEBCAM: GENERAL WEBCAM as /devices/pci0000:00/0000:00:14.0/usb1/1-4/1-4:1.0/input/input25
[ 152.452622] uvcvideo: Failed to query (GET_DEF) UVC control 2 on unit 1: -32 (exp. 1).
[ 152.452780] uvcvideo: Failed to query (GET_DEF) UVC control 2 on unit 1: -32 (exp. 1).
[ 152.483857] usb 1-4: Warning! Unlikely big volume range (=5120), cval->res is probably wrong.
[ 152.483859] usb 1-4: [5] FU [Mic Capture Volume] ch = 1, val = 7680/12800/1
[ 152.484248] usbcore: registered new interface driver snd-usb-audio
czl@czl-RedmiBook-14:~/Workspace$ 2.用到struct v4l2_subdev_internal_ops结构注册的地方 有两个，一个是
另一个是：
有意思的是，这两个一个在PIPEline的首端(sensor),另一个在pipeline的尾端(video), v4l2_subdev_internal_ops提供了4个接口: registered、unregistered、open、close. 这4个接口是给v4l2核心层代码回调用的. 当向核心层注册/注销subdev时(调用v4l2_device_register_subdev）, 核心层会回调这里的registered/unregistered. 当用户空间打开/关闭设备节点时, 核心层会回调这里的open/close.
3. VIN框架中管理有一张表,表中记录了VIN支持的所有可能的格式,JPEG类的是YUV, SRGB类的是RGB.
V4L2_COLORSPACE_JPEG表示YUV像素格式.
使用的地方再这里:
再创建pipeline的时候会看是否支持上面配置下来的格式,不支持会返回失败.
4.V4L2设备初始化的时通过I2C和sensor的交互过程.
MIPI CSI的传输是这样的，如下图，它通过CCI总线对Sensor的寄存器进行配置。
一般情况下，CCI总线是通过I2C总线实现的，所以从驱动角度来讲，它实际上调用的是I2C驱动。下面调试一下Melis系统上Sensor初始化过程中，对I2C的调用时序.
首先看一下打印,在hal_twi_xfer里面加打印：
__builtin_return_address(LEVEL)的作用是返回调用栈第level层的返回值。
可以看到，貌似GCC只能返回当前函数的返回地址。
所以看起来是从cci_read_a16_d8调用下来的，我们看一下这个调用是怎么发生的。
可以看到，这里就是上面说的internal_ops的handler, sensor_registered，调用链中，会调用sensor_detect尝试读取sensor_id,根据ID来判断sensor是否存在。
返回的是IMX的 sensor id.
sensor bringup 的第二步是上电:
第三步的是init寄存器
初始化的是register的列表
下一步，设置默认帧率，还是同样的调用栈：
wsize->regs是默认的帧率配置参数中的一个，每个帧率对应一种配置
接下来，设置增益：
打开阀门，让水流进来，开启sensor抓图。
到这里，画面实际上已经出来了，之后，就是ISP常规化的控制了，这个设置gain的调用会一直存在。
解决的一个问题记录
客户要求可以通过melis shell命令来读取sensor寄存器，由于sensor是板载资源，访问方式是通过cci总线（I2C）的方式进行的。需求很简单，但是在实现的时候遇到了一个技术性问题，感觉对理解v4l2 sensor注册的流程挺有帮助的，这里记录一下。
实现很简单，以为读取sensor id为例，写完后是这样子的，根据上面的分析,0x16,0x17寄存器是ID寄存器，况且启动过程中是可以访问的，那么我把它封装成一个shell命令，应该也可以读取才对。
但是测试情况出乎意料：
执行read_sensor，I2C接口访问竟然是失败的。
而启动过程中的访问是成功的：
启动过程和手敲命令测试流程上是相距不远的逻辑，一定是中间发生了什么才导致的问题。处理这种问题最好使用二分法，不断在从OK到failure的路径中加上打印，最后发现了明显的差异点的位置：
如上图，正常启动流程中的访问是在67行中进行的，然后直到73行中的调用还是OK的，但是到了81行，就无法成功读取到ID了，这说明问题就发生在73行到81行中间。
很明显，最值得怀疑的逻辑是77行，看起来它是将sensor电源关闭了。添加上打印调试一下：
编译启动验证：
LOG证实了我们的猜测，PWR_ON和PWR_OFF中间的代码读取是成功的，恰好是PWR_OFF之后的，读取都失败了。
所以，可以说明，流程中为了节省功耗，对电源进行了按需管理，启动过程中，如果需要对sensor寄存器进行访问，就上电，如果不需要访问sensor了，就关闭电源。
这个例子也可以从侧面说明，启动过程中访问sensor的地方只有一个，就是这里读取sensor id.
接下来把pwr off这行关闭，确认一下是否OK：
OK，看打印结果，每次的读取都是成功的了。
Video Device:
无论是subdevice还是video device，都需要创建struct video_device结构并注册。
static inline int __must_check video_register_device(struct video_device *vdev, …)
向核心层注册一个video_device, 此时核心层会创建字符设备驱动. 后面用户空间就可以通过字符设备节点与video_device交互了.
V4L2设备逻辑图
参考资料：
Linux V4L2子系统 | 思学网V4L2, Camera, subdev
http://www.mysixue.com/?p=131
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