docs: Add debugging guide for the media subsystem

Provide a guide for developers on how to debug code with a focus on the
media subsystem. This document aims to provide a rough overview over the
possibilities and a rational to help choosing the right tool for the
given circumstances.

Signed-off-by: Sebastian Fricke <[e-mail]>
Signed-off-by: Jonathan Corbet <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]

Documentation/admin-guide/media/index.rst

···
   - for driver development information and Kernel APIs used by
     media devices;
 
+Documentation/process/debugging/media_specific_debugging_guide.rst
+
+  - for advice about essential tools and techniques to debug drivers on this
+    subsystem
+
 .. toctree::
 	:caption: Table of Contents
 	:maxdepth: 2

Documentation/process/debugging/index.rst

···
 Debugging advice for Linux Kernel developers
 ============================================
 
+general guides
+--------------
+
 .. toctree::
    :maxdepth: 1
 
    driver_development_debugging_guide
    userspace_debugging_guide
+
+.. only::  subproject and html
+
+subsystem specific guides
+-------------------------
+
+.. toctree::
+   :maxdepth: 1
+
+   media_specific_debugging_guide
 
 .. only::  subproject and html
 

Documentation/process/debugging/media_specific_debugging_guide.rst

···
+.. SPDX-License-Identifier: GPL-2.0
+
+============================================
+Debugging and tracing in the media subsystem
+============================================
+
+This document serves as a starting point and lookup for debugging device
+drivers in the media subsystem and to debug these drivers from userspace.
+
+.. contents::
+    :depth: 3
+
+General debugging advice
+------------------------
+
+For general advice see the :doc:`general advice document
+</process/debugging/index>`.
+
+The following sections show you some of the available tools.
+
+dev_debug module parameter
+--------------------------
+
+Every video device provides a ``dev_debug`` parameter, which allows to get
+further insights into the IOCTLs in the background.::
+
+  # cat /sys/class/video4linux/video3/name
+  rkvdec
+  # echo 0xff > /sys/class/video4linux/video3/dev_debug
+  # dmesg -wH
+  [...] videodev: v4l2_open: video3: open (0)
+  [  +0.000036] video3: VIDIOC_QUERYCAP: driver=rkvdec, card=rkvdec,
+  bus=platform:rkvdec, version=0x00060900, capabilities=0x84204000,
+  device_caps=0x04204000
+
+For the full documentation see :ref:`driver-api/media/v4l2-dev:video device
+debugging`
+
+dev_dbg() / v4l2_dbg()
+----------------------
+
+Two debug print statements, which are specific for devices and for the v4l2
+subsystem, avoid adding these to your final submission unless they have
+long-term value for investigations.
+
+For a general overview please see the
+:ref:`process/debugging/driver_development_debugging_guide:printk() & friends`
+guide.
+
+- Difference between both?
+
+  - v4l2_dbg() utilizes v4l2_printk() under the hood, which further uses
+    printk() directly, thus it cannot be targeted by dynamic debug
+  - dev_dbg() can be targeted by dynamic debug
+  - v4l2_dbg() has a more specific prefix format for the media subsystem, while
+    dev_dbg only highlights the driver name and the location of the log
+
+Dynamic debug
+-------------
+
+A method to trim down the debug output to your needs.
+
+For general advice see the
+:ref:`process/debugging/userspace_debugging_guide:dynamic debug` guide.
+
+Here is one example, that enables all available pr_debug()'s within the file::
+
+  $ alias ddcmd='echo $* > /proc/dynamic_debug/control'
+  $ ddcmd '-p; file v4l2-h264.c +p'
+  $ grep =p /proc/dynamic_debug/control
+   drivers/media/v4l2-core/v4l2-h264.c:372 [v4l2_h264]print_ref_list_b =p
+   "ref_pic_list_b%u (cur_poc %u%c) %s"
+   drivers/media/v4l2-core/v4l2-h264.c:333 [v4l2_h264]print_ref_list_p =p
+   "ref_pic_list_p (cur_poc %u%c) %s\n"
+
+Ftrace
+------
+
+An internal kernel tracer that can trace static predefined events, function
+calls, etc. Very useful for debugging problems without changing the kernel and
+understanding the behavior of subsystems.
+
+For general advice see the
+:ref:`process/debugging/userspace_debugging_guide:ftrace` guide.
+
+DebugFS
+-------
+
+This tool allows you to dump or modify internal values of your driver to files
+in a custom filesystem.
+
+For general advice see the
+:ref:`process/debugging/driver_development_debugging_guide:debugfs` guide.
+
+Perf & alternatives
+-------------------
+
+Tools to measure the various stats on a running system to diagnose issues.
+
+For general advice see the
+:ref:`process/debugging/userspace_debugging_guide:perf & alternatives` guide.
+
+Example for media devices:
+
+Gather statistics data for a decoding job: (This example is on a RK3399 SoC
+with the rkvdec codec driver using the `fluster test suite
+<https://github.com/fluendo/fluster>`__)::
+
+  perf stat -d python3 fluster.py run -d GStreamer-H.264-V4L2SL-Gst1.0 -ts
+  JVT-AVC_V1 -tv AUD_MW_E -j1
+  ...
+  Performance counter stats for 'python3 fluster.py run -d
+  GStreamer-H.264-V4L2SL-Gst1.0 -ts JVT-AVC_V1 -tv AUD_MW_E -j1 -v':
+
+         7794.23 msec task-clock:u                     #    0.697 CPUs utilized
+               0      context-switches:u               #    0.000 /sec
+               0      cpu-migrations:u                 #    0.000 /sec
+           11901      page-faults:u                    #    1.527 K/sec
+       882671556      cycles:u                         #    0.113 GHz                         (95.79%)
+       711708695      instructions:u                   #    0.81  insn per cycle              (95.79%)
+        10581935      branches:u                       #    1.358 M/sec                       (15.13%)
+         6871144      branch-misses:u                  #   64.93% of all branches             (95.79%)
+       281716547      L1-dcache-loads:u                #   36.144 M/sec                       (95.79%)
+         9019581      L1-dcache-load-misses:u          #    3.20% of all L1-dcache accesses   (95.79%)
+ <not supported>      LLC-loads:u
+ <not supported>      LLC-load-misses:u
+
+    11.180830431 seconds time elapsed
+
+     1.502318000 seconds user
+     6.377221000 seconds sys
+
+The availability of events and metrics depends on the system you are running.
+
+Error checking & panic analysis
+-------------------------------
+
+Various Kernel configuration options to enhance error detection of the Linux
+Kernel with the cost of lowering performance.
+
+For general advice see the
+:ref:`process/debugging/driver_development_debugging_guide:kasan, ubsan,
+lockdep and other error checkers` guide.
+
+Driver verification with v4l2-compliance
+----------------------------------------
+
+To verify, that a driver adheres to the v4l2 API, the tool v4l2-compliance is
+used, which is part of the `v4l_utils
+<https://git.linuxtv.org/v4l-utils.git>`__, a suite of userspace tools to work
+with the media subsystem.
+
+To see the detailed media topology (and check it) use::
+
+  v4l2-compliance -M /dev/mediaX --verbose
+
+You can also run a full compliance check for all devices referenced in the
+media topology with::
+
+  v4l2-compliance -m /dev/mediaX
+
+Debugging problems with receiving video
+---------------------------------------
+
+Implementing vidioc_log_status in the driver: this can log the current status
+to the kernel log. It's called by v4l2-ctl --log-status. Very useful for
+debugging problems with receiving video (TV/S-Video/HDMI/etc) since the video
+signal is external (so unpredictable). Less useful with camera sensor inputs
+since you have control over what the camera sensor does.
+
+Usually you can just assign the default::
+
+  .vidioc_log_status  = v4l2_ctrl_log_status,
+
+But you can also create your own callback, to create a custom status log.
+
+You can find an example in the cobalt driver
+(`drivers/media/pci/cobalt/cobalt-v4l2.c <https://elixir.bootlin.com/linux/v6.11.6/source/drivers/media/pci/cobalt/cobalt-v4l2.c#L567>`__).
+
+**Copyright** ©2024 : Collabora