[PATCH RFC v2 3/3] docs: media: Debugging guide for the media subsystem

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Create a guides section for all documentation material, that isn't
strictly related to a specific piece of code.

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 <sebastian.fricke@xxxxxxxxxxxxx>
---
 Documentation/media/guides/debugging_issues.rst | 174 ++++++++++++++++++++++++
 Documentation/media/guides/index.rst            |  11 ++
 Documentation/media/index.rst                   |   1 +
 3 files changed, 186 insertions(+)

diff --git a/Documentation/media/guides/debugging_issues.rst b/Documentation/media/guides/debugging_issues.rst
new file mode 100644
index 000000000000..5f37801dd4ba
--- /dev/null
+++ b/Documentation/media/guides/debugging_issues.rst
@@ -0,0 +1,174 @@
+.. SPDX-License-Identifier: GPL-2.0
+.. include:: <isonum.txt>
+
+============================================
+Debugging and tracing in the media subsystem
+============================================
+
+This document serves as a starting point and lookup for debugging device
+drivers in the media subsystem.
+
+.. contents::
+    :depth: 3
+
+General debugging advice
+========================
+
+For general advice see the `general-debugging-guide <../../debugging/index.html>`__.
+
+Available tools
+===============
+
+dev_debug module parameter
+--------------------------
+
+For a general overview please see the `driver-development-debugging-guide <../../debugging/driver_development_debugging_guide.html>`__.
+
+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
+
+`Full documentation <../../driver-api/media/v4l2-dev.html#video-device-debugging>`__
+
+dev_dbg / v4l2_dbg
+------------------
+
+- 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
+-------------
+
+For general advice see the `userspace-debugging-guide <../../debugging/userspace_debugging_guide.html>`__.
+
+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
+------
+
+For general advice see the `userspace-debugging-guide <../../debugging/userspace_debugging_guide.html>`__.
+
+Trace whenever the `rkvdec_try_ctrl` function is called
+::
+
+  $ cd /sys/kernel/tracing
+  $ echo function > /sys/kernel/tracing/current_tracer
+  $ echo rkvdec_try_ctrl > set_ftrace_filter
+  $ echo 1 > tracing_on
+  $ cat trace
+   h264parse0:sink-6359    [001] ...1. 172714.547523: rkvdec_try_ctrl <-try_or_set_cluster
+   h264parse0:sink-6359    [005] ...1. 172714.567386: rkvdec_try_ctrl <-try_or_set_cluster
+
+Find out from where the calls originate
+::
+
+  $ echo 1 > options/func_stack_trace
+   h264parse0:sink-6715    [002] ..... 172837.967762: rkvdec_try_ctrl <-try_or_set_cluster
+   h264parse0:sink-6715    [002] ..... 172837.967773: <stack trace>
+   => rkvdec_try_ctrl
+   => try_or_set_cluster
+   => try_set_ext_ctrls_common
+   => try_set_ext_ctrls
+   => v4l2_s_ext_ctrls
+   => v4l_s_ext_ctrls
+   ...
+   h264parse0:sink-6715    [004] ..... 172837.985747: rkvdec_try_ctrl <-try_or_set_cluster
+   h264parse0:sink-6715    [004] ..... 172837.985750: <stack trace>
+   => rkvdec_try_ctrl
+   => try_or_set_cluster
+   => v4l2_ctrl_request_setup
+   => rkvdec_run_preamble
+   => rkvdec_h264_run
+   => rkvdec_device_run
+   ...
+
+Trace the children of a function call and show the return values (requires config `FUNCTION_GRAPH_RETVAL`)
+::
+
+  echo function_graph > current_tracer
+  echo rkvdec_h264_run > set_graph_function
+  echo 4 > max_graph_depth
+  echo do_interrupt_handler mutex_* > set_graph_notrace
+  echo 1 > options/funcgraph-retval
+   ...
+   4)               |  rkvdec_h264_run [rockchip_vdec]() {
+   4)               |    v4l2_ctrl_find [videodev]() {
+   ...
+   4)               |    rkvdec_run_preamble [rockchip_vdec]() {
+   4)   4.666 us    |      v4l2_m2m_next_buf [v4l2_mem2mem](); /* = 0xffff000005782000 */
+   ...
+   4)               |      v4l2_ctrl_request_setup [videodev]() {
+   4)   4.667 us    |        media_request_object_find [mc](); /* = 0xffff000005e3aa98 */
+   4)   1.750 us    |        find_ref [videodev](); /* = 0xffff00000833b2a0 */
+   ...
+   4)   1.750 us    |      v4l2_m2m_buf_copy_metadata [v4l2_mem2mem](); /* = 0x0 */
+   4) ! 114.333 us  |    } /* rkvdec_run_preamble [rockchip_vdec] = 0x0 */
+   4)   2.334 us    |    v4l2_h264_init_reflist_builder [v4l2_h264](); /* = 0x3e */
+   ...
+   4)               |    v4l2_h264_build_b_ref_lists [v4l2_h264]() {
+   ...
+   4)               |    rkvdec_run_postamble [rockchip_vdec]() {
+   ...
+   4) ! 444.208 us  |  } /* rkvdec_h264_run [rockchip_vdec] = 0x0 */
+
+DebugFS
+-------
+
+For general advice see the `driver-development-debugging-guide <../../debugging/driver_development_debugging_guide.html>`__.
+
+Perf & alternatives
+-------------------
+
+For general advice see the `userspace-debugging-guide <../../debugging/userspace_debugging_guide.html>`__.
+
+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
+-------------------------------
+
+For general advice see the `driver-development-debugging-guide <../../debugging/driver_development_debugging_guide.html>`__.
+
+**Copyright** |copy| 2024 : Collabora
diff --git a/Documentation/media/guides/index.rst b/Documentation/media/guides/index.rst
new file mode 100644
index 000000000000..0008966c0862
--- /dev/null
+++ b/Documentation/media/guides/index.rst
@@ -0,0 +1,11 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+============
+Media Guides
+============
+
+.. toctree::
+    :caption: Table of Contents
+    :maxdepth: 1
+
+    debugging_issues
diff --git a/Documentation/media/index.rst b/Documentation/media/index.rst
index d056a9e99dca..5461876fc401 100644
--- a/Documentation/media/index.rst
+++ b/Documentation/media/index.rst
@@ -7,6 +7,7 @@ Media Subsystem Documentation
 .. toctree::
    :maxdepth: 2
 
+   guides/index
    ../userspace-api/media/index
    ../driver-api/media/index.rst
    ../admin-guide/media/index

-- 
2.25.1




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