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

[Steve Cho]

Thank you Sebastian. I found this overview to be helpful.
Information about different tools are scattered around, so sometimes
not easy to find.

On Tue, Sep 24, 2024 at 1:47 AM Sebastian Fricke
<sebastian.fricke@xxxxxxxxxxxxx> wrote:
>
> 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.
Also can be used for debugging from user space as you mentioned in the
cover letter.

> +
> +.. 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
mentioning limitations or known issues for tools would be helpful.
Yes, I saw you did it for some of tools.
e.g. I find dmesg to be helpful to scan through different ioctl calls
happening,
but it doesn't necessarily provide details about detailed setups for
each ioctl call.

> +
> +`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"
As you know, dynamic debug was not consistently working for our team
for multiple reasons.
Mentioning few known issues or FAQ would be helpful.

> +
> +Ftrace
> +------
> +
> +For general advice see the `userspace-debugging-guide <../../debugging/userspace_debugging_guide.html>`__.
general advice link is here, but shouldn't we at least provide 1
sentence summary for each tool?

Also, brief explanation about the below examples would be helpful too.
Sometimes people can get lost what you are trying to highlight here.

> +
> +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
Some people might have no idea what this cmd is trying to do. :)

> +  ...
> +  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
>