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Message-ID: <20241023122900.GA17733@pendragon.ideasonboard.com>
Date: Wed, 23 Oct 2024 15:29:00 +0300
From: Laurent Pinchart <laurent.pinchart@...asonboard.com>
To: Hans Verkuil <hverkuil-cisco@...all.nl>
Cc: Sebastian Fricke <sebastian.fricke@...labora.com>,
Jonathan Corbet <corbet@....net>, linux-doc@...r.kernel.org,
linux-kernel@...r.kernel.org, linux-media@...r.kernel.org,
mauro.chehab@...ux.intel.com, kernel@...labora.com,
bob.beckett@...labora.com, nicolas.dufresne@...labora.com
Subject: Re: [PATCH RFC v2 3/3] docs: media: Debugging guide for the media
subsystem
On Wed, Oct 23, 2024 at 01:43:34PM +0200, Hans Verkuil wrote:
> On 24/09/2024 10:45, Sebastian Fricke 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@...labora.com>
> > ---
> > 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
>
> I would add a few more:
>
> - 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.
To avoid unnecessary complexity in drivers, should we encourage
implementing log_status for receivers but discourage it for camera
sensors ? I haven't seen many people attempting to do so, but I have
pushed back against the debug read/write register ops in sensor drivers.
> - Run v4l2-compliance to verify the driver. 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 by
> running v4l2-compliance -m /dev/mediaX
>
> > 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
--
Regards,
Laurent Pinchart
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