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Message-Id: <20240529-b4-media_docs_improve-v2-3-66318b2da726@collabora.com>
Date: Tue, 24 Sep 2024 10:45:59 +0200
From: Sebastian Fricke <sebastian.fricke@...labora.com>
To: Jonathan Corbet <corbet@....net>
Cc: linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
linux-media@...r.kernel.org, laurent.pinchart@...asonboard.com,
hverkuil-cisco@...all.nl, mauro.chehab@...ux.intel.com, kernel@...labora.com,
bob.beckett@...labora.com, nicolas.dufresne@...labora.com,
Sebastian Fricke <sebastian.fricke@...labora.com>
Subject:
[PATCH RFC v2 3/3] docs: media: Debugging guide for the media subsystem
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
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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