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SPDX-License-Identifier: GPL-2.0 2 3============================================ 4Debugging and tracing in the media subsystem 5============================================ 6 7This document serves as a starting point and lookup for debugging device 8drivers in the media subsystem and to debug these drivers from userspace. 9 10.. contents:: 11 :depth: 3 12 13General debugging advice 14------------------------ 15 16For general advice see the :doc:`general advice document 17</process/debugging/index>`. 18 19The following sections show you some of the available tools. 20 21dev_debug module parameter 22-------------------------- 23 24Every video device provides a ``dev_debug`` parameter, which allows to get 25further insights into the IOCTLs in the background.:: 26 27 # cat /sys/class/video4linux/video3/name 28 rkvdec 29 # echo 0xff > /sys/class/video4linux/video3/dev_debug 30 # dmesg -wH 31 [...] videodev: v4l2_open: video3: open (0) 32 [ +0.000036] video3: VIDIOC_QUERYCAP: driver=rkvdec, card=rkvdec, 33 bus=platform:rkvdec, version=0x00060900, capabilities=0x84204000, 34 device_caps=0x04204000 35 36For the full documentation see :ref:`driver-api/media/v4l2-dev:video device 37debugging` 38 39dev_dbg() / v4l2_dbg() 40---------------------- 41 42Two debug print statements, which are specific for devices and for the v4l2 43subsystem, avoid adding these to your final submission unless they have 44long-term value for investigations. 45 46For a general overview please see the 47:ref:`process/debugging/driver_development_debugging_guide:printk() & friends` 48guide. 49 50- Difference between both? 51 52 - v4l2_dbg() utilizes v4l2_printk() under the hood, which further uses 53 printk() directly, thus it cannot be targeted by dynamic debug 54 - dev_dbg() can be targeted by dynamic debug 55 - v4l2_dbg() has a more specific prefix format for the media subsystem, while 56 dev_dbg only highlights the driver name and the location of the log 57 58Dynamic debug 59------------- 60 61A method to trim down the debug output to your needs. 62 63For general advice see the 64:ref:`process/debugging/userspace_debugging_guide:dynamic debug` guide. 65 66Here is one example, that enables all available pr_debug()'s within the file:: 67 68 $ alias ddcmd='echo $* > /proc/dynamic_debug/control' 69 $ ddcmd '-p; file v4l2-h264.c +p' 70 $ grep =p /proc/dynamic_debug/control 71 drivers/media/v4l2-core/v4l2-h264.c:372 [v4l2_h264]print_ref_list_b =p 72 "ref_pic_list_b%u (cur_poc %u%c) %s" 73 drivers/media/v4l2-core/v4l2-h264.c:333 [v4l2_h264]print_ref_list_p =p 74 "ref_pic_list_p (cur_poc %u%c) %s\n" 75 76Ftrace 77------ 78 79An internal kernel tracer that can trace static predefined events, function 80calls, etc. Very useful for debugging problems without changing the kernel and 81understanding the behavior of subsystems. 82 83For general advice see the 84:ref:`process/debugging/userspace_debugging_guide:ftrace` guide. 85 86DebugFS 87------- 88 89This tool allows you to dump or modify internal values of your driver to files 90in a custom filesystem. 91 92For general advice see the 93:ref:`process/debugging/driver_development_debugging_guide:debugfs` guide. 94 95Perf & alternatives 96------------------- 97 98Tools to measure the various stats on a running system to diagnose issues. 99 100For general advice see the 101:ref:`process/debugging/userspace_debugging_guide:perf & alternatives` guide. 102 103Example for media devices: 104 105Gather statistics data for a decoding job: (This example is on a RK3399 SoC 106with the rkvdec codec driver using the `fluster test suite 107<https://github.com/fluendo/fluster>`__):: 108 109 perf stat -d python3 fluster.py run -d GStreamer-H.264-V4L2SL-Gst1.0 -ts 110 JVT-AVC_V1 -tv AUD_MW_E -j1 111 ... 112 Performance counter stats for 'python3 fluster.py run -d 113 GStreamer-H.264-V4L2SL-Gst1.0 -ts JVT-AVC_V1 -tv AUD_MW_E -j1 -v': 114 115 7794.23 msec task-clock:u # 0.697 CPUs utilized 116 0 context-switches:u # 0.000 /sec 117 0 cpu-migrations:u # 0.000 /sec 118 11901 page-faults:u # 1.527 K/sec 119 882671556 cycles:u # 0.113 GHz (95.79%) 120 711708695 instructions:u # 0.81 insn per cycle (95.79%) 121 10581935 branches:u # 1.358 M/sec (15.13%) 122 6871144 branch-misses:u # 64.93% of all branches (95.79%) 123 281716547 L1-dcache-loads:u # 36.144 M/sec (95.79%) 124 9019581 L1-dcache-load-misses:u # 3.20% of all L1-dcache accesses (95.79%) 125 <not supported> LLC-loads:u 126 <not supported> LLC-load-misses:u 127 128 11.180830431 seconds time elapsed 129 130 1.502318000 seconds user 131 6.377221000 seconds sys 132 133The availability of events and metrics depends on the system you are running. 134 135Error checking & panic analysis 136------------------------------- 137 138Various Kernel configuration options to enhance error detection of the Linux 139Kernel with the cost of lowering performance. 140 141For general advice see the 142:ref:`process/debugging/driver_development_debugging_guide:kasan, ubsan, 143lockdep and other error checkers` guide. 144 145Driver verification with v4l2-compliance 146---------------------------------------- 147 148To verify, that a driver adheres to the v4l2 API, the tool v4l2-compliance is 149used, which is part of the `v4l_utils 150<https://git.linuxtv.org/v4l-utils.git>`__, a suite of userspace tools to work 151with the media subsystem. 152 153To see the detailed media topology (and check it) use:: 154 155 v4l2-compliance -M /dev/mediaX --verbose 156 157You can also run a full compliance check for all devices referenced in the 158media topology with:: 159 160 v4l2-compliance -m /dev/mediaX 161 162Debugging problems with receiving video 163--------------------------------------- 164 165Implementing vidioc_log_status in the driver: this can log the current status 166to the kernel log. It's called by v4l2-ctl --log-status. Very useful for 167debugging problems with receiving video (TV/S-Video/HDMI/etc) since the video 168signal is external (so unpredictable). Less useful with camera sensor inputs 169since you have control over what the camera sensor does. 170 171Usually you can just assign the default:: 172 173 .vidioc_log_status = v4l2_ctrl_log_status, 174 175But you can also create your own callback, to create a custom status log. 176 177You can find an example in the cobalt driver 178(`drivers/media/pci/cobalt/cobalt-v4l2.c <https://elixir.bootlin.com/linux/v6.11.6/source/drivers/media/pci/cobalt/cobalt-v4l2.c#L567>`__). 179 180**Copyright** ©2024 : Collabora 181