Hi Sebastian, On 29.05.2024 18:22, 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. Just like you, I'm not an English native speaker myself, so sometimes I make style mistakes that only seem obvious to someone whose native tongue is English. What I do in the case of documentation patches is running the whole thing through languagetool (https://www.languagetool.org). It's LGPL and I suppose some vim versions might have a plugin that lets you interface with it from within the buffer you're editing. On my side, when running it on the previous paragraph, it showed the following warning: [ADVISE_VBG[3] premium: false] The verb ‘help’ is used with an infinitive. -> (to choose, choose) > Signed-off-by: Sebastian Fricke <sebastian.fricke@xxxxxxxxxxxxx> > --- > Documentation/media/guides/debugging_issues.rst | 477 ++++++++++++++++++++++++ > Documentation/media/guides/index.rst | 11 + > Documentation/media/index.rst | 1 + > 3 files changed, 489 insertions(+) > > diff --git a/Documentation/media/guides/debugging_issues.rst b/Documentation/media/guides/debugging_issues.rst > new file mode 100644 > index 000000000000..8fee749e7e6a > --- /dev/null > +++ b/Documentation/media/guides/debugging_issues.rst > @@ -0,0 +1,477 @@ > +.. SPDX-License-Identifier: GPL-2.0 I've noticed you've included the licence header in all your files. I also noticed though that Documentation/gpu/index.rst doesn't have it. I suppose it might've become a requiremente after that file was first submitted, but I'm not sure so I guess it's best to ask you directly. > +.. 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 > +======================== > + > +Depending on the issue, a different set of tools is available to track down the > +problem or even to realize whether there is one in the first place. > + > +As a first step you have to figure out what kind of issue you want to debug. > +Depending on the answer, your methodology and choice of tools may vary. > +Some typical issues are listed below. The list is not exhaustive. I've noticed a lot of the information in this guide isn't specific to the Media subsystem. Isn't there a chance that you might be reduplicating this information? I guess a lot of it could fit into some sort of 'general driver debugging techniques' file, although I'm not sure where that would fit into the documentation hierarchy. > +What is the issue? > +------------------ > +1. Driver doesn't behave as expected > + > + TBD > + > +2. Running out of memory > + > + In this case you should use `Linux CLI tools <linux-tools_>`_ first to verify > + whether the issue stems from the Kernel or from userspace. > + > + TBD > + > +3. Kernel panic > + > + The kernel provides a number of `tools <kernel_panic_analysis_tools_>`_ to > + analyse a Kernel panic, use these first, once you have identified where the > + error occurs you can further analyse the issue by adding `debug logs <Printk > + & friends_>`_ to the codebase. > + > +4. Device doesn't probe > + > + TBD > + > +5. Driver too slow > + > + You should start with a `performance analysis <performance>`__, to pin down the problem area. > + > +Is timing a factor? > +------------------- > + > +It is important to understand if the problem you want to debug manifests itself > +consistently (i.e. given a set of inputs you always get the same, incorrect > +output), or inconsistently. If it manifests itself inconsistently, some timing > +factor might be at play. If inserting delays into the code does change the > +behavior, then quite likely timing is a factor. > + > + In this case using a simple `printk`_ won't work, a similar alternative is > + too use `trace_printk`_, which logs the debug messages to the trace file > + instead of the kernel log. > + > +Do I have root access to the system? > +------------------------------------ > + > +Are you easily able to replace the module in question or to install a new kernel? > + > +TBD > + > +.. _live_debug: > + > +Do I need to debug on a live system? > +------------------------------------ > + > +When the kernel is compiled with `Ftrace`_, this tool might be a good start, as you can trace specific functions to dial down to the sequence of actions that causes the misbehavior. > + > +TBD > + > +Is the outcome flaky? > +--------------------- > + > +We will call the outcome "flaky" if multiple iterations of the same routine with the same parameters result in different outcomes. > + > +TBD > + > +Available tools > +=============== > + > +Printk & friends > +---------------- > + > +These are derivatives of printf() with varying destination and support for being dynamically turned on or off, or lack thereof. > + > +.. _printk: > + > +**Simple printk** > +~~~~~~~~~~~~~~~~~ > + > +The classic, this can be used to great effect for quick and dirty development > +of new modules or to extract arbitrary necessary data for troubleshooting. > + > +Prerequisite: `CONFIG_PRINTK` (usually enabled by default) > + > +**Pros**: > + > +- No need to learn anything, simple to use > +- Easy to modify exactly to your needs (formatting of the data (See: `format specifiers <../../core-api/printk-formats.html#printk-specifiers>`__), visibility in the log) > +- Can cause delays in the execution of the code (beneficial to confirm whether timing is a factor) > + > +**Cons**: > + > +- Requires rebuilding the kernel/module, (See: `live_debug`_) > +- Can cause delays in the execution of the code > + > +`Full documentation <../../core-api/printk-basics.rst>`__ > + > +.. _trace_printk: > + > +**Trace_printk** > +~~~~~~~~~~~~~~~~ > + > +Prerequisite: `CONFIG_DYNAMIC_FTRACE` & `#include <linux/ftrace.h>` > + > +A tiny bit less comfortable to use than `printk`_, because you will have to > +read the messages from the trace file (See: `Reading the ftrace log`_ instead > +of from the kernel log, but very useful when printk adds unwanted delays into > +the code execution, causing issues to be flaky or hidden.) > + > +If the processing of this still causes timing issues then you can try `trace_puts()`. > + > +`Full Documentation <../../driver-api/basics.html#c.trace_printk>`__ > + > +**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 > + > +**The dev_debug module parameter** > + > +Every video device provides a `dev_debug` parameter, which allows to get further insights into the IOCTLs in the background. > + > +`Full documentation <../../driver-api/media/v4l2-dev.html#video-device-debugging>`__ > + > +**When is it appropriate to leave a debug print in the code?** > + > +Permanent debug statements have to be useful for a developer to troubleshoot > +why the driver misbehaves. Judging that is a bit more of an art than a > +science, but some guidelines are in the `Coding style guide > +<../../process/coding-style.html#printing-kernel-messages>`__) > + > +**Custom printk** > +~~~~~~~~~~~~~~~~~ > +Example: > +:: > + > + #define core_dbg(fmt, arg...) do { \ > + if (core_debug) \ > + printk(KERN_DEBUG pr_fmt("core: " fmt), ## arg); \ > + } while (0) > + > +**When should you do this?** > + > +It is better to just use a `pr_debug()`, which can later be turned on/off with > +dynamic debug. Additionally, a lot of drivers activate these prints via a > +variable like `core_debug` set by a module parameter. However, Module > +parameters `are not recommended anymore > +<https://lkml.org/lkml/2024/3/27/163>`_. > + > +Dynamic debug > +------------- > + > +Mechanism to filter what ends up in the kernel log by dis-/en-abling log > +messages. > + > +Prerequisite: `CONFIG_DYNAMIC_DEBUG` > + > +.. _valid_dyndbg_prints: > + > +Dynamic debug is only able to target: > + > +- `pr_debug()` > +- `dev_dbg()` > +- `print_hex_dump_debug()` > +- `print_hex_dump_bytes()` > + > +Therefore the usability of this tool is quite limited in the media subsystem, > +because, as of now, there is no uniform rule for adding debug prints to the codebase, > +resulting in a variety of ways these prints are implemented. > + > +Also, note that most debug statements are implemented as a variation of > +`dprintk`, which have to be activated via a parameter in respective module, > +dynamic debug is unable to do that step for you. > + > +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" > + > +**When should you use this over** `Ftrace`_ **?** > + > +- When the code contains one of the :ref:`valid print statements <valid_dyndbg_prints_>`_ or when you have added multiple pr_debug() statements during development > +- When timing is not an issue, meaning if multiple `pr_debug()` statements in the code won't cause delays > +- When you care more about receiving specific log messages than tracing the pattern of how a function is called > + > +`Full documentation <../../admin-guide/dynamic-debug-howto.rst>`__ > + > +Ftrace > +------ > + > +Prerequisite: `CONFIG_DYNAMIC_FTRACE` > + > +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 */ > + > +**Reading the ftrace log** > +~~~~~~~~~~~~~~~~~~~~~~~~~~ > + > +The `trace` file can be read just like any other file (`cat`, `tail`, `head`, > +`vim`, etc.), the size of the file is limited by the `buffer_size_kb` (``echo > +1000 > buffer_size_kb``). The `trace_pipe` will behave similar to the `trace` > +file, but whenever you read from the file the content is consumed. > + > +**Kernelshark** > +~~~~~~~~~~~~~~~ > + > +`Kernelshark documentation <https://kernelshark.org/Documentation.html>`_ > + > +`Full documentation <../../trace/ftrace.rst>`__ > + > +DebugFS > +------- > + > +Prerequisite: `CONFIG_DEBUG_FS` & `#include <linux/debugfs.h>` > + > +DebugFS differs from the other approaches of debugging, as it doesn't write messages to the kernel log nor add traces to the code. Instead it allows the developer to handle a set of files. > +With these files you can either store values of variables or make register/memory dumps or you can make these files writable and modify values/settings in the driver. > +Possible use-cases among others: > + > +- Store register values > +- Keep track of variables > +- Store errors > +- Store settings > +- Toggle a setting like debug on/off > +- Error injection > + > +This is especially useful, when the size of a data dump would be hard to digest as > +part of the general kernel log (for example when dumping raw bitstream data) or > +when you are not interested in all the values all the time, but with the > +possibility to inspect them. > + > +The general idea is: > + > +- Create a directory during probe (`struct dentry *parent = debugfs_create_dir("my_driver", NULL);`) > +- Create a file (`debugfs_create_u32("my_value", 444, parent, &my_variable);`) > + > + - In this example the file is found in `/sys/kernel/debug/my_driver/my_value` (with read permissions for user/group/all) > + - any update of `my_variable` will update the value in the file > + > +- Clean up the folder when removing the device (`debugfs_remove_recursive(parent);`) > + > +`Full documentation <../../filesystems/debugfs.rst>`__ > + > +Perf & alternatives > +------------------- > + > +The tools mentioned above provide ways to inspect kernel code, results, variable values, etc. > +Sometimes you have to find out first where to look and for those cases a box of performance tracking tools can help you to frame the issue. > + > +.. _performance: > + > +**Why should you do a performance analysis?** > +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ > + > +A performance analysis is a good first step when among other reasons: > + > +- you cannot define the issue > +- you do not know where it occurs > +- the running system should not be interrupted or it is a remote system, where you cannot install a new module/kernel > + > +.. _linux-tools: > + > +**How to do a simple analysis with linux tools?** > +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ > + > +For the start of a performance analysis, you can start with the usual tools like: > + > +- `top` / `htop` / `atop` (*get an overview of the system load, see spikes on specific processes*) > +- `mpstat -P ALL` (*look at the load distribution among CPUs*) > +- `iostat -x` (*observe input and output devices utilization and performance*) > +- `vmstat` (*overview of memory usage on the system*) > +- `pidstat` (*similar to* `vmstat` *but per process, to dial it down to the target*) > +- `strace -tp $PID` (*once you know the process, you can figure out how it communicates with the Kernel*) > + > +These should help to figure out restrict the areas to look at sufficiently. > + > +**Diving deeper with perf** > +~~~~~~~~~~~~~~~~~~~~~~~~~~~ > + > +The **perf** tool provides a series of metrics and events to further dial down on issues. > + > +Prerequisite: TBD > + > +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. > + > +`Full documentation <https://perf.wiki.kernel.org/index.php/Main_Page>`__ > + > +**Perfetto** > +~~~~~~~~~~~~ > + > +TBD > + > +.. _error_checking: > + > +KASAN, UBSAN, lockdep and other error checkers > +---------------------------------------------- > + > +TBD > + > +.. _kernel_panic_analysis_tools: > + > +Kernel panic analysis tools > +--------------------------- > + > + To analyse the crash dump please use `Kdump` & `Kexec`. > + > + TBD > + > + `Full documentation <../../admin-guide/kdump/kdump.rst>`__ > + > + In order to find the corresponding line in the code you can use `faddr2line > + <https://elixir.bootlin.com/linux/latest/source/scripts/faddr2line>`__, note > + that you need to enable `CONFIG_DEBUG_INFO` for that to work. > + > + An alternative to using `faddr2line` is the use of `objdump` (and it's > + derivatives for the different platforms like `aarch64-linux-gnu-objdump`), > + take this line as an example: > + > + `[ +0.000240] rkvdec_device_run+0x50/0x138 [rockchip_vdec]`. > + > + We can find the corresponding line of code by executing: > + :: > + > + aarch64-linux-gnu-objdump -dS drivers/staging/media/rkvdec/rockchip-vdec.ko | grep rkvdec_device_run\>: -A 40 > + 0000000000000ac8 <rkvdec_device_run>: > + ac8: d503201f nop > + acc: d503201f nop > + { > + ad0: d503233f paciasp > + ad4: a9bd7bfd stp x29, x30, [sp, #-48]! > + ad8: 910003fd mov x29, sp > + adc: a90153f3 stp x19, x20, [sp, #16] > + ae0: a9025bf5 stp x21, x22, [sp, #32] > + const struct rkvdec_coded_fmt_desc *desc = ctx->coded_fmt_desc; > + ae4: f9411814 ldr x20, [x0, #560] > + struct rkvdec_dev *rkvdec = ctx->dev; > + ae8: f9418015 ldr x21, [x0, #768] > + if (WARN_ON(!desc)) > + aec: b4000654 cbz x20, bb4 <rkvdec_device_run+0xec> > + ret = pm_runtime_resume_and_get(rkvdec->dev); > + af0: f943d2b6 ldr x22, [x21, #1952] > + ret = __pm_runtime_resume(dev, RPM_GET_PUT); > + af4: aa0003f3 mov x19, x0 > + af8: 52800081 mov w1, #0x4 // #4 > + afc: aa1603e0 mov x0, x22 > + b00: 94000000 bl 0 <__pm_runtime_resume> > + if (ret < 0) { > + b04: 37f80340 tbnz w0, #31, b6c <rkvdec_device_run+0xa4> > + dev_warn(rkvdec->dev, "Not good\n"); > + b08: f943d2a0 ldr x0, [x21, #1952] > + b0c: 90000001 adrp x1, 0 <rkvdec_try_ctrl-0x8> > + b10: 91000021 add x1, x1, #0x0 > + b14: 94000000 bl 0 <_dev_warn> > + *bad = 1; > + b18: d2800001 mov x1, #0x0 // #0 > + ... > + > + > + To find the matching line we just have to add `0x50` (from > + `rkvdec_device_run+0x50`) to `0xac8` (from `0000000000000ac8 > + <rkvdec_device_run>:`), which yields `0xb18` corresponding with `*bad = 1`. > + > +**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 Adrian Larumbe
