Hi Sebastian, > On 24 Sep 2024, at 05:45, 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. > + > +.. 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 */ Maybe have a look at retsnoop? To me, a very frustrating and very common issue is having to figure out where exactly an error code was generated. Ftrace helps a great deal, but I feel that retsnoop just takes it a step further. On top of that, you can retrace the execution on a statement level. That is, with the right options, it can tell you exactly what statements executed, which then lets you get very precise without any extra prints. See [0]. There’s a talk on KR2024 about it too [1]. [0]: https://github.com/anakryiko/retsnoop [1]: https://www.youtube.com/watch?v=NvTBrx6EdF8 — Daniel