Re: [PATCH 0/4] media: Track v4l2 buffers through an allocator

[Diederik de Haas]

Hi,

On Thu Sep 24, 2026 at 2:36 PM CEST, Detlev Casanova wrote:
> On Sunday, September 20, 2026 9:50:24 a.m. Eastern Daylight Time Diederik de 
> Haas wrote:
>> On Wed Sep 16, 2026 at 4:25 PM CEST, Detlev Casanova wrote:
>> > Currently, the only way to track buffers allocated by v4l2 from usespace
>> > is to use the subsystem available debug information (e.g.:
>> > /sys/kernel/debug/dma_buf/bufinfo).
>> > But that information is generic and cannot be matched to a v4l2 driver or
>> > to a userspace application: It is merely information about the allocation.
>> > 
>> > ...
>> > 
>> > Note that this depends on the ftrace support patch series[1] that
>> > provides fd/pid info in the v4l2_fh struct.
>> > That series is a bit old, so this is based on an older linux version, but
>> > that only changes things for the last 2 commits.
>> > 
>> > A v4l2top utility[2] has been made, to be used in parallel with the
>> > fdinfo patch series[3], to show the list of active streams with their HW
>> > and memory usage.
>> > 
>> > With this, debugfs looks like this when decoding a HEVC 1080p stream with
>> > 
>> > rkvdec on rk3588:
>> >   root # cat /sys/kernel/debug/v4l2/fdc38100.video-codec/mem
>> >   created-by                      fd              pid             size    
>> >          label
>> >   -----------------------------------------------------------------------
>> >   ...
>> 
>> I build a kernel with your 3 patch series and I also build the v4l2top
>> utility. Cool tool :-D
>> Especially after I found out I should build the 'upstream' branch ;-)
>> 
>> There were a couple of things I noticed:
>> 1) With 'plain' kernel 7.3-rc3 but also with all my patches included, but
>>    excluding the patches from this series, I see this:
>> ...
>> 
>> Checking v4l2 debug dir again results in this:
>> ```
>> root@nanopc-t6-plus:~# ls -lh /sys/kernel/debug/v4l2/
>> total 0
>> drwxr-xr-x 2 root root 0 Sep 20 15:13 fdb60000.rga
>> drwxr-xr-x 2 root root 0 Sep 20 15:13 fdb80000.rga
>> drwxr-xr-x 2 root root 0 Sep 20 15:13 fdc38000.video-codec
>> drwxr-xr-x 2 root root 0 Sep 20 15:13 fdee0000.hdmi_receiver
>> ```
>> 
>> I noticed my video-codec address was slightly different from yours.
>
> Yes, mine is on rk3588. A different SoC will have a different device register 
> address.

That's why I noticed/mentioned it as mine is rk3588 too. Maybe this commit?
b481c11cd20a ("arm64: dts: rockchip: Update vdec register blocks order on RK3588")

>> Playing a 1080p BBB x264 video with a patched ffmpeg+mpv, results in this:
>> ...
>> 
>> And looking at the v4l2top utility, I noticed the following:
>> 1. I see differing indications of the load while it's playing :-D
>
> \o/

Just did another quick test (for 3.) and noticed sometimes green 'lines' and
sometimes red 'lines'. Any significance to the color?
There also seems to be some kind of 'delay' when stopping a video and playing
another one (can be the same video). It looks like it's 'flatlining' until the
line of the previous video is scrolled off ...  and then it seems to 'repaint'
itself as the 'flatline' comes back alive :-P

But this is REALLY minor and not bothersome at all.

>> 2. The ``Clock rate`` is always 750.0MHz (or 786431991Hz) (expected I
>> think?)
>
> Yes, the clock won't change, for now at least. Some decoders could adapt their 
> clocks depending on the work. The frequency value will also be used to 
> determine exact core usage for those that expose a used cycle count after each 
> job (I did not implement that yet in v4l2top)
>
>> 3. The ``Total Memory`` is always 0B and the ``Memory usage
>> details`` column is always empty. That's also the case when I press one of
>> the arrow keys which seems to select the entry. Am I missing sth or doing
>> sth wrong?
>
> My guess would be that you didn't run it as root (this is usually needed to 
> access the debugfs*). But if you did, we will have to dig a little.

I did indeed not run it as root the first time around. But just did another
test and with root the memory data is displayed :-)

Cheers,
  Diederik

> *: Note that later, we'd like to report global memory usage through fdinfo, so 
> that the metric is accessible as a user, with buffer details still from the 
> debugfs (so that'd need root access)
>
> Detlev.
>
>> 
>> > [1]:
>> > https://lore.kernel.org/all/20260610-v4l2-add-ftrace-v2-0-9756edf72ac1@co
>> > llabora.com/ [2]: https://github.com/cazou/v4l2top/tree/upstream
>> > [3]:
>> > https://lore.kernel.org/all/20260706-v4l2-add-fdinfo-v3-0-d556568cf38e@co
>> > llabora.com/
>> > 
>> > Signed-off-by: Detlev Casanova <detlev.casanova@xxxxxxxxxxxxx>
>> > ---
>> > 
>> > Detlev Casanova (4):
>> >       media: Add a v4l2 memory allocations tracker
>> >       media: Store v4l2_fh in the vb_queue
>> >       media: verisilicon: Switch to tracked dma allocations
>> >       media: rkvdec: Switch to tracked dma allocations
>> >  
>> >  drivers/media/common/videobuf2/Makefile            |   1 +
>> >  drivers/media/common/videobuf2/v4l2-allocator.c    | 186
>> >  +++++++++++++++++++++ .../media/common/videobuf2/videobuf2-dma-contig.c 
>> >  |  27 ++-
>> >  .../media/platform/rockchip/rkvdec/rkvdec-h264.c   |  14 +-
>> >  .../media/platform/rockchip/rkvdec/rkvdec-hevc.c   |  14 +-
>> >  .../media/platform/rockchip/rkvdec/rkvdec-rcb.c    |  21 ++-
>> >  .../platform/rockchip/rkvdec/rkvdec-vdpu381-h264.c |  14 +-
>> >  .../platform/rockchip/rkvdec/rkvdec-vdpu381-hevc.c |  14 +-
>> >  .../platform/rockchip/rkvdec/rkvdec-vdpu383-h264.c |  14 +-
>> >  .../platform/rockchip/rkvdec/rkvdec-vdpu383-hevc.c |  14 +-
>> >  .../media/platform/rockchip/rkvdec/rkvdec-vp9.c    |  33 ++--
>> >  drivers/media/platform/rockchip/rkvdec/rkvdec.c    |   4 +
>> >  drivers/media/platform/verisilicon/hantro.h        |   1 +
>> >  drivers/media/platform/verisilicon/hantro_drv.c    |   4 +
>> >  drivers/media/platform/verisilicon/hantro_h264.c   |   7 +-
>> >  drivers/media/platform/verisilicon/hantro_hevc.c   | 100 ++++++-----
>> >  drivers/media/platform/verisilicon/hantro_mpeg2.c  |  16 +-
>> >  .../media/platform/verisilicon/hantro_postproc.c   |  14 +-
>> >  drivers/media/platform/verisilicon/hantro_vp8.c    |  25 +--
>> >  drivers/media/platform/verisilicon/hantro_vp9.c    |  34 +++-
>> >  .../verisilicon/rockchip_vpu981_hw_av1_dec.c       | 160
>> >  +++++++++++-------
>> >  drivers/media/v4l2-core/v4l2-device.c              |   4 +-
>> >  include/media/v4l2-allocator.h                     |  26 +++
>> >  include/media/v4l2-device.h                        |   2 +
>> >  include/media/videobuf2-core.h                     |   3 +
>> >  25 files changed, 555 insertions(+), 197 deletions(-)
>> > 
>> > ---
>> > base-commit: 66affa37cfac0aec061cc4bcf4a065b0c52f7e19
>> > change-id: 20260612-v4l2-add-mem-tracker-da0088c74a64
>> > prerequisite-change-id: 20260608-v4l2-add-ftrace-aec6e7f60a6c:v2
>> > prerequisite-patch-id: bd45b4df66799a7f9f22b49973a78d1bd3590a4a
>> > prerequisite-patch-id: 6a5ed5615f08257cf854d441785066b5ff4d5d47
>> > prerequisite-patch-id: 02594c869498b9e91e416d70242afbaa6b3a8d6a
>> > prerequisite-patch-id: 9f25ec78f3b99ac1c5d42eb732f5bd9eae73fadd
>> > prerequisite-patch-id: 4ed2c2e8c3abfa7f6328d0ea4f488ca5a3eec8fb
>> > prerequisite-patch-id: 8787d50eb80e1a63a57a123268cade23c9e64618
>> > prerequisite-patch-id: bfe85822378fa771f9bbc3e1e8c8ff6bce0eb7fd
>> > prerequisite-patch-id: 784d0801da320f7b41194f4632ee422c32f1b5b8
>> > prerequisite-patch-id: 362c8366e2efbfb5e6f1a3f71cfddbd46e8d1506
>> > 
>> > Best regards,
>> > --
>> > Detlev Casanova <detlev.casanova@xxxxxxxxxxxxx>
>> > 
>> > 
>> > _______________________________________________
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