Re: [PATCH v2 2/5] docs: media: add documentation for media client usage stats

[Mauro Carvalho Chehab <mchehab+huawei@kernel.org>]

On Wed, Jun 17, 2026 at 02:10:57PM -0400, Detlev Casanova wrote:
> From: Christopher Healy <healych@amazon.com>
> 
> Document the media fdinfo interface for per-file-descriptor usage
> statistics exposed by stateless V4L2 codec drivers via
> /proc/<pid>/fdinfo/<fd>.
> 
> This interface is designed for stateless (request API based) codec
> devices where the kernel driver has per-job visibility into hardware
> execution. Stateful codecs cannot support all of this because their
> firmware manages job scheduling opaquely.
> 
> The specification defines media- prefixed keys for engine utilization
> time, and operating frequency, following the same conventions as the DRM
> fdinfo mechanism documented in drm-usage-stats.rst.
> 
> More fields can be added later.
> 
> Signed-off-by: Christopher Healy <healych@amazon.com>
> Signed-off-by: Detlev Casanova <detlev.casanova@collabora.com>
> ---
>  .../userspace-api/media/drivers/index.rst          |  1 +
>  .../media/drivers/media-usage-stats.rst            | 85 ++++++++++++++++++++++

Please notice that sysfs/procfs APIs are usually documented at
Documentation/ABI/, using a machine-readable format[1]. Still, we do want
it also at media uAPI. So, please add it there as well on your next spin.

[1] As those are new, Documentation/ABI/testing/ is probably the best place.

>  2 files changed, 86 insertions(+)
> 
> diff --git a/Documentation/userspace-api/media/drivers/index.rst b/Documentation/userspace-api/media/drivers/index.rst
> index 02967c9b18d6..61879738836c 100644
> --- a/Documentation/userspace-api/media/drivers/index.rst
> +++ b/Documentation/userspace-api/media/drivers/index.rst
> @@ -34,6 +34,7 @@ For more details see the file COPYING in the source distribution of Linux.
>  	imx-uapi
>  	mali-c55
>  	max2175
> +	media-usage-stats
>  	npcm-video
>  	omap3isp-uapi
>  	thp7312
> diff --git a/Documentation/userspace-api/media/drivers/media-usage-stats.rst b/Documentation/userspace-api/media/drivers/media-usage-stats.rst
> new file mode 100644
> index 000000000000..d3dc07002f62
> --- /dev/null
> +++ b/Documentation/userspace-api/media/drivers/media-usage-stats.rst
> @@ -0,0 +1,85 @@
> +.. SPDX-License-Identifier: GPL-2.0
> +
> +.. _media-usage-stats:
> +
> +==========================
> +Media client usage stats
> +==========================
> +
> +Stateless V4L2 codec drivers can optionally expose per-file-descriptor usage
> +statistics via ``/proc/<pid>/fdinfo/<fd>``. This is analogous to the DRM fdinfo
> +mechanism documented in :ref:`drm-client-usage-stats`, but uses the ``media-``
> +key prefix for V4L2 media devices.
> +
> +This interface is specific to stateless (request API based) codec devices,
> +including both decoders and encoders. With stateless codecs, the kernel driver
> +explicitly submits each frame to the hardware and receives a completion
> +interrupt, providing a clean per-job boundary that can be attributed to the
> +submitting file descriptor.
> +
> +Stateful codec devices cannot support this interface because their firmware
> +manages job scheduling internally. The kernel driver submits bitstream data
> +but has no visibility into per-frame hardware execution timing.
> +
> +Implementation
> +==============
> +
> +The V4L2 core provides the plumbing: drivers implement the ``show_fdinfo``
> +callback in ``struct v4l2_file_operations``, and the core wires it into the
> +kernel ``struct file_operations`` so that ``/proc/<pid>/fdinfo/<fd>`` output
> +includes the driver-provided keys.
> +
> +File format specification
> +=========================
> +
> +- File shall contain one key value pair per one line of text.
> +- Colon character (``:``) must be used to delimit keys and values.
> +- All standardised keys shall be prefixed with ``media-``.
> +- Driver-specific keys shall be prefixed with ``driver_name-``.
> +
> +Mandatory keys
> +--------------
> +
> +- media-driver: <valstr>
> +
> +  String shall contain the name of the media driver.
> +
> +- media-type: <valstr>
> +
> +  String shall identify the type of media engine exposed through this file
> +  descriptor. Standard values are ``decoder`` and ``encoder``.
> +
> +Utilization keys
> +----------------
> +
> +- media-engine-usage: <uint> ns
> +
> +  Time in nanoseconds that the hardware engine spent busy processing work
> +  belonging to this file descriptor. The engine being measured is identified
> +  by the ``media-type`` key.
> +
> +  Values are not required to be constantly monotonic if it makes the driver
> +  implementation easier, but are required to catch up with the previously
> +  reported larger value within a reasonable period.
> +
> +Frequency keys
> +--------------
> +
> +- media-maxfreq: <uint> Hz
> +
> +  Maximum operating frequency of the main engine clock.
> +
> +- media-curfreq: <uint> Hz
> +
> +  Current operating frequency of the main engine clock.
> +
> +Example output
> +==============
> +
> +::
> +
> +  media-driver:           hantro-vpu
> +  media-type:             decoder
> +  media-engine-usage:     123456789 ns
> +  media-maxfreq:          600000000 Hz
> +  media-curfreq:          600000000 Hz
> 
> -- 
> 2.54.0
> 

-- 
Thanks,
Mauro