media: Add fdinfo support for v4l2 drivers

fdinfo is useful to poll information from the driver.
Unlike the ftrace interface, it lets userspace tools get a snapshot of
the state of the driver at their own pace, instead of getting events for
each driver status change.

This works better to get information like HW usage time or clock
frequencies, as well as memory usage.
This patch set focuses on mem2mem drivers that are per-frame based and
an implementation is proposed for the Verisilicon Hantro and Rockchip
rkvdec drivers, limited to main clock frequency and HW usage time.

To ease support in drivers, a v4l2_stats struct is added.
It stores statistics information and provides helper functions to write
the fdinfo file in a standard way.

An example of usage of this from userspace is implemented in v4l2top:

https://github.com/cazou/v4l2top (Check the upstream branch)

checkpatch.pl warning: The 2 arguments in the show_fdinfo callback
definition are kept unnamed to match with the rest of the struct
definition.

Signed-off-by: Detlev Casanova <detlev.casanova@collabora.com>
---
Changes in v2:
- Simplify key/value pairs by removing repeated <eng> suffix
- Add media type support
- Add v4l2_stats helpers
- Add support in rkvdec too
- Link to v1: https://patch.msgid.link/20260612-v4l2-add-fdinfo-v1-0-72...

To: Mauro Carvalho Chehab <mchehab@kernel.org>
To: Nicolas Dufresne <nicolas.dufresne@collabora.com>
To: Benjamin Gaignard <benjamin.gaignard@collabora.com>
To: Philipp Zabel <p.zabel@pengutronix.de>
To: Detlev Casanova <detlev.casanova@collabora.com>
To: Ezequiel Garcia <ezequiel@vanguardiasur.com.ar>
To: Heiko Stuebner <heiko@sntech.de>
Cc: kernel@collabora.com
Cc: linux-media@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Cc: linux-rockchip@lists.infradead.org
Cc: linux-arm-kernel@lists.infradead.org

---
Christopher Healy (2):
      docs: media: add documentation for media client usage stats
      media: hantro: add per-context fdinfo usage stats

Detlev Casanova (3):
      media: v4l2: Add callback for show_fdinfo
      media: v4l2-core: Add v4l2-stats interface
      media: rkvdec: Add per-context fdinfo usage stats

 .../userspace-api/media/drivers/index.rst          |  1 +
 .../media/drivers/media-usage-stats.rst            | 85 ++++++++++++++++++++++
 drivers/media/platform/rockchip/rkvdec/rkvdec.c    | 19 +++++
 drivers/media/platform/rockchip/rkvdec/rkvdec.h    |  1 +
 drivers/media/platform/verisilicon/hantro.h        |  3 +
 drivers/media/platform/verisilicon/hantro_drv.c    | 19 +++++
 drivers/media/v4l2-core/Makefile                   |  2 +-
 drivers/media/v4l2-core/v4l2-dev.c                 | 12 +++
 drivers/media/v4l2-core/v4l2-fh.c                  |  3 +
 drivers/media/v4l2-core/v4l2-stats.c               | 65 +++++++++++++++++
 include/media/v4l2-dev.h                           |  1 +
 include/media/v4l2-fh.h                            |  2 +
 include/media/v4l2-stats.h                         | 44 +++++++++++
 13 files changed, 256 insertions(+), 1 deletion(-)
---
base-commit: 66affa37cfac0aec061cc4bcf4a065b0c52f7e19
change-id: 20260610-v4l2-add-fdinfo-a790fceab329

Best regards,
--  
Detlev Casanova <detlev.casanova@collabora.com>