Re: [RFC PATCH 1/7] media: v4l2-ctrls: Add V4L2_CID_MEMORY_USAGE control

[Ming Qian(OSS)]

Hi Nicolas,

On 4/1/2026 10:23 AM, Ming Qian(OSS) wrote:
> Hi Nicolas,
>
> On 3/31/2026 10:54 PM, Nicolas Dufresne wrote:
> > Le mardi 31 mars 2026 à 10:33 -0400, Frank Li a écrit :
> > > On Tue, Mar 31, 2026 at 03:23:11PM +0800, ming.qian@xxxxxxxxxxx wrote:
> > > > From: Ming Qian <ming.qian@xxxxxxxxxxx>
> > > >
> > > > Add a new read-only control V4L2_CID_MEMORY_USAGE that allows
> > > > applications to query the total amount of memory currently used
> > > > by a device instance.
> > > >
> > > > This control reports the memory consumption in bytes, including
> > > > internal buffers, intermediate processing data, and other
> > > > driver-managed allocations. Applications can use this information
> > > > for debugging, resource monitoring, or making informed decisions
> > > > about buffer allocation strategies.
> > > >
> > > > Signed-off-by: Ming Qian <ming.qian@xxxxxxxxxxx>
> > > > ---
> > >
> > > Not sure why not export these information by debugfs, or any benefit vs
> > > debugfs?
> >
> > There is also a on-going proposal that uses fdinfo.
> >
> > Nicolas
>
> Thanks for the reminder about the ongoing fdinfo proposal.
>
> Just to confirm, you are referring to Detlev’s ongoing fdinfo proposal,
> specifically this series:
> https://lore.kernel.org/lkml/20260212162328.192217-1- 
> detlev.casanova@xxxxxxxxxxxxx/
>
> I will align my work with it and switch to using fdinfo.
> Once the show_fdinfo support from that series is merged, I will prepare
> the next revision of my patch accordingly.
>
> Regards,
> Ming

Regarding the discussion about using fdinfo instead of a V4L2 control, I
have two questions:

	1. Key consistency in fdinfo
	fdinfo uses key–value pairs, which is flexible, but if multiple
	drivers want to expose the same “memory usage” information,
	they need to agree on a common key name and meaning. Otherwise
	user‑space must handle each driver differently. A V4L2 control
	naturally provides a unified interface without this coordination
	effort.


	2. Lack of notification in fdinfo
	With a control, user‑space can subscribe to control events and
	receive notifications when the memory usage changes. fdinfo does
	not have a built‑in event mechanism, so users must either poll
	or rely on additional eventfd‑like or custom event mechanisms.

Do you have any suggestions or existing practices to address these two
issues when using fdinfo?

Thanks again for your time and comments.

Regards,
Ming

> > > Generanlly document should be first patch, then driver change.
> > >
> > > Frank
> > >
> > > >   drivers/media/v4l2-core/v4l2-ctrls-defs.c | 8 ++++++++
> > > >   include/uapi/linux/v4l2-controls.h        | 4 +++-
> > > >   2 files changed, 11 insertions(+), 1 deletion(-)
> > > >
> > > > diff --git a/drivers/media/v4l2-core/v4l2-ctrls-defs.c b/drivers/ 
> > > > media/v4l2-core/v4l2-ctrls-defs.c
> > > > index 551426c4cd01..053db78ff661 100644
> > > > --- a/drivers/media/v4l2-core/v4l2-ctrls-defs.c
> > > > +++ b/drivers/media/v4l2-core/v4l2-ctrls-defs.c
> > > > @@ -831,6 +831,7 @@ const char *v4l2_ctrl_get_name(u32 id)
> > > >       case V4L2_CID_ALPHA_COMPONENT:        return "Alpha Component";
> > > >       case V4L2_CID_COLORFX_CBCR:        return "Color Effects, CbCr";
> > > >       case V4L2_CID_COLORFX_RGB:              return "Color Effects, 
> > > > RGB";
> > > > +    case V4L2_CID_MEMORY_USAGE:        return "Memory Usage";
> > > >
> > > >       /*
> > > >        * Codec controls
> > > > @@ -1476,6 +1477,13 @@ void v4l2_ctrl_fill(u32 id, const char 
> > > > **name, enum v4l2_ctrl_type *type,
> > > >           *min = 0;
> > > >           *max = 0xffff;
> > > >           break;
> > > > +    case V4L2_CID_MEMORY_USAGE:
> > > > +        *type = V4L2_CTRL_TYPE_INTEGER64;
> > > > +        *flags |= V4L2_CTRL_FLAG_READ_ONLY;
> > > > +        *min = 0;
> > > > +        *max = S64_MAX;
> > > > +        *step = 1;
> > > > +        break;
> > > >       case V4L2_CID_FLASH_FAULT:
> > > >       case V4L2_CID_JPEG_ACTIVE_MARKER:
> > > >       case V4L2_CID_3A_LOCK:
> > > > diff --git a/include/uapi/linux/v4l2-controls.h b/include/uapi/ 
> > > > linux/v4l2-controls.h
> > > > index 68dd0c4e47b2..02c6f960d38e 100644
> > > > --- a/include/uapi/linux/v4l2-controls.h
> > > > +++ b/include/uapi/linux/v4l2-controls.h
> > > > @@ -110,8 +110,10 @@ enum v4l2_colorfx {
> > > >   #define V4L2_CID_COLORFX_CBCR            (V4L2_CID_BASE+42)
> > > >   #define V4L2_CID_COLORFX_RGB            (V4L2_CID_BASE+43)
> > > >
> > > > +#define V4L2_CID_MEMORY_USAGE            (V4L2_CID_BASE+44)
> > > > +
> > > >   /* last CID + 1 */
> > > > -#define V4L2_CID_LASTP1                         (V4L2_CID_BASE+44)
> > > > +#define V4L2_CID_LASTP1                         (V4L2_CID_BASE+45)
> > > >
> > > >   /* USER-class private control IDs */
> > > >
> > > > --
> > > > 2.53.0