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

From: Detlev Casanova

Date: Thu Apr 09 2026 - 10:35:28 EST

On 4/8/26 17:11, Nicolas Dufresne wrote:

Le jeudi 02 avril 2026 à 11:14 +0800, Ming Qian(OSS) a écrit :

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.

Added Detlev in CC. You can also refer to his work through:

https://lore.kernel.org/all/20260212162328.192217-1-detlev.casanova@xxxxxxxxxxxxx/

Nicolas

Hi Ming !

One of the reasons for using fdinfo is that it's already being used in the drm subsystem and it is working well.
Of course, in DRM, drivers don't allocate a lot of memory themselves, userspace drivers (in mesa) go through the DRM uAPI to allocate buffers, making the DRM subsystem aware of all allocated memory.
That lets DRM show memory stats in a standard way for all drm drivers. In v4l2, memory allocation is shared between userspace and the driver.
We could have drivers report memory usage through a callback and v4l2-core can add the standard field based on that.

For notifications, I don't really see a need for that, most tracing tools will use polling (I'm thinking perfetto, but also top-like tools).
We could have a max-mem-usage field if we'd want to make sure we don't miss the maximum memory usage between 2 polls.

Finally, I think v4l2 controls should only be used to control, configure and exchange data with video devices, not get stat information on what the driver is doing.

Detlev.

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