[RFC PATCH 2/7] docs: media: v4l2-ctrls: Add V4L2_CID_MEMORY_USAGE control

[ming.qian@oss.nxp.com]

From: Ming Qian <ming.qian@oss.nxp.com>

Add documentation for the V4L2_CID_MEMORY_USAGE control in the
userspace API documentation. This read-only control allows applications
to query the total amount of memory currently used by a device instance.

The documentation explains:
- The control reports memory usage in bytes for driver-managed allocations
- It includes internal buffers, intermediate processing data, and other
  driver allocations associated with the instance
- Applications can use it for debugging, resource monitoring, and making
  informed buffer allocation decisions
- The value may change dynamically during device operation
- It only reports driver-managed memory, not application-allocated buffers

Update V4L2_CID_LASTP1 definition to reflect the new control as the
last predefined control ID.

Signed-off-by: Ming Qian <ming.qian@oss.nxp.com>
---
 .../userspace-api/media/v4l/control.rst       | 22 ++++++++++++++++++-
 1 file changed, 21 insertions(+), 1 deletion(-)

diff --git a/Documentation/userspace-api/media/v4l/control.rst b/Documentation/userspace-api/media/v4l/control.rst
index 19372bb32c4b..6904c5bdbf37 100644
--- a/Documentation/userspace-api/media/v4l/control.rst
+++ b/Documentation/userspace-api/media/v4l/control.rst
@@ -316,9 +316,29 @@ Control IDs
     set the alpha component value of all pixels for further processing
     in the device.
 
+``V4L2_CID_MEMORY_USAGE`` ``(integer64)``
+    This is an optional read-only control that reports the total amount of
+    memory, in bytes, currently allocated and used by this device instance.
+    This includes memory used for internal buffers, intermediate processing
+    data, and any other driver-managed allocations associated with the
+    instance.
+
+    Applications can use this control to monitor memory consumption for
+    debugging purposes, resource management, or to make informed decisions
+    about buffer allocation strategies. The value may change dynamically
+    as the device operates, for example when buffers are allocated or
+    released during streaming.
+
+    Note that this control only reports memory managed by the driver for
+    this specific instance; it does not include memory allocated by the
+    application itself (e.g., userspace buffers passed to the driver).
+
+    Not all drivers support this control. Applications should query for
+    its availability before use.
+
 ``V4L2_CID_LASTP1``
     End of the predefined control IDs (currently
-    ``V4L2_CID_ALPHA_COMPONENT`` + 1).
+    ``V4L2_CID_MEMORY_USAGE`` + 1).
 
 ``V4L2_CID_PRIVATE_BASE``
     ID of the first custom (driver specific) control. Applications
-- 
2.53.0