linux-kernel - [PATCH v2 2/3] media: Documentation: Add note about UVCH length field

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Message-Id: <20250306-uvc-metadata-v2-2-7e939857cad5@chromium.org>
Date: Thu, 06 Mar 2025 15:51:02 +0000
From: Ricardo Ribalda <ribalda@...omium.org>
To: Mauro Carvalho Chehab <mchehab@...nel.org>, 
 Laurent Pinchart <laurent.pinchart@...asonboard.com>, 
 Hans de Goede <hdegoede@...hat.com>, 
 Guennadi Liakhovetski <guennadi.liakhovetski@...el.com>
Cc: linux-media@...r.kernel.org, linux-kernel@...r.kernel.org, 
 Ricardo Ribalda <ribalda@...omium.org>
Subject: [PATCH v2 2/3] media: Documentation: Add note about UVCH length
 field

The documentation currently describes the UVC length field as the "length
of the rest of the block", which can be misleading. The driver limits the
data copied to a maximum of 12 bytes.

This change adds a clarifying sentence to the documentation to make this
restriction explicit.

Signed-off-by: Ricardo Ribalda <ribalda@...omium.org>
---
 Documentation/userspace-api/media/v4l/metafmt-uvc.rst | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/Documentation/userspace-api/media/v4l/metafmt-uvc.rst b/Documentation/userspace-api/media/v4l/metafmt-uvc.rst
index 784346d14bbdbf28348262084d5b0646d30bd1da..42599875331c0066cf529153caccb731148023b9 100644
--- a/Documentation/userspace-api/media/v4l/metafmt-uvc.rst
+++ b/Documentation/userspace-api/media/v4l/metafmt-uvc.rst
@@ -44,7 +44,9 @@ Each individual block contains the following fields:
         them
     * - :cspan:`1` *The rest is an exact copy of the UVC payload header:*
     * - __u8 length;
-      - length of the rest of the block, including this field
+      - length of the rest of the block, including this field. Please note that
+        regardless of the this value, for V4L2_META_FMT_UVC the kernel will
+        never copy more than 2-12 bytes.
     * - __u8 flags;
       - Flags, indicating presence of other standard UVC fields
     * - __u8 buf[];

-- 
2.48.1.711.g2feabab25a-goog