linux-kernel - [PATCH v6 2/4] media: Documentation: Add note about UVCH length field

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Message-Id: <20250604-uvc-meta-v6-2-7141d48c322c@chromium.org>
Date: Wed, 04 Jun 2025 12:16:03 +0000
From: Ricardo Ribalda <ribalda@...omium.org>
To: Laurent Pinchart <laurent.pinchart@...asonboard.com>, 
 Hans de Goede <hdegoede@...hat.com>, 
 Mauro Carvalho Chehab <mchehab@...nel.org>, 
 Guennadi Liakhovetski <guennadi.liakhovetski@...el.com>, 
 Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc: linux-media@...r.kernel.org, linux-kernel@...r.kernel.org, 
 linux-usb@...r.kernel.org, Ricardo Ribalda <ribalda@...omium.org>, 
 Hans de Goede <hansg@...nel.org>
Subject: [PATCH v6 2/4] 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.

Reviewed-by: Hans de Goede <hansg@...nel.org>
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..4c05e9e54683a2bf844ddf26f99d0d9713ef05de 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 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[];

-- 
2.50.0.rc0.604.gd4ff7b7c86-goog