linux-kernel - Re: [PATCH v8 8/8] media: Documentation: kapi: Add v4l2 generic ISP support

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Message-ID: <20251107233415.GK5558@pendragon.ideasonboard.com>
Date: Sat, 8 Nov 2025 01:34:15 +0200
From: Laurent Pinchart <laurent.pinchart@...asonboard.com>
To: Jacopo Mondi <jacopo.mondi@...asonboard.com>
Cc: Dafna Hirschfeld <dafna@...tmail.com>, Keke Li <keke.li@...ogic.com>,
	Mauro Carvalho Chehab <mchehab@...nel.org>,
	Heiko Stuebner <heiko@...ech.de>,
	Dan Scally <dan.scally@...asonboard.com>,
	Sakari Ailus <sakari.ailus@...ux.intel.com>,
	Antoine Bouyer <antoine.bouyer@....com>,
	linux-kernel@...r.kernel.org, linux-media@...r.kernel.org,
	linux-rockchip@...ts.infradead.org,
	linux-arm-kernel@...ts.infradead.org,
	Michael Riesch <michael.riesch@...labora.com>
Subject: Re: [PATCH v8 8/8] media: Documentation: kapi: Add v4l2 generic ISP
 support

Hi Jacopo,

Thank you for the patch.

On Mon, Oct 20, 2025 at 10:24:54AM +0200, Jacopo Mondi wrote:
> Add to the driver-api documentation the v4l2-isp.h types and
> helpers documentation.
> 
> Reviewed-by: Daniel Scally <dan.scally@...asonboard.com>
> Reviewed-by: Michael Riesch <michael.riesch@...labora.com>
> Acked-by: Sakari Ailus <sakari.ailus@...ux.intel.com>
> Signed-off-by: Jacopo Mondi <jacopo.mondi@...asonboard.com>
> ---
>  Documentation/driver-api/media/v4l2-core.rst |  1 +
>  Documentation/driver-api/media/v4l2-isp.rst  | 49 ++++++++++++++++++++++++++++
>  MAINTAINERS                                  |  1 +
>  3 files changed, 51 insertions(+)
> 
> diff --git a/Documentation/driver-api/media/v4l2-core.rst b/Documentation/driver-api/media/v4l2-core.rst
> index ad987c34ad2a8460bb95e97adc4d850d624e0b81..a5f5102c64cca57b57b54ab95882b26286fb27de 100644
> --- a/Documentation/driver-api/media/v4l2-core.rst
> +++ b/Documentation/driver-api/media/v4l2-core.rst
> @@ -27,3 +27,4 @@ Video4Linux devices
>      v4l2-common
>      v4l2-tveeprom
>      v4l2-jpeg
> +    v4l2-isp
> diff --git a/Documentation/driver-api/media/v4l2-isp.rst b/Documentation/driver-api/media/v4l2-isp.rst
> new file mode 100644
> index 0000000000000000000000000000000000000000..150ba39b257b23e6a8ca1a348047f5b55588fbf7
> --- /dev/null
> +++ b/Documentation/driver-api/media/v4l2-isp.rst
> @@ -0,0 +1,49 @@
> +.. SPDX-License-Identifier: GPL-2.0
> +
> +V4L2 generic ISP parameters and statistics support
> +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
> +
> +Design rationale
> +================
> +
> +ISP configuration parameters and statistics are processed and collected by
> +drivers and exchanged with userspace through data types that usually
> +reflect the ISP peripheral registers layout.
> +
> +Each ISP driver defines its own metadata output format for parameters and
> +a metadata capture format for statistics. The buffer layout is realized by a
> +set of C structures that reflects the registers layout. The number and types
> +of C structures is fixed by the format definition and becomes part of the Linux
> +kernel uAPI/uABI interface.
> +
> +Because of the hard requirement of backward compatibility when extending the
> +user API/ABI interface, modifying an ISP driver capture or output metadata
> +format after it has been accepted by mainline is very hard if not impossible.
> +
> +It generally happens, in fact, that after the first accepted revision of an ISP
> +driver the buffers layout need to be modified, either to support new hardware
> +blocks, to fix bugs or to support different revisions of the hardware.
> +
> +Each of these situations would require defining a new metadata format, making it
> +really hard to maintain and extend drivers and requiring userspace to use
> +the correct format depending on the kernel revision in use.
> +
> +V4L2 ISP configuration parameters
> +=================================
> +
> +For these reasons, Video4Linux2 defines generic types for ISP configuration
> +parameters and statistics. Drivers are still expected to define their own
> +formats for their metadata output and capture nodes, but the buffers layout can
> +be defined using the extensible and versioned types defined by
> +include/uapi/linux/media/v4l2-isp.h.
> +
> +Drivers are expected to provide the definitions of their supported ISP blocks,
> +the control flags and the expected maximum size of a buffer.

What are the control flags here ? Is it a leftover from a previous
version ?

With this addressed,

Reviewed-by: Laurent Pinchart <laurent.pinchart+renesas@...asonboard.com>

> +
> +For driver developers a set of helper functions to assist them with validation
> +of the buffer received from userspace is available in
> +drivers/media/v4l2-core/v4l2-isp.c
> +
> +V4L2 ISP support driver documentation
> +=====================================
> +.. kernel-doc:: include/media/v4l2-isp.h
> diff --git a/MAINTAINERS b/MAINTAINERS
> index 5833f82caa7f2f734bb0e1be144ade2109b23988..cd1137c7754538d02bd72521fec6c89e082246d2 100644
> --- a/MAINTAINERS
> +++ b/MAINTAINERS
> @@ -26856,6 +26856,7 @@ V4L2 GENERIC ISP PARAMETERS AND STATISTIC FORMATS
>  M:	Jacopo Mondi <jacopo.mondi@...asonboard.com>
>  L:	linux-media@...r.kernel.org
>  S:	Maintained
> +F:	Documentation/driver-api/media/v4l2-isp.rst
>  F:	Documentation/userspace-api/media/v4l/v4l2-isp.rst
>  F:	drivers/media/v4l2-core/v4l2-isp.c
>  F:	include/media/v4l2-isp.h

-- 
Regards,

Laurent Pinchart