From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from perceval.ideasonboard.com (perceval.ideasonboard.com [213.167.242.64]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 5A3DE3595B; Sat, 8 Nov 2025 00:17:21 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=213.167.242.64 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1762561043; cv=none; b=usfKd/6Rf4l/JmNIuPWwKI7HKzbKYRJYZTVSlI6wSjEPV/IrRVn+HA6UZu2gO5RkAGaTdeCQK8LC/zLHf79MseLhHARDs5OJH0RtdyD9D6qzJRRYTlYvxg7ZuAXihYrdPz4kyEOdtUU3QLr2jcatvzpSIiYrmQ5xGbvFED5R8mk= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1762561043; c=relaxed/simple; bh=ojB+a2DTst1XfKFJV78Vgu0HOH/aW8mkmjJDGhjVS88=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=KRHCkcDUSxFjlr4bRhv5XvZa5ut17hA+w2gPDBp9yQYh4fcLuGkMl4mKYOzlumzVf53HlGS7hzdJfbSZPmTAs1vORozaD3Rv7Dmv55VVyATicRL2VXh5v0a4CUKt1WhiY7H0kk7nn4vnS+cPJjsbj8EnBTdxCOwD76reQbeeXew= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=ideasonboard.com; spf=pass smtp.mailfrom=ideasonboard.com; dkim=pass (1024-bit key) header.d=ideasonboard.com header.i=@ideasonboard.com header.b=dXcqXRgT; arc=none smtp.client-ip=213.167.242.64 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=ideasonboard.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=ideasonboard.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=ideasonboard.com header.i=@ideasonboard.com header.b="dXcqXRgT" Received: from pendragon.ideasonboard.com (82-203-161-95.bb.dnainternet.fi [82.203.161.95]) by perceval.ideasonboard.com (Postfix) with UTF8SMTPSA id D9948B3; Sat, 8 Nov 2025 01:15:22 +0100 (CET) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=ideasonboard.com; s=mail; t=1762560923; bh=ojB+a2DTst1XfKFJV78Vgu0HOH/aW8mkmjJDGhjVS88=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=dXcqXRgTwgvkImn2+w7CpcGZrDRAEErxLOct0yKJycod7ino1Mz20CPU13jFLfwhw SRGiVrTchZun6ttRqi/Vt5PAq46h4J/1Ic/GsfoRDzHZFtDPRkJEC9Fp9svuZAy1QJ vQ+2qf6TrdNxVIqT7nFWb1OummJIFJNfm+IJpSKA= Date: Sat, 8 Nov 2025 02:17:13 +0200 From: Laurent Pinchart To: Jacopo Mondi Cc: Dafna Hirschfeld , Keke Li , Mauro Carvalho Chehab , Heiko Stuebner , Dan Scally , Sakari Ailus , Antoine Bouyer , linux-kernel@vger.kernel.org, linux-media@vger.kernel.org, linux-rockchip@lists.infradead.org, linux-arm-kernel@lists.infradead.org, Michael Riesch Subject: Re: [PATCH v8 4/8] media: Documentation: uapi: Add V4L2 ISP documentation Message-ID: <20251108001713.GL5558@pendragon.ideasonboard.com> References: <20251020-extensible-parameters-validation-v8-0-afba4ba7b42d@ideasonboard.com> <20251020-extensible-parameters-validation-v8-4-afba4ba7b42d@ideasonboard.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <20251020-extensible-parameters-validation-v8-4-afba4ba7b42d@ideasonboard.com> Hi Jacopo, Thank you for the patch. On Mon, Oct 20, 2025 at 10:24:50AM +0200, Jacopo Mondi wrote: > Add userspace documentation for V4L2 ISP generic parameters and > statistics formats. > > Reviewed-by: Daniel Scally > Reviewed-by: Michael Riesch > Acked-by: Sakari Ailus > Signed-off-by: Jacopo Mondi > --- > .../userspace-api/media/v4l/meta-formats.rst | 1 + > Documentation/userspace-api/media/v4l/v4l2-isp.rst | 120 +++++++++++++++++++++ > MAINTAINERS | 1 + > 3 files changed, 122 insertions(+) > > diff --git a/Documentation/userspace-api/media/v4l/meta-formats.rst b/Documentation/userspace-api/media/v4l/meta-formats.rst > index d9868ee88a0717c1acaa4ee477eaed96a6411f73..7b758ea9eb4ac3c4b354bf8e2f319985ed9e2b37 100644 > --- a/Documentation/userspace-api/media/v4l/meta-formats.rst > +++ b/Documentation/userspace-api/media/v4l/meta-formats.rst > @@ -25,3 +25,4 @@ These formats are used for the :ref:`metadata` interface only. > metafmt-vivid > metafmt-vsp1-hgo > metafmt-vsp1-hgt > + v4l2-isp > diff --git a/Documentation/userspace-api/media/v4l/v4l2-isp.rst b/Documentation/userspace-api/media/v4l/v4l2-isp.rst > new file mode 100644 > index 0000000000000000000000000000000000000000..b53df722ed29117c3827314e844fc4de61343f40 > --- /dev/null > +++ b/Documentation/userspace-api/media/v4l/v4l2-isp.rst > @@ -0,0 +1,120 @@ > +.. SPDX-License-Identifier: GFDL-1.1-no-invariants-or-later > + > +.. _v4l2-isp: > + > +************************ > +Generic V4L2 ISP formats > +************************ > + > +ISP configuration and statistics: theory of operations > +====================================================== > + > +ISP configuration parameters are computed by userspace and programmed into a > +*parameters buffer* which is queued to the ISP driver on a per-frame basis. > + > +ISP statistics are collected at a specific time point and drivers use them to > +populate a *statistics buffer* which is then returned to userspace. > + > +The parameters and statistics buffers are organized in a driver-specific > +way, and their data layout differs between one driver and another. > + > +ISP drivers generally exchange parameters and statistics with userspace through > +a metadata output and capture node respectively, implementing the > +:c:type:`v4l2_meta_format` interface. Each ISP driver defines one metadata > +capture format and one metadata output format to be used on those video nodes, > +and the buffer content layout and organization is fixed by the format definition. > + > +The uAPI/ABI problem > +-------------------- > + > +By upstreaming the metadata formats that describe the parameters and statistics > +buffers layout, driver developers make them part of the Linux kernel ABI. As for > +most peripherals, ISP driver development in Linux is often an iterative process, > +in which not all of the hardware features are supported in the first version. > + > +The support for new features and/or bug fixes may land in the kernel at a later > +stage and require changes to the metadata formats definition. This is > +considered an ABI breakage that is strictly forbidden by the Linux kernel > +policies. For this reason, any change in the ISP parameters and statistics > +buffer layout would require defining a new metadata format. > + > +For these reasons Video4Linux2 has introduced support for generic ISP parameters > +and statistics data types, designed with the goal of being: > + > +- Extensible: new features can be added later on without breaking the existing > + interface > +- Versioned: different versions of the format can be defined without > + breaking the existing interface > + > +ISP configuration > +================= > + > +Before the introduction of generic formats > +------------------------------------------ > + > +Metadata output formats that describe ISP configuration parameters were > +typically realized by defining C structures that reflect the ISP registers > +layout and get populated by userspace before queueing the buffer to the ISP. > +Each C structure usually corresponds to one ISP *processing block*, with each > +block implementing one of the ISP supported features. > + > +The number of supported ISP blocks, the layout of their configuration data are > +fixed by the format definition, incurring in the above described uAPI/uABI > +problem. > + > +Generic ISP parameters > +---------------------- > + Most of the text above is a design rationale that in my opinion doesn't belong to the UAPI documentation. You already include a design rationale in the kernel documentation, in patch 8/8. If some of the text above contains a more verbose explanation, it could be moved there. I would shorten all this to ************************ Generic V4L2 ISP formats ************************ Generic ISP formats are metadata formats that define a mechanism to pass ISP parameters and statistics between userspace and drivers in V4L2 buffers. They are designed to allow extending the data in a backward-compatible way. ISP configuration ================= > +The generic ISP configuration parameters format is realized by a defining a > +single C structure that contains a header, followed by a binary buffer where > +userspace programs a variable number of ISP configuration data block, one for > +each supported ISP feature. > + > +The :c:type:`v4l2_isp_params_buffer` structure defines the parameters buffer > +header which is followed by a binary buffer of ISP configuration parameters. > +Userspace shall correctly populate the buffer header with the versioning s/versioning information/extensible parameters format version/ > +information and with the size (in bytes) of the binary data buffer where it will > +store the ISP blocks configuration. > + > +Each *ISP configuration block* is preceded by an header implemented by the > +:c:type:`v4l2_isp_params_block_header` structure, followed by the configuration > +parameters for that specific block, defined by the ISP driver specific data > +types. > + > +Userspace applications are responsible for correctly populating each block's > +header fields (type, flags and size) and the block-specific parameters. > + > +ISP Block enabling, disabling and configuration > +----------------------------------------------- > + > +When userspace wants to configure and enable an ISP block it shall fully > +populate the block configuration and set the V4L2_ISP_PARAMS_FL_BLOCK_ENABLE > +bit in the block header's `flags` field. > + > +When userspace simply wants to disable an ISP block the > +V4L2_ISP_PARAMS_FL_BLOCK_DISABLE bit should be set in block header's `flags` > +field. Drivers accept a configuration parameters block with no additional > +data after the header in this case. > + > +If the configuration of an already active ISP block has to be updated, > +userspace shall fully populate the ISP block parameters and omit setting the > +V4L2_ISP_PARAMS_FL_BLOCK_ENABLE and V4L2_ISP_PARAMS_FL_BLOCK_DISABLE bits in the > +header's `flags` field. > + > +Setting both the V4L2_ISP_PARAMS_FL_BLOCK_ENABLE and > +V4L2_ISP_PARAMS_FL_BLOCK_DISABLE bits in the flags field is not allowed and not > +accepted. s/and not accepted/and returns an error/ > + > +Any further extension to the parameters layout that happens after the ISP driver s/Any further extension/Extensions/ > +has been merged in Linux can be implemented by adding new blocks definition s/after the ISP driver has been merged in Linux // Reviewed-by: Laurent Pinchart > +without invalidating the existing ones. > + > +ISP statistics > +============== > + > +Support for generic statistics format is not yet implemented in Video4Linux2. > + > +V4L2 ISP uAPI data types > +======================== > + > +.. kernel-doc:: include/uapi/linux/media/v4l2-isp.h > diff --git a/MAINTAINERS b/MAINTAINERS > index d925745077f21e5a1388a30217a24beeb4fff3b5..f52237d57710cadff78b297d2b4610b508f55092 100644 > --- a/MAINTAINERS > +++ b/MAINTAINERS > @@ -26856,6 +26856,7 @@ V4L2 GENERIC ISP PARAMETERS AND STATISTIC FORMATS > M: Jacopo Mondi > L: linux-media@vger.kernel.org > S: Maintained > +F: Documentation/userspace-api/media/v4l/v4l2-isp.rst > F: include/uapi/linux/media/v4l2-isp.h > > VF610 NAND DRIVER -- Regards, Laurent Pinchart