Message-ID: <20251005000602.GA13055@pendragon.ideasonboard.com>
Date: Sun, 5 Oct 2025 03:06:02 +0300
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
Subject: Re: [PATCH v5 1/8] media: uapi: Introduce V4L2 generic ISP types

Hi Jacopo,

Thank you for the patch.

On Mon, Sep 15, 2025 at 07:18:10PM +0200, Jacopo Mondi wrote:
> Introduce v4l2-isp.h in the Linux kernel uAPI.
> 
> The header includes types for generic ISP configuration parameters
> and will be extended in future with support for generic ISP statistics

s/in future/in the future/

(and you can reflow the commit message)

> formats.
> 
> Generic ISP parameters support is provided by introducing two new
> types that represent an extensible and versioned buffer of ISP
> configuration parameters.
> 
> The v4l2_params_block_header structure represents the header to be
> prepend to each ISP configuration block and the v4l2_params_buffer type
> represents the base type for the configuration parameters buffer.

The second part of the sentence describes the same structure as the next
paragraph.

> The v4l2_params_buffer represents the container for the ISP
> configuration data block. The generic type is defined with a 0-sized
> data member that the ISP driver implementations shall properly size
> according to their capabilities.

This will be easier to understand if you describe v4l2_params_buffer
first.

The commit message would benefit from being rewritten.

> [Add v4l2_params_buffer_size()]
> Signed-off-by: Daniel Scally <dan.scally@...asonboard.com>
> Reviewed-by: Daniel Scally <dan.scally@...asonboard.com>
> Signed-off-by: Jacopo Mondi <jacopo.mondi@...asonboard.com>
> ---
>  MAINTAINERS                         |   6 +++
>  include/uapi/linux/media/v4l2-isp.h | 100 ++++++++++++++++++++++++++++++++++++
>  2 files changed, 106 insertions(+)
> 
> diff --git a/MAINTAINERS b/MAINTAINERS
> index ee8cb2db483f6a5e96b62b6f2edd05b1427b69f5..e82c3d0758d6033fe8fcd56ffde2c03c4319fd11 100644
> --- a/MAINTAINERS
> +++ b/MAINTAINERS
> @@ -26410,6 +26410,12 @@ F:	drivers/media/i2c/vd55g1.c
>  F:	drivers/media/i2c/vd56g3.c
>  F:	drivers/media/i2c/vgxy61.c
>  
> +V4L2 GENERIC ISP PARAMETERS AND STATISTIC FORMATS
> +M:	Jacopo Mondi <jacopo.mondi@...asonboard.com>
> +L:	linux-media@...r.kernel.org
> +S:	Maintained
> +F:	include/uapi/linux/media/v4l2-isp.h
> +
>  VF610 NAND DRIVER
>  M:	Stefan Agner <stefan@...er.ch>
>  L:	linux-mtd@...ts.infradead.org
> diff --git a/include/uapi/linux/media/v4l2-isp.h b/include/uapi/linux/media/v4l2-isp.h
> new file mode 100644
> index 0000000000000000000000000000000000000000..b838555dce2b290a14136ab09ea4d2dfdc95b26b
> --- /dev/null
> +++ b/include/uapi/linux/media/v4l2-isp.h
> @@ -0,0 +1,100 @@
> +/* SPDX-License-Identifier: GPL-2.0 WITH Linux-syscall-note */
> +/*
> + * Video4Linux2 generic ISP parameters and statistics support
> + *
> + * Copyright (C) 2025 Ideas On Board Oy
> + * Author: Jacopo Mondi <jacopo.mondi@...asonboard.com>
> + */
> +
> +#ifndef _UAPI_V4L2_ISP_H_
> +#define _UAPI_V4L2_ISP_H_
> +
> +#include <linux/stddef.h>
> +#include <linux/types.h>
> +
> +#define V4L2_PARAMS_FL_BLOCK_DISABLE	(1U << 0)
> +#define V4L2_PARAMS_FL_BLOCK_ENABLE	(1U << 1)
> +
> +/*
> + * Reserve the first 8 bits for V4L2_PARAMS_FL_* flag.
> + *
> + * Driver-specific flags should be defined as:
> + * #define PLATFORM_SPECIFIC_FLAG0     ((1U << V4L2_PARAMS_FL_DRIVER_FLAGS(0))

s/PLATFORM/DRIVER/

> + * #define PLATFORM_SPECIFIC_FLAG1     ((1U << V4L2_PARAMS_FL_DRIVER_FLAGS(1))
> + */
> +#define V4L2_PARAMS_FL_DRIVER_FLAGS(n)       ((n) + 8)
> +
> +/**
> + * struct v4l2_params_block_header - V4L2 extensible parameters block header
> + *
> + * This structure represents the common part of all the ISP configuration
> + * blocks. Each parameters block shall embed an instance of this structure type
> + * as its first member, followed by the block-specific configuration data. The
> + * driver inspects this common header to discern the block type and its size and
> + * properly handle the block content.

The last sentence is not relevant for the UAPI.

> + *
> + * The @type field is an ISP driver-specific value that identifies the block
> + * type. The @size field specifies the size of the parameters block.
> + *
> + * The @flags field is a bitmask of per-block flags V4L2_PARAMS_FL_* and
> + * driver-specific flags specified by the driver header.
> + *
> + * @type: The parameters block type (driver-specific)
> + * @flags: A bitmask of block flags (driver-specific)
> + * @size: Size (in bytes) of the parameters block, including this header

I think the fields usually go right after the structure name, followed
by the rest of the documentation.

> + */
> +struct v4l2_params_block_header {
> +	__u16 type;
> +	__u16 flags;
> +	__u32 size;
> +} __attribute__((aligned(8)));
> +
> +/**
> + * v4l2_params_buffer_size - Calculate size of v4l2_params_buffer for a platform
> + *
> + * Users of the v4l2 extensible parameters will have differing sized data arrays
> + * depending on their specific parameter buffers. Drivers and userspace will
> + * need to be able to calculate the appropriate size of the struct to
> + * accommodate all ISP configuration blocks provided by the platform.
> + * This macro provides a convenient tool for the calculation.
> + *
> + * @max_params_size: The total size of the ISP configuration blocks
> + */
> +#define v4l2_params_buffer_size(max_params_size) \
> +	(offsetof(struct v4l2_params_buffer, data) + (max_params_size))

This isn't used in this series as far as I can tell, and neither is it
used in your libcamera implementation. I'd drop the macro (as well as
the mention in the commit message).

> +
> +/**
> + * struct v4l2_params_buffer - V4L2 extensible parameters configuration
> + *
> + * This struct contains the configuration parameters of the ISP algorithms,

s/struct/structure/

> + * serialized by userspace into a data buffer. Each configuration parameter
> + * block is represented by a block-specific structure which contains a
> + * :c:type:`v4l2_params_block_header` entry as first member. Userspace populates
> + * the @data buffer with configuration parameters for the blocks that it intends
> + * to configure. As a consequence, the data buffer effective size changes
> + * according to the number of ISP blocks that userspace intends to configure and
> + * is set by userspace in the @data_size field.
> + *
> + * The parameters buffer is versioned by the @version field to allow modifying
> + * and extending its definition. Userspace shall populate the @version field to
> + * inform the driver about the version it intends to use. The driver will parse
> + * and handle the @data buffer according to the data layout specific to the
> + * indicated version and return an error if the desired version is not
> + * supported.
> + *
> + * For each ISP block that userspace wants to configure, a block-specific
> + * structure is appended to the @data buffer, one after the other without gaps
> + * in between nor overlaps. Userspace shall populate the @data_size field with

I think you can drop "nor overlaps", nobody in their right mind should
think the blocks could be overlayed :-)

> + * the effective size, in bytes, of the @data buffer.
> + *
> + * @version: The parameters buffer version (driver-specific)
> + * @data_size: The configuration data effective size, excluding this header
> + * @data: The configuration data

Move the fields up just after the structure name.

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

> + */
> +struct v4l2_params_buffer {
> +	__u32 version;
> +	__u32 data_size;
> +	__u8 data[] __counted_by(data_size);
> +};
> +
> +#endif /* _UAPI_V4L2_ISP_H_ */

-- 
Regards,

Laurent Pinchart

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