Message-ID: <246ae834-3077-4e38-b419-5c8773139e65@ideasonboard.com>
Date: Mon, 6 Oct 2025 09:46:26 +0100
From: Dan Scally <dan.scally@...asonboard.com>
To: Laurent Pinchart <laurent.pinchart@...asonboard.com>
Cc: Jacopo Mondi <jacopo.mondi@...asonboard.com>,
 Dafna Hirschfeld <dafna@...tmail.com>, Keke Li <keke.li@...ogic.com>,
 Mauro Carvalho Chehab <mchehab@...nel.org>, Heiko Stuebner
 <heiko@...ech.de>, 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

On 06/10/2025 09:27, Laurent Pinchart wrote:
> On Mon, Oct 06, 2025 at 09:15:47AM +0100, Daniel Scally wrote:
>> On 05/10/2025 01:06, Laurent Pinchart wrote:
>>> 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).
>>
>> This is because the rkisp1 and c3 ISP implementations are already merged with a custom parameters
>> buffer struct defined at [1] and [2]. There the array size is set to the max size macro. This series
>> just asserts that the header part is a size match for struct v4l2_params_buffer to guarantee
>> compatibility, so throughout those drivers they can use sizeof(struct c3_isp_params_cfg) or the
>> rkisp equivalent and it's fine.
>>
>> For new users that don't have a custom struct in their uAPI and are relying on struct
>> v4l2_params_buffer we can't just do sizeof(), so the idea was for this to provide a canonical way
>> for the size calculation to be done across both the kernel and userspace.
>>
>> If we decide that it's worth keeping but want a user in this series to justify its inclusion, it
>> could be used in patches 2 and 3 - I'll reply to patch 2 in a second to indicate where.
> 
> Will it be used by userspace too for the Mali C55, or only by the kernel

I have used it in userspace too.

Thanks
Dan

> driver ?
> 
>> [1] https://elixir.bootlin.com/linux/v6.17/source/include/uapi/linux/rkisp1-config.h#L1675
>> [2] https://elixir.bootlin.com/linux/v6.17/source/include/uapi/linux/media/amlogic/c3-isp-config.h#L558>
>>
>>>> +
>>>> +/**
>>>> + * 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_ */
> 

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