Message-ID: <badf4971-0916-45f9-952c-09963c6cb19a@ideasonboard.com>
Date: Mon, 6 Oct 2025 09:15:47 +0100
From: Dan Scally <dan.scally@...asonboard.com>
To: Laurent Pinchart <laurent.pinchart@...asonboard.com>,
 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>, 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

Morning Laurent, Jacopo

On 05/10/2025 01:06, Laurent Pinchart wrote:
> 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).

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.

Thanks
Dan

[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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