Message-ID: <10f04dfd-ce2f-42c1-b926-153f419add89@collabora.com>
Date: Wed, 7 Feb 2024 17:35:26 +0100
From: Benjamin Gaignard <benjamin.gaignard@...labora.com>
To: Hans Verkuil <hverkuil@...all.nl>, mchehab@...nel.org
Cc: linux-kernel@...r.kernel.org, linux-media@...r.kernel.org,
 kernel@...labora.com
Subject: Re: [PATCH v19 0/9] Add DELETE_BUF ioctl


Le 07/02/2024 à 12:32, Hans Verkuil a écrit :
> On 07/02/2024 12:25, Benjamin Gaignard wrote:
>> Le 07/02/2024 à 11:28, Hans Verkuil a écrit :
>>> On 06/02/2024 09:58, Hans Verkuil wrote:
>>>> On 06/02/2024 09:02, Benjamin Gaignard wrote:
>>>>> Unlike when resolution change on keyframes, dynamic resolution change
>>>>> on inter frames doesn't allow to do a stream off/on sequence because
>>>>> it is need to keep all previous references alive to decode inter frames.
>>>>> This constraint have two main problems:
>>>>> - more memory consumption.
>>>>> - more buffers in use.
>>>>> To solve these issue this series introduce DELETE_BUFS ioctl and remove
>>>>> the 32 buffers limit per queue.
>>>> This v19 looks good. There are three outstanding issues that I need to take a
>>>> look at:
>>>>
>>>> 1) Can we still signal support for DELETE_BUFS in the V4L2_BUF_CAP_ caps?
>>>>      It would be nice to have, but I'm not sure if and how that can be done.
>>>>
>>>> 2) I want to take another look at providing a next version of the VIDIOC_CREATE_BUFS
>>>>      ioctl and how that would possibly influence the design of DELETE_BUFS.
>>>>      Just to make sure we're not blocking anything or making life harder.
>>> So I just tried creating a new version of the VIDIOC_CREATE_BUFS ioctl
>>> that is greatly simplified. This just updates the header, no real code yet:
>>>
>>> Signed-off-by: Hans Verkuil <hverkuil-cisco@...all.nl>
>>> ---
>>> diff --git a/include/uapi/linux/videodev2.h b/include/uapi/linux/videodev2.h
>>> index 03443833aaaa..a7398e4c85e7 100644
>>> --- a/include/uapi/linux/videodev2.h
>>> +++ b/include/uapi/linux/videodev2.h
>>> @@ -2624,6 +2624,39 @@ struct v4l2_create_buffers {
>>>        __u32            reserved[5];
>>>    };
>>>
>>> +/**
>>> + * struct v4l2_create_buffers_v2 - VIDIOC_CREATE_BUFFERS argument
>>> + * @type:    enum v4l2_buf_type
>>> + * @memory:    enum v4l2_memory; buffer memory type
>>> + * @count:    entry: number of requested buffers,
>>> + *        return: number of created buffers
>>> + * @num_planes:    requested number of planes for each buffer
>>> + * @sizes:    requested plane sizes for each buffer
>>> + * @start_index:on return, index of the first created buffer
>>> + * @total_count:on return, the total number of allocated buffers
>>> + * @capabilities: capabilities of this buffer type.
>>> + * @flags:    additional buffer management attributes (ignored unless the
>>> + *        queue has V4L2_BUF_CAP_SUPPORTS_MMAP_CACHE_HINTS capability
>>> + *        and configured for MMAP streaming I/O).
>>> + * @max_num_buffers: if V4L2_BUF_CAP_SUPPORTS_MAX_NUM_BUFFERS capability flag is set
>>> + *        this field indicate the maximum possible number of buffers
>>> + *        for this queue.
>>> + * @reserved:    future extensions
>>> + */
>>> +struct v4l2_create_buffers_v2 {
>>> +    __u32            type;
>>> +    __u32            memory;
>>> +    __u32            count;
>>> +    __u32            num_planes;
>>> +    __u32            size[VIDEO_MAX_PLANES];
>>> +    __u32            start_index;
>>> +    __u32            total_count;
>>> +    __u32            capabilities;
>>> +    __u32            flags;
>>> +    __u32            max_num_buffers;
>>> +    __u32            reserved[7];
>>> +};
>>> +
>>>    /**
>>>     * struct v4l2_delete_buffers - VIDIOC_DELETE_BUFS argument
>>>     * @index:    the first buffer to be deleted
>>> @@ -2738,6 +2771,7 @@ struct v4l2_delete_buffers {
>>>
>>>    #define VIDIOC_QUERY_EXT_CTRL    _IOWR('V', 103, struct v4l2_query_ext_ctrl)
>>>    #define VIDIOC_DELETE_BUFS    _IOWR('V', 104, struct v4l2_delete_buffers)
>>> +#define VIDIOC_CREATE_BUFFERS    _IOWR('V', 105, struct v4l2_create_buffers_v2)
>>>
>>>
>>>    /* Reminder: when adding new ioctls please add support for them to
>>>
>>>
>>> Sadly, struct v4l2_create_buffers was already used for the existing
>>> VIDIOC_CREATE_BUFS (I wish it was called v4l2_create_bufs...), so I did
>>> have to add a _v2 suffix. Compared to the old struct it just moves the
>>> type, num_planes and sizes from v4l2_format into the new struct and drops
>>> the format field. Note that those fields are the only v4l2_format fields
>>> that VIDIOC_CREATE_BUFS used, so getting rid of the format makes live
>>> much easier. I also renamed 'index' to 'start_index' and added a new 'total_count'
>>> field to report the total number of buffers.
>>>
>>> The reason for adding 'total_count' is that VIDIOC_CREATE_BUFS with
>>> count == 0 would report the total number of buffers in the 'index' field,
>>> which was rather odd. Adding a specific field for this purpose is nicer.
>>>
>>> One thing that might impact your series is the name of the ioctls.
>>>
>>> We have:
>>>
>>> VIDIOC_CREATE_BUFS/v4l2_create_buffers
>>> VIDIOC_DELETE_BUFS/v4l2_delete_buffers
>>> VIDIOC_CREATE_BUFFERS/v4l2_create_buffers_v2 (TBD)
>>>
>>> I'm wondering if VIDIOC_DELETE_BUFS should be renamed to
>>> VIDIOC_DELETE_BUFFERS, that would make it more consistent with
>>> the proposed VIDIOC_CREATE_BUFFERS.
>>>
>>> Alternatively, instead of calling it VIDIOC_CREATE_BUFFERS we
>>> might call it VIDIOC_CREATE_BUFS_V2.
>>>
>>> Or perhaps some other naming convention?
>> Maybe VIDIOC_NEW_BUFS/v4l2_new_buffers ?
>>
>> I will wait until naming convention is fixed before send v20.
> How about VIDIOC_ADD_BUFS/REMOVE_BUFS?
>
> And struct v4l2_add_bufs/v4l2_remove_bufs.

I'm really bad from naming thing like that.
I will until the next and, if nobody complain, I will change my code to
use your proposal.

Regards,
Benjamin

>
> One thing that I always found debatable about the names CREATE and DELETE
> is that it suggests that buffer memory is also created and deleted. While
> true for MMAP, that's not the case for DMABUF and USERPTR.
>
> Regards,
>
> 	Hans
>
>> Regards,
>> Benjamin
>>
>>> Ideas are welcome.
>>>
>>> Regards,
>>>
>>>      Hans
>>>

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