linux-kernel - Re: [PATCH v5 01/10] media: videobuf2: Rework offset 'cookie' encoding pattern

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Message-ID: <01c299f2-8118-5d86-e9b6-a459c1b6c467@xs4all.nl>
Date:   Wed, 30 Aug 2023 14:25:09 +0200
From:   Hans Verkuil <hverkuil-cisco@...all.nl>
To:     Benjamin Gaignard <benjamin.gaignard@...labora.com>,
        mchehab@...nel.org, tfiga@...omium.org, m.szyprowski@...sung.com,
        ming.qian@....com, ezequiel@...guardiasur.com.ar,
        p.zabel@...gutronix.de, gregkh@...uxfoundation.org,
        nicolas.dufresne@...labora.com
Cc:     linux-media@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org,
        linux-mediatek@...ts.infradead.org, linux-arm-msm@...r.kernel.org,
        linux-rockchip@...ts.infradead.org, linux-staging@...ts.linux.dev,
        kernel@...labora.com
Subject: Re: [PATCH v5 01/10] media: videobuf2: Rework offset 'cookie'
 encoding pattern

On 24/08/2023 11:21, Benjamin Gaignard wrote:
> Change how offset 'cookie' field value is computed to make possible
> to use more buffers (up to 0xffff).
> With this encoding pattern we know the maximum number that a queue
> could store so we can check ing at queue init time.
> It also make easier and faster to find buffer and plane from using
> the offset field.
> 
> Signed-off-by: Benjamin Gaignard <benjamin.gaignard@...labora.com>
> ---
> v5:
> - I haven't change DST_QUEUE_OFF_BASE definition because it used in
>   v4l2-mem2mem and s5p_mfc driver with a shift.
> 
>  .../media/common/videobuf2/videobuf2-core.c   | 48 +++++++++----------
>  1 file changed, 24 insertions(+), 24 deletions(-)
> 
> diff --git a/drivers/media/common/videobuf2/videobuf2-core.c b/drivers/media/common/videobuf2/videobuf2-core.c
> index cf6727d9c81f..e06905533ef4 100644
> --- a/drivers/media/common/videobuf2/videobuf2-core.c
> +++ b/drivers/media/common/videobuf2/videobuf2-core.c
> @@ -31,6 +31,10 @@
>  
>  #include <trace/events/vb2.h>
>  
> +#define PLANE_INDEX_SHIFT	3
> +#define PLANE_INDEX_MASK	0x7
> +#define MAX_BUFFERS		0xffff

Very poor name, see below.

> +
>  static int debug;
>  module_param(debug, int, 0644);
>  
> @@ -358,21 +362,23 @@ static void __setup_offsets(struct vb2_buffer *vb)
>  	unsigned int plane;
>  	unsigned long off = 0;
>  
> -	if (vb->index) {
> -		struct vb2_buffer *prev = q->bufs[vb->index - 1];
> -		struct vb2_plane *p = &prev->planes[prev->num_planes - 1];
> -
> -		off = PAGE_ALIGN(p->m.offset + p->length);
> -	}
> +	/*
> +	 * Offsets cookies value have the following constraints:
> +	 * - a buffer could have up to 8 planes.
> +	 * - v4l2 mem2mem use bit 30 to distinguish between source and destination buffers.
> +	 * - must be page aligned
> +	 * That led to this bit mapping:
> +	 * |30                |29        15|14       12|11 0|
> +	 * |DST_QUEUE_OFF_BASE|buffer index|plane index| 0  |
> +	 * where there is 16 bits to store buffer index.

16 -> 15: there are 15 (not 16!) bits available for buffer indices. So the maximum
number of buffers is 32768, given that the indices start at 0.

> +	 */
> +	off = vb->index << (PLANE_INDEX_SHIFT + PAGE_SHIFT);
>  
>  	for (plane = 0; plane < vb->num_planes; ++plane) {
> -		vb->planes[plane].m.offset = off;
> +		vb->planes[plane].m.offset = off + (plane << PAGE_SHIFT);
>  
>  		dprintk(q, 3, "buffer %d, plane %d offset 0x%08lx\n",
>  				vb->index, plane, off);
> -
> -		off += vb->planes[plane].length;
> -		off = PAGE_ALIGN(off);
>  	}
>  }
>  
> @@ -2209,21 +2215,15 @@ static int __find_plane_by_offset(struct vb2_queue *q, unsigned long off,
>  		return -EBUSY;
>  	}
>  
> -	/*
> -	 * Go over all buffers and their planes, comparing the given offset
> -	 * with an offset assigned to each plane. If a match is found,
> -	 * return its buffer and plane numbers.
> -	 */
> -	for (buffer = 0; buffer < q->num_buffers; ++buffer) {
> -		vb = q->bufs[buffer];
> +	/* Get buffer and plane from the offset */
> +	buffer = (off >> (PLANE_INDEX_SHIFT + PAGE_SHIFT)) & MAX_BUFFERS;

Hmm, you use it as a mask. The name MAX_BUFFERS is really confusing.
How about BUFFER_INDEX_MASK? That is consistent with PLANE_INDEX_MASK.

> +	plane = (off >> PAGE_SHIFT) & PLANE_INDEX_MASK;
>  
> -		for (plane = 0; plane < vb->num_planes; ++plane) {
> -			if (vb->planes[plane].m.offset == off) {
> -				*_buffer = buffer;
> -				*_plane = plane;
> -				return 0;
> -			}
> -		}
> +	vb = q->bufs[buffer];
> +	if (vb->planes[plane].m.offset == off) {
> +		*_buffer = buffer;
> +		*_plane = plane;
> +		return 0;
>  	}
>  
>  	return -EINVAL;

Regards,

	Hans