linux-kernel - Re: [PATCH v8 01/53] media: videobuf2: Rework offset 'cookie' encoding pattern

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Date:   Sat, 30 Sep 2023 10:52:40 +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 v8 01/53] media: videobuf2: Rework offset 'cookie'
 encoding pattern

On 27/09/2023 17:35, Benjamin Gaignard wrote:
> Change how offset 'cookie' field value is computed to make possible
> to use more buffers (up to 0x7fff)
> 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.
> Change __find_plane_by_offset() prototype to return the video buffer
> itself rather than it index.
> 
> Signed-off-by: Benjamin Gaignard <benjamin.gaignard@...labora.com>
> ---
>  .../media/common/videobuf2/videobuf2-core.c   | 74 +++++++++----------
>  1 file changed, 35 insertions(+), 39 deletions(-)
> 
> diff --git a/drivers/media/common/videobuf2/videobuf2-core.c b/drivers/media/common/videobuf2/videobuf2-core.c
> index cf6727d9c81f..6eeddb3a01c7 100644
> --- a/drivers/media/common/videobuf2/videobuf2-core.c
> +++ b/drivers/media/common/videobuf2/videobuf2-core.c
> @@ -31,6 +31,14 @@
>  
>  #include <trace/events/vb2.h>
>  
> +#define PLANE_INDEX_SHIFT	(PAGE_SHIFT + 3)
> +#define PLANE_INDEX_MASK	0x7
> +#define BUFFER_INDEX_MASK	0x7fff
> +
> +#if PAGE_SHIFT != 12
> +#error Expected PAGE_SHIFT to be 12

So it turns out this can actually be something other than 12.
Search for CONFIG_ARM64_PAGE_SHIFT and CONFIG_PPC_PAGE_SHIFT.

arm64 supports 12, 14, 16 and powerpc64 supports 12, 14, 16, 18.

I think we should calculate the BUFFER_INDEX_MASK based on the
PAGE_SHIFT value. Even with a PAGE_SHIFT of 18, you can still
allocate up to 512 buffers.

Regards,

	Hans