Re: [PATCH] media: mediatek: vcodec: Alloc DMA memory with DMA_ATTR_ALLOC_SINGLE_PAGES

[Nicolas Dufresne <nicolas.dufresne@collabora.com>]

Hi,

Le lundi 22 avril 2024 à 10:03 -0700, Douglas Anderson a écrit :
> As talked about in commit 14d3ae2efeed ("ARM: 8507/1: dma-mapping: Use
> DMA_ATTR_ALLOC_SINGLE_PAGES hint to optimize alloc"), it doesn't
> really make sense to try to allocate contiguous chunks of memory for
> video encoding/decoding. Let's switch the Mediatek vcodec driver to
> pass DMA_ATTR_ALLOC_SINGLE_PAGES and take some of the stress off the
> memory subsystem.
> 
> Signed-off-by: Douglas Anderson <dianders@chromium.org>
> ---
> NOTE: I haven't personally done massive amounts of testing with this
> change, but I originally added the DMA_ATTR_ALLOC_SINGLE_PAGES flag
> specifically for the video encoding / decoding cases and I know it
> helped avoid memory problems in the past on other systems. Colleagues
> of mine have told me that with this change memory problems are harder
> to reproduce, so it seems like we should consider doing it.

One thing to improve in your patch submission is to avoid abstracting the
problems. Patch review and pulling is based on a technical rational and very
rarely on the trust that it helps someone somewhere in some unknown context.
What kind of memory issues are you facing ? What is the technical advantage of
using DMA_ATTR_ALLOC_SINGLE_PAGES over the current approach that helps fixing
the issue? I do expect this to be documented in the commit message itselfé.

regards,
Nicolas

> 
>  .../media/platform/mediatek/vcodec/common/mtk_vcodec_util.c    | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/media/platform/mediatek/vcodec/common/mtk_vcodec_util.c b/drivers/media/platform/mediatek/vcodec/common/mtk_vcodec_util.c
> index 9ce34a3b5ee6..3fb1d48c3e15 100644
> --- a/drivers/media/platform/mediatek/vcodec/common/mtk_vcodec_util.c
> +++ b/drivers/media/platform/mediatek/vcodec/common/mtk_vcodec_util.c
> @@ -64,7 +64,8 @@ int mtk_vcodec_mem_alloc(void *priv, struct mtk_vcodec_mem *mem)
>  		id = dec_ctx->id;
>  	}
>  
> -	mem->va = dma_alloc_coherent(&plat_dev->dev, size, &mem->dma_addr, GFP_KERNEL);
> +	mem->va = dma_alloc_attrs(&plat_dev->dev, size, &mem->dma_addr,
> +				  GFP_KERNEL, DMA_ATTR_ALLOC_SINGLE_PAGES);
>  	if (!mem->va) {
>  		mtk_v4l2_err(plat_dev, "%s dma_alloc size=%ld failed!",
>  			     dev_name(&plat_dev->dev), size);