linux-kernel - Re: [PATCH v3 5/6] media: uvcvideo: Use dma_alloc

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Message-ID: <20201126114450.GB3723071@google.com>
Date:   Thu, 26 Nov 2020 20:44:50 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:     Ricardo Ribalda <ribalda@...omium.org>
Cc:     Marek Szyprowski <m.szyprowski@...sung.com>,
        Robin Murphy <robin.murphy@....com>,
        Christoph Hellwig <hch@....de>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        IOMMU DRIVERS <iommu@...ts.linux-foundation.org>,
        Joerg Roedel <joro@...tes.org>,
        Linux Doc Mailing List <linux-doc@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux Media Mailing List <linux-media@...r.kernel.org>,
        Tomasz Figa <tfiga@...omium.org>,
        Sergey Senozhatsky <senozhatsky@...gle.com>
Subject: Re: [PATCH v3 5/6] media: uvcvideo: Use dma_alloc_noncontiguos API

On (20/11/25 23:19), Ricardo Ribalda wrote:
[..]
> +	if (uvc_urb->pages)
> +		dma_sync_sgtable_for_device(stream_to_dmadev(uvc_urb->stream),
> +					    &uvc_urb->sgt, DMA_FROM_DEVICE);

[..]

> +	if (uvc_urb->pages)
> +		dma_sync_sgtable_for_cpu(stream_to_dmadev(stream),
> +					 &uvc_urb->sgt, DMA_FROM_DEVICE);

[..]

> +	uvc_urb->pages = dma_alloc_noncontiguous(dma_dev, stream->urb_size,
> +						 &uvc_urb->dma,
> +						 gfp_flags | __GFP_NOWARN, 0);

Do we need to pass __GFP_NOWARN? It seems that

dma_alloc_noncontiguous()
  __iommu_dma_alloc_noncontiguous()
    __iommu_dma_alloc_pages()

does this internally.

> +	if (!uvc_urb->pages)
> +		return false;
> +
> +	uvc_urb->buffer = vmap(uvc_urb->pages,
> +			       PAGE_ALIGN(stream->urb_size) >> PAGE_SHIFT,
> +			       VM_DMA_COHERENT, PAGE_KERNEL);

This is not related to Ricardo's patch, just a side note:

  I think VM_DMA_COHERENT needs to be renamed. I found it a bit confusing
  to see DMA_COHERENT mapping being dma_sync-ed. It turned out that the
  flag has different meaning.

	-ss