linux-kernel - Re: [PATCH] MA-21654 Use dma_alloc_pages in vb2_dma_sg_alloc

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Date:   Tue, 26 Sep 2023 09:21:15 +0100
From:   Robin Murphy <robin.murphy@....com>
To:     Christoph Hellwig <hch@....de>
Cc:     Tomasz Figa <tfiga@...omium.org>, Fang Hui <hui.fang@....com>,
        m.szyprowski@...sung.com, mchehab@...nel.org,
        linux-media@...r.kernel.org, linux-kernel@...r.kernel.org,
        anle.pan@....com, xuegang.liu@....com
Subject: Re: [PATCH] MA-21654 Use dma_alloc_pages in
 vb2_dma_sg_alloc_compacted

On 2023-09-26 07:51, Christoph Hellwig wrote:
> On Wed, Sep 20, 2023 at 05:54:26PM +0100, Robin Murphy wrote:
>> As I mentioned before, I think it might make the most sense to make the
>> whole thing into a "proper" dma_alloc_sgtable() function, which can then be
>> used with dma_sync_sgtable_*() as dma_alloc_pages() is used with
>> dma_sync_single_*() (and then dma_alloc_noncontiguous() clearly falls as
>> the special in-between case).
> 
> Why not just use dma_alloc_noncontiguous if the caller wants an sgtable
> anyway?

Because we don't need the restriction of the allocation being 
DMA-contiguous (and thus having to fall back to physically-contiguous in 
the absence of an IOMMU). That's what vb2_dma_contig already does, 
whereas IIUC vb2_dma_sg is for devices which can handle genuine 
scatter-gather DMA (and so are less likely to have an IOMMU, and more 
likely to need the best shot at piecing together large allocations).

Thanks,
Robin.