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Message-ID: <4d0f3de5-1d34-d998-cb55-7ce7bfaf3f49@arm.com>
Date:   Tue, 26 Sep 2023 09:21:15 +0100
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Subject: Re: [PATCH] MA-21654 Use dma_alloc_pages in
 vb2_dma_sg_alloc_compacted
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To:     Christoph Hellwig <hch@lst.de>
Cc:     Tomasz Figa <tfiga@chromium.org>, Fang Hui <hui.fang@nxp.com>,
        m.szyprowski@samsung.com, mchehab@kernel.org,
        linux-media@vger.kernel.org, linux-kernel@vger.kernel.org,
        anle.pan@nxp.com, xuegang.liu@nxp.com
References: <20230914145812.12851-1-hui.fang@nxp.com>
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 <353919fd-932e-5d81-6ac5-7b51117366cd@arm.com> <20230926065143.GB5606@lst.de>
From:   Robin Murphy <robin.murphy@arm.com>
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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.