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

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Date:   Tue, 26 Sep 2023 11:46:16 +0200
From:   Christoph Hellwig <hch@....de>
To:     Robin Murphy <robin.murphy@....com>
Cc:     Christoph Hellwig <hch@....de>, 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 Tue, Sep 26, 2023 at 09:21:15AM +0100, Robin Murphy wrote:
> 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).

Let's just extent dma_alloc_noncontiguous with a max_dma_segments
parameter instead of adding yet another API.