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

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Message-ID: <06d476e5-ba85-1504-d69b-a8c1cf617d54@arm.com>
Date:   Tue, 26 Sep 2023 15:38:33 +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 26/09/2023 10:46 am, Christoph Hellwig wrote:
> 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.

Sure, that could work equally well, and might even help make its 
existing usage a bit clearer.

Cheers,
Robin.