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Date:   Tue, 26 Sep 2023 15:38:33 +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
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From:   Robin Murphy <robin.murphy@arm.com>
In-Reply-To: <20230926094616.GA14877@lst.de>
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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.