Re: [PATCH] MA-21654 Use dma_alloc_pages in vb2_dma_sg_alloc_compacted

[Robin Murphy]

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.