Re: [PATCH v3 5/6] media: uvcvideo: Use dma_alloc_noncontiguos API

[Ricardo Ribalda <ribalda@chromium.org>]

Hi Christoph

On Mon, Nov 30, 2020 at 9:34 AM Christoph Hellwig <hch@lst.de> wrote:
>
> > +#ifndef CONFIG_DMA_NONCOHERENT
>
> I think you need to drop this ifdef.  This code should work just fine
> on noncoherent mips and sh platforms.
>
> > +     uvc_urb->pages = dma_alloc_noncontiguous(dma_dev, stream->urb_size,
> > +                                              &uvc_urb->dma,
> > +                                              gfp_flags | __GFP_NOWARN, 0);
> > +     if (!uvc_urb->pages)
> > +             return false;
> > +
> > +     uvc_urb->buffer = vmap(uvc_urb->pages,
> > +                            PAGE_ALIGN(stream->urb_size) >> PAGE_SHIFT,
> > +                            VM_DMA_COHERENT, PAGE_KERNEL);
> > +     if (!uvc_urb->buffer) {
> > +             dma_free_noncontiguous(dma_dev, stream->urb_size,
> > +                                    uvc_urb->pages, uvc_urb->dma);
> > +             return false;
> > +     }
> > +
> > +     if (sg_alloc_table_from_pages(&uvc_urb->sgt, uvc_urb->pages,
> > +                             PAGE_ALIGN(stream->urb_size) >> PAGE_SHIFT, 0,
> > +                             stream->urb_size, GFP_KERNEL)) {
> > +             vunmap(uvc_urb->buffer);
> > +             dma_free_noncontiguous(dma_dev, stream->urb_size,
> > +                                    uvc_urb->pages, uvc_urb->dma);
> > +             return false;
> > +     }
> > +
> > +     return true;
> > +}
>
> I wonder if we should lift this into a helper.  On the one hand I had
> proliferating struct scatterlist usage, on the other hand it is all over
> the media and drm code anyway, and duplicating this doesn't help anyone.
>
> Possibly including the fallback to the coherent allocating.

Probably Sergey has best opinion of this than mine. I only had to look
into one driver, he has been working with the vb2, which uses the API
much more.



-- 
Ricardo Ribalda