Re: [PATCH v4 1/1] drm: allow limiting the scatter list size.

[Gerd Hoffmann <kraxel@xxxxxxxxxx>]

On Mon, Sep 07, 2020 at 03:53:02PM +0200, Daniel Vetter wrote:
> On Mon, Sep 7, 2020 at 1:24 PM Gerd Hoffmann <kraxel@xxxxxxxxxx> wrote:
> >
> > Add drm_device argument to drm_prime_pages_to_sg(), so we can
> > call dma_max_mapping_size() to figure the segment size limit
> > and call into __sg_alloc_table_from_pages() with the correct
> > limit.
> >
> > This fixes virtio-gpu with sev.  Possibly it'll fix other bugs
> > too given that drm seems to totaly ignore segment size limits
> > so far ...
> >
> > v2: place max_segment in drm driver not gem object.
> > v3: move max_segment next to the other gem fields.
> > v4: just use dma_max_mapping_size().
> >
> > Signed-off-by: Gerd Hoffmann <kraxel@xxxxxxxxxx>
> 
> Uh, are you sure this works in all cases for virtio?

Sure, I've tested it ;)

> The comments I've found suggest very much not ... Or is that all very
> old stuff only that no one cares about anymore?

I think these days it is possible to override dma_ops per device, which
in turn allows virtio to deal with the quirks without the rest of the
kernel knowing about these details.

I also think virtio-gpu can drop the virtio_has_dma_quirk() checks, just
use the dma api path unconditionally and depend on virtio core having
setup dma_ops in a way that it JustWorks[tm].  I'll look into that next.

take care,
  Gerd