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

[Daniel Vetter <daniel@xxxxxxxx>]

On Tue, Sep 08, 2020 at 07:48:58AM +0200, Gerd Hoffmann wrote:
> 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.

The comment above vring_use_dma_api() suggests that this has not yet
happened, that's why I'm asking. If this has happened then I think it'd be
best if you remove that todo entry and update it, as part of the overall
series to add dma_max_mapping_size and remove the quirks.

Otherwise this all is a bit wtf material :-)
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch