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

From: Daniel Vetter
Date: Tue Sep 08 2020 - 09:04:03 EST

On Tue, Sep 08, 2020 at 12:02:53PM +0200, Gerd Hoffmann wrote:
> > > > 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.
> Hmm, wading through the code, seems it indeed happen yet, even though my
> testing didn't show any issues. Probably pure luck because devices and
> cpus have the same memory view on x86. Guess I need to try this on
> ppc64 to see it actually failing ...
> So dropping the virtio_has_dma_quirk() checks isn't going to fly.
> Using dma_max_mapping_size() should be fine though. It might use a
> lower limit than needed for virtio, but it should not break things.

Makes sense. On this patch here:

Reviewed-by: Daniel Vetter <daniel.vetter@xxxxxxxx>

And I guess would be good if virtio pushes a bit more towards using the
dma api abstraction fully so we can get rid of these hacks. Virtio feels
like a driver that really should be using dma-api and not dig around
behind it because "it' makes stuff 0.5% faster" or so, since being
virtualized it's already not the king of speed anyway :-)

Cheers, Daniel
Daniel Vetter
Software Engineer, Intel Corporation