Re: [PATCH v3 3/3] drm/vgem: use normal cached mmap'ings

[Daniel Vetter]

On Tue, Jul 16, 2019 at 05:13:10PM -0700, Rob Clark wrote:
> On Tue, Jul 16, 2019 at 4:39 PM Eric Anholt <eric@xxxxxxxxxx> wrote:
> >
> > Rob Clark <robdclark@xxxxxxxxx> writes:
> >
> > > From: Rob Clark <robdclark@xxxxxxxxxxxx>
> > >
> > > Since there is no real device associated with VGEM, it is impossible to
> > > end up with appropriate dev->dma_ops, meaning that we have no way to
> > > invalidate the shmem pages allocated by VGEM.  So, at least on platforms
> > > without drm_cflush_pages(), we end up with corruption when cache lines
> > > from previous usage of VGEM bo pages get evicted to memory.
> > >
> > > The only sane option is to use cached mappings.
> >
> > This may be an improvement, but...
> >
> > pin/unpin is only on attaching/closing the dma-buf, right?  So, great,
> > you flushed the cached map once after exporting the vgem dma-buf to the
> > actual GPU device, but from then on you still have no interface for
> > getting coherent access through VGEM's mapping again, which still
> > exists.
> 
> In *theory* one would detach before doing further CPU access to
> buffer, and then re-attach when passing back to GPU.
> 
> Ofc that isn't how actual drivers do things.  But maybe it is enough
> for vgem to serve it's purpose (ie. test code).
> 
> > I feel like this is papering over something that's really just broken,
> > and we should stop providing VGEM just because someone wants to write
> > dma-buf test code without driver-specific BO alloc ioctl code.
> 
> yup, it is vgem that is fundamentally broken (or maybe more
> specifically doesn't fit in w/ dma-mappings view of how to do cache
> maint), and I'm just papering over it because people and CI systems
> want to be able to use it to do some dma-buf tests ;-)
> 
> I'm kinda wondering, at least for arm/dt based systems, if there is a
> way (other than in early boot) that we can inject a vgem device node
> into the dtb.  That isn't a thing drivers should normally do, but (if
> possible) since vgem is really just test infrastructure, it could be a
> way to make dma-mapping happily think vgem is a real device.

Or we just extend drm_cflush_pages with the cflushing we need (at least
for those arms where this is possible, let's ignore the others) and accept
for a few more years that dma-api doesn't fit?

Note this would need to be a full copypasta of what the arch code has
(since just exporting the function was shot down before), but I really
don't care about the resulting wailing if we do this.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch