[PATCH v2] drm/gem: Fix mmap fake offset handling for drm_gem_object_funcs.mmap

[Gerd Hoffmann]

On Fri, Nov 15, 2019 at 11:18:28AM +0100, Daniel Vetter wrote:
> On Fri, Nov 15, 2019 at 10:37 AM Gerd Hoffmann <kraxel at redhat.com> wrote:
> >
> > > You need memory pressure, to force ttm to unmap the bo, not userspace. So
> > > roughly
> > > 1. create bo
> > > 2. mmap it through drm fd, write some stuff
> > > 3. export to dma-buf, mmap it, verify stuff is there
> > > 4. create a pile more bo, mmap them write to them
> > > 5. once you've thrashed all of vram enough, recheck your original bo. If
> > > I'm right you should get the following:
> > >    - drm fd mmap still show right content
> > >    - dma-buf fd mmap shows random crap that you've written into other
> > >      buffers
> > >
> > > Ofc you need to make sure that an mmap actually forces the buffer into
> > > vram. So might need a combo of modeset+mmap, to make that happen. Plain
> > > mmap might just give you ptes that point into system memory, which is not
> > > managed by ttm like vram.
> >
> > Is any move buffer good enough to trigger this, i.e. will SYSTEM -> VRAM
> > work too?  That'll be easier because all I need to do is map the buffer
> > to a crtc to force pinning to vram, then check if the mappings are
> > intact still ...
> 
> I think that should work too.

Seems to work ok for ttm/vram.

Test code:
  https://git.kraxel.org/cgit/drminfo/commit/?id=a9eb96504dc17376e07b5c6edf5296b41a48bbfe

> Just make sure you actually write
> through SYSTEM first (maybe with some printk or whatever).

That works fine.  Sprinkled ...

	system("cat /sys/kernel/debug/dri/0/vram-mm"); 

... into the test code, and drmModeSetCrtc() indeed makes the gem object show
up there.

cheers,
  Gerd