Re: [PATCH 3/4] drm/i915: opt-out CPU and WC mmaps from FBC

[Daniel Vetter <daniel@ffwll.ch>]

On Fri, Mar 25, 2016 at 02:05:42PM +0000, Zanoni, Paulo R wrote:
> Em Qui, 2016-03-24 às 21:20 +0000, chris@chris-wilson.co.uk escreveu:
> > On Thu, Mar 24, 2016 at 09:03:59PM +0000, chris@chris-wilson.co.uk
> > wrote:
> > > 
> > > On Thu, Mar 24, 2016 at 08:53:21PM +0000, Zanoni, Paulo R wrote:
> > > > 
> > > > Em Qui, 2016-03-24 às 19:31 +0000, Chris Wilson escreveu:
> > > > > 
> > > > > On Thu, Mar 24, 2016 at 04:16:11PM -0300, Paulo Zanoni wrote:
> > > > > > 
> > > > > > 
> > > > > > FBC and the frontbuffer tracking infrastructure were designed
> > > > > > assuming
> > > > > > that user space applications would follow a specific set of
> > > > > > rules
> > > > > > regarding frontbuffer management and mmapping. I recently
> > > > > > discovered
> > > > > > that current user space is not exactly following these rules:
> > > > > > my
> > > > > > investigation led me to the conclusion that the generic
> > > > > > backend
> > > > > > from
> > > > > > SNA - used by SKL and the other new platforms without a
> > > > > > specific
> > > > > > backend - is not issuing sw_finish/dirty_fb IOCTLs when using
> > > > > > the
> > > > > > CPU
> > > > > > and WC mmaps. I discovered this when running lightdm: I would
> > > > > > type
> > > > > > the
> > > > > > password and nothing would appear on the screen unless I
> > > > > > moved the
> > > > > > mouse over the place where the letters were supposed to
> > > > > > appear.
> > > > > Yes, that is a kernel bug. The protocol we said the kernel
> > > > > would
> > > > > follow
> > > > > is to disable FBC/WC when userspace marks the object for
> > > > > writing by
> > > > > the
> > > > > CPU and would only reestablish FBC/WC upon dirtyfb.
> > > > But on WC mmaps we mark the object for writing by the GTT instead
> > > > of
> > > > the CPU, and while the tracking engine is able to see "normal"
> > > > GTT mmap
> > > > writes, it's not able to see WC mmap writes, so we established
> > > > that
> > > > we'd call dirtyfb after frontbuffer drawing through WC mmaps,
> > > > which is
> > > > something that the DDX never implemented. This was discussed on
> > > > #intel-
> > > > gfx on Nov 5 2014, and also possibly other places, but I can't
> > > > find the
> > > > logs. Daniel also confirmed this to me again on private IRC on
> > > > Jun 16
> > > > 2015. So I still don't understand why this is a Kernel bug
> > > > instead of a
> > > > DDX bug.
> > > Because we said that once invalidated, it would not be restored
> > > until
> > > dirtyfb. The kernel is not doing that. Your patch does not do that.
> > > To
> > > be even close, you should be setting the origin flag based on the
> > > existence
> > > of wc mmaping for the object inside set-to-gtt-domain. Otherwise,
> > > you
> > > are not implementing even close to the protocol you say you are.
> > > That is
> > > invalidate on set-domain, flush on dirtyfb.
> > > 
> > > The kernel's bug is that is not cancelling FBC. Userspace's bug is
> > > not
> > > signalling when to reenable it.
> > diff --git a/drivers/gpu/drm/i915/i915_drv.h
> > b/drivers/gpu/drm/i915/i915_drv.h
> > index 8dec2e8..0314346 100644
> > --- a/drivers/gpu/drm/i915/i915_drv.h
> > +++ b/drivers/gpu/drm/i915/i915_drv.h
> > @@ -2145,6 +2145,7 @@ struct drm_i915_gem_object {
> >         unsigned int cache_dirty:1;
> >  
> >         unsigned int frontbuffer_bits:INTEL_FRONTBUFFER_BITS;
> > +       unsigned int has_wc_mmap:1;
> >  
> >         /** Count of VMA actually bound by this object */
> >         unsigned int bind_count;
> > diff --git a/drivers/gpu/drm/i915/i915_gem.c
> > b/drivers/gpu/drm/i915/i915_gem.c
> > index 05ae706..29ca96d 100644
> > --- a/drivers/gpu/drm/i915/i915_gem.c
> > +++ b/drivers/gpu/drm/i915/i915_gem.c
> > @@ -1310,6 +1310,13 @@ unlock:
> >         return ret;
> >  }
> >  
> > +static enum fb_op_origin
> > +write_origin(struct drm_i915_gem_object *obj, unsigned domain)
> > +{
> > +       return domain == I915_GEM_DOMAIN_GTT && !obj->has_wc_mmap ?
> > +             ORIGIN_GTT : ORIGIN_CPU;
> 
> What if I have both a WC mmap and a GTT mmap, and I'm actually using
> the GTT mmap now? My set_domain call will be treated as WC mmap usage,
> while in fact it should be treated as GTT usage. Is there a way to
> differentiate between them with the current set_domain API?

*Blonk* or whatever the sound for suddenly realization is. Totally forgot
that we're reuseding set_domain(GTT) for wc mmaps because this it's a nice
trick.

Otoh, is that trick the reason why wc mmaps aren't coherent enough? One
possible difference is that this won't do the magic chipset flush in
intel-gtt.c that we have on gen3-5. Let's pretend wbinv doesn't exist on
gen2 ;-) But that's just an aside really ...

Anyway, now that I can see again, ack on the trick to decide on ORIGIN_GTT
vs. ORIGIN_CPU.
-Daniel

> 
> 
> > +}
> > +
> >  /**
> >   * Called when user space prepares to use an object with the CPU,
> > either
> >   * through the mmap ioctl's mapping or a GTT mapping.
> > @@ -1363,9 +1370,7 @@ i915_gem_set_domain_ioctl(struct drm_device
> > *dev, void *data,
> >                 ret = i915_gem_object_set_to_cpu_domain(obj,
> > write_domain != 0);
> >  
> >         if (write_domain != 0)
> > -               intel_fb_obj_invalidate(obj,
> > -                                       write_domain ==
> > I915_GEM_DOMAIN_GTT ?
> > -                                       ORIGIN_GTT : ORIGIN_CPU);
> > +               intel_fb_obj_invalidate(obj, write_origin(obj,
> > write_domain));
> >  
> >  unref:
> >         drm_gem_object_unreference(&obj->base);
> > @@ -1466,6 +1471,9 @@ i915_gem_mmap_ioctl(struct drm_device *dev,
> > void *data,
> >                 else
> >                         addr = -ENOMEM;
> >                 up_write(&mm->mmap_sem);
> > +
> > +               /* This may race, but that's ok, it only gets set */
> > +               to_intel_bo(obj)->has_wc_mmap = true;
> >         }
> > 

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch
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