[Intel-gfx] [PATCH v5 3/4] drm/i915/bdw: Pin the context backing objects to GGTT on-demand

[Daniel, Thomas]

> -----Original Message-----
> From: Daniel Vetter [mailto:daniel.vetter at ffwll.ch] On Behalf Of Daniel
> Vetter
> Sent: Tuesday, November 18, 2014 2:33 PM
> To: Daniel, Thomas
> Cc: Daniel Vetter; intel-gfx at lists.freedesktop.org
> Subject: Re: [Intel-gfx] [PATCH v5 3/4] drm/i915/bdw: Pin the context
> backing objects to GGTT on-demand
> 
> On Tue, Nov 18, 2014 at 10:48:09AM +0000, Daniel, Thomas wrote:
> > > -----Original Message-----
> > > From: Intel-gfx [mailto:intel-gfx-bounces at lists.freedesktop.org] On
> > > Behalf Of Daniel, Thomas
> > > Sent: Tuesday, November 18, 2014 9:28 AM
> > > To: Daniel Vetter
> > > Cc: intel-gfx at lists.freedesktop.org
> > > Subject: Re: [Intel-gfx] [PATCH v5 3/4] drm/i915/bdw: Pin the
> > > context backing objects to GGTT on-demand
> > >
> > > > -----Original Message-----
> > > > From: Daniel Vetter [mailto:daniel.vetter at ffwll.ch] On Behalf Of
> > > > Daniel Vetter
> > > > Sent: Monday, November 17, 2014 6:09 PM
> > > > To: Daniel, Thomas
> > > > Cc: intel-gfx at lists.freedesktop.org
> > > > Subject: Re: [Intel-gfx] [PATCH v5 3/4] drm/i915/bdw: Pin the
> > > > context backing objects to GGTT on-demand
> > > >
> > > > On Thu, Nov 13, 2014 at 10:28:10AM +0000, Thomas Daniel wrote:
> > > > > diff --git a/drivers/gpu/drm/i915/i915_drv.h
> > > > > b/drivers/gpu/drm/i915/i915_drv.h index 059330c..3c7299d 100644
> > > > > --- a/drivers/gpu/drm/i915/i915_drv.h
> > > > > +++ b/drivers/gpu/drm/i915/i915_drv.h
> > > > > @@ -655,6 +655,7 @@ struct intel_context {
> > > > >  	struct {
> > > > >  		struct drm_i915_gem_object *state;
> > > > >  		struct intel_ringbuffer *ringbuf;
> > > > > +		int unpin_count;
> > > >
> > > > Pinning is already refcounted. Why this additional refcount?
> > >
> > > The vma.pin_count is only allocated 4 bits of storage.  If this
> > > restriction can be lifted then I can use that.
> 
> Those 4 bits are good enough for legacy contexts, so I wonder a bit what's so
> massively different for execlist contexts.
With execlists, in order to dynamically unpin the LRC backing object and ring buffer object when not required we take a reference for each execlist request that uses them (remember that the execlist request lifecycle is currently different from the execbuffer request).  This can be a lot, especially in some of the less sane i-g-t tests.

> > Actually I just tried to implement this, it causes a problem for patch
> > 4 of this set as the unpin_count is also used for the ringbuffer
> > object which has an ioremap as well as a ggtt pin.
> 
> Yeah, ioremap needs to be redone every time we pin/unpin. But on sane
> archs it's almost no overhead really. And if this does start to matter (shudder
> for 32bit kernels on gen8) then we can fix it ...
Hm, so the CPU vaddr of the ring buffer will move around as more requests reference it which I suppose is not a problem.  We will use a lot of address space (again, especially with the i-g-t stress tests which can submit tens of thousands of requests in a very short space of time).  What would the fix be?  An extra reference count for the ioremap?  Looks familiar :)

I still think it's best to keep the context unpin_count for execlists mode.

Thomas.

> -Daniel
> --
> Daniel Vetter
> Software Engineer, Intel Corporation
> +41 (0) 79 365 57 48 - http://blog.ffwll.ch