[Intel-gfx] [PATCH 1/7] drm/i915: Specify bsd rings through exec flag

[Zhao, Yakui]

On Wed, 2014-12-10 at 08:55 -0700, Dave Gordon wrote:
> On 10/12/14 09:11, Daniel Vetter wrote:
> > On Wed, Dec 10, 2014 at 02:18:15AM +0000, Gong, Zhipeng wrote:
> >> On Tue, 2014-12-09 at 10:46 +0100, Daniel Vetter wrote:
> >>> On Mon, Dec 08, 2014 at 01:55:56PM -0800, Rodrigo Vivi wrote:
> 
> [snip]
> 
> >>>> diff --git a/drivers/gpu/drm/i915/i915_gem_execbuffer.c b/drivers/gpu/drm/i915/i915_gem_execbuffer.c
> >>>> index e1ed85a..d9081ec 100644
> >>>> --- a/drivers/gpu/drm/i915/i915_gem_execbuffer.c
> >>>> +++ b/drivers/gpu/drm/i915/i915_gem_execbuffer.c
> >>>> @@ -1273,8 +1273,23 @@ i915_gem_do_execbuffer(struct drm_device *dev, void *data,
> >>>>       else if ((args->flags & I915_EXEC_RING_MASK) == I915_EXEC_BSD) {
> >>>>               if (HAS_BSD2(dev)) {
> >>>>                       int ring_id;
> >>>> -                     ring_id = gen8_dispatch_bsd_ring(dev, file);
> >>>> -                     ring = &dev_priv->ring[ring_id];
> >>>> +
> >>>> +                     switch (args->flags & I915_EXEC_BSD_MASK) {
> >>>> +                     case I915_EXEC_BSD_DEFAULT:
> >>>> +                             ring_id = gen8_dispatch_bsd_ring(dev, file);
> >>>> +                             ring = &dev_priv->ring[ring_id];
> >>>> +                             break;
> >>>> +                     case I915_EXEC_BSD_RING1:
> >>>> +                             ring = &dev_priv->ring[VCS];
> >>>
> >>> Do we have any use-case for selecting ring1 specifically? I've thought
> >>> it's only ring2 that is special?
> >> The HEVC GPU commands should be dispatched to BSD RING 1 instead of BSD
> >> RING2 as the two rings are asymmetrical. 
> >> For the H264 decoding/encoding either ring is OK.
> > 
> > Well then same arguments applies with ring2 since only ring1 is special?
> > It's just to minimize abi and reduce the amount of rope we hand to
> > userspace.
> 
> Anyone who knows to use any of these flags is taking responsibility for
> doing explicit engine allocation, so why not give them all the options
> -- if for no other reason, more symmetry is good.

Agree with Dave's point. The override flag is initiated by the SKL GT3
platform, which requires that the HEVC GPU command can only be
dispatched to the BSD ring1 explicitly as the two BSD rings are not
symmetric. And the override flag can also provide the user-space
app/driver with more flexibility to explicitly determine which BSD ring
should be used to dispatch video GPU command instead of kernel ping-pong
mode. And it benefits the platform with two BSD rings.

> 
> As an examle, there could be a case where userspace knows better than
> the kernel how long each batch will take, and can predict an optimal
> allocation pattern rather than just flip-flopping. So even when a batch
> *can* run on either engine, there might be a reason to pick a specific one.
> 
> e.g.	short-1 -> ring 1
> 	short-2 -> ring 1
> 	long-1  -> ring 2
> 	short-3 -> ring 1
> 	long-2  -> ring 1
> 
> because the program knows that the three short batches together will
> take less time than the one first long one.
> 
> .Dave.
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