[Intel-gfx] [PATCH v4 10/38] drm/i915: Force MMIO flips when scheduler enabled

Chris Wilson chris at chris-wilson.co.uk
Tue Jan 12 13:53:35 PST 2016

On Tue, Jan 12, 2016 at 03:07:03PM +0100, Daniel Vetter wrote:
> On Tue, Jan 12, 2016 at 11:19:26AM +0000, John Harrison wrote:
> > On 11/01/2016 22:16, Chris Wilson wrote:
> > >On Mon, Jan 11, 2016 at 06:42:39PM +0000, John.C.Harrison at Intel.com wrote:
> > >>From: John Harrison <John.C.Harrison at Intel.com>
> > >>
> > >>MMIO flips are the preferred mechanism now but more importantly,
> > >Says who?
> > 
> > I asked this exact question at the linux architecture forum quite some time
> > ago - does the scheduler need to worry about managing non-batch buffer work
> > such as page flips. The answer from everyone present was no, MMIO flips are
> > the way to go so don't over complicate the scheduler trying to support ring
> > flips. Indeed, execlist mode already forces MMIO flips anyway.

Two wrongs do not make a right, as they say. CS flips work very nicely
with execlists.

> Atomic will kill CS flips. We can mourn them and scream about the loss,
> but imo best is to just skip that all and move on to acceptance. So mmio
> flips (or well, atomic flips) is still the way to go for everything.

The real issue I think here is that not trying to feed a request into the
scheduler for the flip has lead to a poor interface into the scheduler.
For a CS flip request, we know the ordering, it's contents, we have to
choose the context though, but we have a good idea of the deadline which
gives a good challenge to a scheduler. That was my take.

Chris Wilson, Intel Open Source Technology Centre

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