[Intel-gfx] [PATCH 1/8] drm/i915: Get rid of the synchronous flag in switch_mm

Mateo Lozano, Oscar oscar.mateo at intel.com
Thu Jun 19 16:00:24 CEST 2014

> -----Original Message-----
> From: Daniel Vetter [mailto:daniel.vetter at ffwll.ch] On Behalf Of Daniel
> Vetter
> Sent: Wednesday, June 18, 2014 8:51 PM
> To: Mateo Lozano, Oscar
> Cc: intel-gfx at lists.freedesktop.org
> Subject: Re: [Intel-gfx] [PATCH 1/8] drm/i915: Get rid of the synchronous flag
> in switch_mm
> On Wed, Jun 18, 2014 at 05:15:35PM +0100, oscar.mateo at intel.com wrote:
> > From: Oscar Mateo <oscar.mateo at intel.com>
> >
> > As a first step, split switch_mm into two different functions:
> > sync_switch_mm (currently used by ppgtt_enable) and switch_mm (used by
> > the outside world).
> >
> > Signed-off-by: Oscar Mateo <oscar.mateo at intel.com>
> I guess that jira task description was a bit unclear - removing the bool
> synchronous argument isn't the goal itself. The idea was to rework the gpu
> reset sequence so that it matches the same sequence we use at driver load
> and so can use the same code. This patch removes the bool synchronous, but
> the logical split with two different ways to load ppgtt is still there.

No, I realized I didn´t really do anything, but I saw the possibility of saving one special casing in Execlists and I jumped for it, forgetting about the JIRA task (psychological fixation, I guess)
Anyway, I was going to ask for more details on the JIRA...

> The reason for aiming as hard as possible to use the exact same code for
> driver load, gpu reset and runtime pm/system resume is that we've simply
> seen too many bugs due to slight variations and unintended omissions.
> If for some really odd reason this is impossible (e.g. when the gpu needs a
> special dance when coming out of reset) then we need a giant comment
> explaining the magic. But without such a clear reason the goal should always
> be to have just 1 single autorative piece of code for a given state transition.

Ok. How about this: on ring init, I manually set the Aliasing PPGTT and the default context object synchronously via MMIO writes (the Aliasing PPGTT via the ppgtt->sync_switch_mm() and the default context directly using the CCID). I assume that, whenever we are in ring init, the GPU is sufficiently idle to manually frob all the required registers. That way, I get rid of i915_gem_context_enable() completely and I don´t have to do any special casing at all.
Ring commands (PPGTT switch and MI_SET_CONTEXT) are then left completely for the normal do_switch().
What do you think?

-- Oscar

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