Re: [PATCH v8 2/2] drm/i915: Wait for PSR exit before checking for vblank evasion

From: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
To: Tarun Vyas <tarun.vyas@intel.com>, intel-gfx@lists.freedesktop.org
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>, rodrigo.vivi@intel.com
Subject: Re: [PATCH v8 2/2] drm/i915: Wait for PSR exit before checking for vblank evasion
Date: Mon, 02 Jul 2018 11:40:51 -0700	[thread overview]
Message-ID: <1530556851.2736.4.camel@intel.com> (raw)
In-Reply-To: <1530314774.25977.5.camel@intel.com>

On Fri, 2018-06-29 at 16:26 -0700, Dhinakaran Pandiyan wrote:
> On Wed, 2018-06-27 at 13:02 -0700, Tarun Vyas wrote:
> > 
> > The PIPEDSL freezes on PSR entry and if PSR hasn't fully exited,
> > then
> > the pipe_update_start call schedules itself out to check back
> > later.
> > 
> > On ChromeOS-4.4 kernel, which is fairly up-to-date w.r.t drm/i915
> > but
> > lags w.r.t core kernel code, hot plugging an external display
> > triggers
> > tons of "potential atomic update errors" in the dmesg, on *pipe A*.
> > A
> > closer analysis reveals that we try to read the scanline 3 times
> > and
> > eventually timeout, b/c PSR hasn't exited fully leading to a
> > PIPEDSL
> > stuck @ 1599. This issue is not seen on upstream kernels, b/c for
> > *some*
> > reason we loop inside intel_pipe_update start for ~2+ msec which in
> > this
> > case is more than enough to exit PSR fully, hence an *unstuck*
> > PIPEDSL
> > counter, hence no error. On the other hand, the ChromeOS kernel
> > spends
> > ~1.1 msec looping inside intel_pipe_update_start and hence errors
> > out
> > b/c the source is still in PSR.
> > 
> > Regardless, we should wait for PSR exit (if PSR is disabled, we
> > incur
> > a ~1-2 usec penalty) before reading the PIPEDSL, b/c if we haven't
> > fully exited PSR, then checking for vblank evasion isn't actually
> > applicable.
> > 
> > v4: Comment explaining psr_wait after enabling VBL interrupts (DK)
> > 
> > v5: CAN_PSR() to handle platforms that don't support PSR.
> > 
> > v6: Handle local_irq_disable on early return (Chris)
>  
> Series Reviewed-by: Dhinakaran Pandiyan
> <dhinakaran.pandiyan@intel.com>
> 
> Daniel's questions were addressed over IRC, I'll push this version if
> they aren't any other concerns.

and pushed to -dinq with Daniel's irc ack.

"<dhnkrn> ickle: danvet ack on the PSR wait_for_idle series? 
<danvet> sure"

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