[Intel-gfx] [PATCH v5 2/2] drm/i915: Wait for PSR exit before checking for vblank evasion

[Tarun Vyas]

On Mon, Jun 25, 2018 at 01:56:24PM +0100, Chris Wilson wrote:
> Quoting Tarun Vyas (2018-06-25 08:09:18)
> > 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.
> > 
> > Signed-off-by: Tarun Vyas <tarun.vyas at intel.com>
> > ---
> >  drivers/gpu/drm/i915/intel_sprite.c | 12 ++++++++++--
> >  1 file changed, 10 insertions(+), 2 deletions(-)
> > 
> > diff --git a/drivers/gpu/drm/i915/intel_sprite.c b/drivers/gpu/drm/i915/intel_sprite.c
> > index 344c0e709b19..8982a69a13dd 100644
> > --- a/drivers/gpu/drm/i915/intel_sprite.c
> > +++ b/drivers/gpu/drm/i915/intel_sprite.c
> > @@ -107,14 +107,22 @@ void intel_pipe_update_start(const struct intel_crtc_state *new_crtc_state)
> >                                                       VBLANK_EVASION_TIME_US);
> >         max = vblank_start - 1;
> >  
> > -       local_irq_disable();
> > -
> >         if (min <= 0 || max <= 0)
> >                 return;
> >  
> >         if (WARN_ON(drm_crtc_vblank_get(&crtc->base)))
> >                 return;
> >  
> > +       /*
> > +        * Wait for psr to idle out after enabling the VBL interrupts
> > +        * VBL interrupts will start the PSR exit and prevent a PSR
> > +        * re-entry as well.
> > +        */
> > +       if (CAN_PSR(dev_priv) && intel_psr_wait_for_idle(dev_priv))
> > +               DRM_ERROR("PSR idle timed out, atomic update may fail\n");
> > +
> > +       local_irq_disable();
> 
> The function must return with irqs disabled as the update_end is always
> called to re-enable irqs.
> 
> The pair of early returns may be unjustified, but they still do exist...
> -Chris
Oops ! Missed it, will handle in v6 :( . I am curious as to how the atomic update will go ahead in the event of these early returns.