Re: [PATCH v3] drm/i915/skl: Add support for the SAGV, fix underrun hangs

["Ville Syrjälä" <ville.syrjala@linux.intel.com>]

On Wed, Jul 13, 2016 at 11:08:46AM -0700, Matt Roper wrote:
> On Tue, Jul 12, 2016 at 11:21:39AM -0700, Matt Roper wrote:
> > On Tue, Jul 12, 2016 at 01:36:03PM -0400, Lyude wrote:
> > > Since the watermark calculations for Skylake are still broken, we're apt
> > > to hitting underruns very easily under multi-monitor configurations.
> > > While it would be lovely if this was fixed, it's not. Another problem
> > > that's been coming from this however, is the mysterious issue of
> > > underruns causing full system hangs. An easy way to reproduce this with
> > > a skylake system:
> > > 
> > > - Get a laptop with a skylake GPU, and hook up two external monitors to
> > >   it
> > > - Move the cursor from the built-in LCD to one of the external displays
> > >   as quickly as you can
> > > - You'll get a few pipe underruns, and eventually the entire system will
> > >   just freeze.
> > > 
> > > After doing a lot of investigation and reading through the bspec, I
> > > found the existence of the SAGV, which is responsible for adjusting the
> > > system agent voltage and clock frequencies depending on how much power
> > > we need. According to the bspec:
> > > 
> > > "The display engine access to system memory is blocked during the
> > >  adjustment time. SAGV defaults to enabled. Software must use the
> > >  GT-driver pcode mailbox to disable SAGV when the display engine is not
> > >  able to tolerate the blocking time."
> > > 
> > > The rest of the bspec goes on to explain that software can simply leave
> > > the SAGV enabled, and disable it when we use interlaced pipes/have more
> > > then one pipe active.
> > > 
> > > Sure enough, with this patchset the system hangs resulting from pipe
> > > underruns on Skylake have completely vanished on my T460s. Additionally,
> > > the bspec mentions turning off the SAGV	with more then one pipe enabled
> > > as a workaround for display underruns. While this patch doesn't entirely
> > > fix that, it looks like it does improve the situation a little bit so
> > > it's likely this is going to be required to make watermarks on Skylake
> > > fully functional.
> > > 
> > > Changes since v2:
> > >  - Really apply minor style nitpicks to patch this time
> > > Changes since v1:
> > >  - Added comments about this probably being one of the requirements to
> > >    fixing Skylake's watermark issues
> > >  - Minor style nitpicks from Matt Roper
> > >  - Disable these functions on Broxton, since it doesn't have an SAGV
> > > 
> > > Cc: Matt Roper <matthew.d.roper@intel.com>
> > > Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
> > > Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
> > > Signed-off-by: Lyude <cpaul@redhat.com>
> > 
> > I don't have a SKL to try this out on (only BXT here), but this matches
> > my interpretation of the current bspec text, so
> > 
> > Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
> > 
> > I think this also applies to
> > 
> > Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=94625
> > 
> > And maybe a +cc stable?
> > 
> 
> Oops, one more minor issue below that I missed the first time around.
> 
> ...
> > > +	/* bspec says to keep retrying for at least 1 ms */
> > > +	timeout = jiffies + msecs_to_jiffies(1);
> > > +	do {
> > > +		ret = sandybridge_pcode_write(dev_priv, GEN9_PCODE_SAGV_CONTROL,
> > > +					      GEN9_SAGV_DISABLE);
> > > +		if (ret) {
> > > +			DRM_ERROR("Failed to disable the SAGV\n");
> > > +			goto out;
> > > +		}
> > > +
> > > +		ret = sandybridge_pcode_read(dev_priv, GEN9_PCODE_SAGV_CONTROL,
> > > +					     &temp);
> > > +		if (ret) {
> > > +			DRM_ERROR("Failed to check the status of the SAGV\n");
> > > +			goto out;
> > > +		}
> > > +	} while (!(temp & 0x1) && jiffies < timeout);
> 
> We shouldn't compare jiffies directly here due to wraparound; you can
> use time_before() to handle that safely.

We have wait_for()/_wait_for() for polling stuff.

> 
> 
> Matt
> 
> > > +
> > > +	if (temp & 0x1) {
> > > +		dev_priv->skl_sagv_enabled = false;
> > > +	} else {
> > > +		ret = -1;
> > > +		DRM_ERROR("Request to disable SAGV timed out\n");
> > > +	}
> > > +
> > > +out:
> > > +	mutex_unlock(&dev_priv->rps.hw_lock);
> > > +	return ret;
> > > +}
> > > +
> > > +static void
> > >  skl_ddb_get_pipe_allocation_limits(struct drm_device *dev,
> > >  				   const struct intel_crtc_state *cstate,
> > >  				   struct skl_ddb_entry *alloc, /* out */
> > > @@ -3686,6 +3789,11 @@ static void skl_write_wm_values(struct drm_i915_private *dev_priv,
> > >  	struct drm_device *dev = &dev_priv->drm;
> > >  	struct intel_crtc *crtc;
> > >  
> > > +	if (dev_priv->active_crtcs == 1)
> > > +		skl_enable_sagv(dev_priv);
> > > +	else
> > > +		skl_disable_sagv(dev_priv);
> > > +
> > >  	for_each_intel_crtc(dev, crtc) {
> > >  		int i, level, max_level = ilk_wm_max_level(dev);
> > >  		enum pipe pipe = crtc->pipe;
> > > @@ -4228,6 +4336,8 @@ void skl_wm_get_hw_state(struct drm_device *dev)
> > >  		/* Easy/common case; just sanitize DDB now if everything off */
> > >  		memset(ddb, 0, sizeof(*ddb));
> > >  	}
> > > +
> > > +	skl_sagv_get_hw_state(dev_priv);
> > >  }
> > >  
> > >  static void ilk_pipe_wm_get_hw_state(struct drm_crtc *crtc)
> > > -- 
> > > 2.7.4
> > > 
> > 
> > -- 
> > Matt Roper
> > Graphics Software Engineer
> > IoTG Platform Enabling & Development
> > Intel Corporation
> > (916) 356-2795
> > _______________________________________________
> > Intel-gfx mailing list
> > Intel-gfx@lists.freedesktop.org
> > https://lists.freedesktop.org/mailman/listinfo/intel-gfx
> 
> -- 
> Matt Roper
> Graphics Software Engineer
> IoTG Platform Enabling & Development
> Intel Corporation
> (916) 356-2795
> _______________________________________________
> Intel-gfx mailing list
> Intel-gfx@lists.freedesktop.org
> https://lists.freedesktop.org/mailman/listinfo/intel-gfx

-- 
Ville Syrjälä
Intel OTC
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