Re: [RFC 3/4] drm/i915: valleyview: Make intel_set_rps get FORCEWAKE_MEDIA

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

On Mon, Jan 02, 2017 at 05:02:25PM +0200, Ville Syrjälä wrote:
> On Mon, Jan 02, 2017 at 02:53:59PM +0000, Chris Wilson wrote:
> > On Mon, Jan 02, 2017 at 04:40:05PM +0200, Ville Syrjälä wrote:
> > > On Mon, Jan 02, 2017 at 02:21:58PM +0000, Chris Wilson wrote:
> > > > On Mon, Jan 02, 2017 at 04:10:49PM +0200, Ville Syrjälä wrote:
> > > > > On Mon, Jan 02, 2017 at 11:37:59AM +0000, Chris Wilson wrote:
> > > > > > On Sun, Jan 01, 2017 at 09:48:53PM +0100, Hans de Goede wrote:
> > > > > > > Hi,
> > > > > > > 
> > > > > > > On 01-01-17 21:24, Chris Wilson wrote:
> > > > > > > >On Sun, Jan 01, 2017 at 09:14:02PM +0100, Hans de Goede wrote:
> > > > > > > >>All callers of valleyview_set_rps() get at least FORCEWAKE_MEDIA, except
> > > > > > > >>for intel_set_rps(). Since intel_set_rps can for example be called from
> > > > > > > >>sysfs store functions, there is no guarantee this is already done, so add
> > > > > > > >>an intel_uncore_forcewake_get(FORCEWAKE_MEDIA) call to intel_set_rps.
> > > > > > > >>
> > > > > > > >>Signed-off-by: Hans de Goede <hdegoede@redhat.com>
> > > > > > > >>---
> > > > > > > >> drivers/gpu/drm/i915/intel_pm.c | 9 +++++++--
> > > > > > > >> 1 file changed, 7 insertions(+), 2 deletions(-)
> > > > > > > >>
> > > > > > > >>diff --git a/drivers/gpu/drm/i915/intel_pm.c b/drivers/gpu/drm/i915/intel_pm.c
> > > > > > > >>index 4b12637..cc4fbd7 100644
> > > > > > > >>--- a/drivers/gpu/drm/i915/intel_pm.c
> > > > > > > >>+++ b/drivers/gpu/drm/i915/intel_pm.c
> > > > > > > >>@@ -5096,9 +5096,14 @@ void gen6_rps_boost(struct drm_i915_private *dev_priv,
> > > > > > > >>
> > > > > > > >> void intel_set_rps(struct drm_i915_private *dev_priv, u8 val)
> > > > > > > >> {
> > > > > > > >>-	if (IS_VALLEYVIEW(dev_priv) || IS_CHERRYVIEW(dev_priv))
> > > > > > > >>+	if (IS_VALLEYVIEW(dev_priv) || IS_CHERRYVIEW(dev_priv)) {
> > > > > > > >>+		/* Wake up the media well, as that takes a lot less
> > > > > > > >>+		 * power than the Render well.
> > > > > > > >>+		 */
> > > > > > > >>+		intel_uncore_forcewake_get(dev_priv, FORCEWAKE_MEDIA);
> > > > > > > >> 		valleyview_set_rps(dev_priv, val);
> > > > > > > >
> > > > > > > >Both powerwells are woken for rps. (Taking one but not the other has no
> > > > > > > >benefit, and very misleading.)
> > > > > > > 
> > > > > > > The comment on why FORCEWAKE_MEDIA is used + code is copy pasted from the
> > > > > > > existing code in vlv_set_rps_idle().
> > > > > > 
> > > > > > It's not correct there either...
> > > > > 
> > > > > Why not? If the render well is in rc6 already we don't want to waste
> > > > > power by waking it. The only reason we have to wake up *something* is
> > > > > so that the gpll frequency actually gets changed.
> > > > 
> > > > If the register write requires the powerwell, the mmio access will take
> > > > the powerwell.
> > > 
> > > The register write doesn't require a power well. It's a sideband access.
> > > The punit will simply not change the GPLL frequency if the GPLL is
> > > currently not running (which will/can happen when all power wells are
> > > asleep). That in itself doesn't sound too back (why change the
> > > frequency if the thing isn't even running, right?). But the real problem
> > > is that the punit will not let the voltage on the rail to drop
> > > either until the new frequency gets programmed into the GPLL. Hence if
> > > the GPU goes idle before we've dropped the GPLL frequency to the
> > > minimum value, we will waste power by having a needlessly high voltage.
> > > 
> > > Originally we tried to avoid this problem via vlv_force_gfx_clock(),
> > > which should just force the GPLL to turn on without powering on
> > > any power wells. But that caused some spurious WARNs or something
> > > IIRC, so we had to come up with another workaround. And since powering
> > > either well is sufficient we chose to use the cheaper media well.
> > 
> > That explains set_idle() (and would be a better comment that the one
> > there). But not set_rps(), since there we don't care if the write is
> > delayed until the GPU is next active.
> 
> I don't see any forcewakes in set_rps()

Ah, it was this patch which wants to add that. And indeed that doesn't
make sense.

-- 
Ville Syrjälä
Intel OTC
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