Re: [PATCH] drm/i915: Don't load context at driver init time on SNB

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

On Mon, Aug 12, 2013 at 10:33:37AM -0700, Stéphane Marchesin wrote:
> On Fri, Aug 9, 2013 at 9:55 PM, Ben Widawsky <ben@bwidawsk.net> wrote:
> > On Fri, Aug 09, 2013 at 08:32:54PM -0700, Stéphane Marchesin wrote:
> >> This is a partial revert of b4ae3f22d238617ca11610b29fde16cf8c0bc6e0
> >> (drm/i915: load boot context at driver init time)
> >>
> >> This bit breaks hardware video decode for me after resume.
> >>
> >> Signed-off-by: Stéphane Marchesin <marcheu@chromium.org>
> >> ---
> >>  drivers/gpu/drm/i915/intel_pm.c | 4 ----
> >>  1 file changed, 4 deletions(-)
> >>
> >> diff --git a/drivers/gpu/drm/i915/intel_pm.c b/drivers/gpu/drm/i915/intel_pm.c
> >> index f895d15..ffa4ab2 100644
> >> --- a/drivers/gpu/drm/i915/intel_pm.c
> >> +++ b/drivers/gpu/drm/i915/intel_pm.c
> >> @@ -4614,10 +4614,6 @@ static void gen6_init_clock_gating(struct drm_device *dev)
> >>                  ILK_DPARBUNIT_CLOCK_GATE_ENABLE  |
> >>                  ILK_DPFDUNIT_CLOCK_GATE_ENABLE);
> >>
> >> -     /* WaMbcDriverBootEnable:snb */
> >> -     I915_WRITE(GEN6_MBCTL, I915_READ(GEN6_MBCTL) |
> >> -                GEN6_MBCTL_ENABLE_BOOT_FETCH);
> >> -
> >>       g4x_disable_trickle_feed(dev);
> >>
> >>       /* The default value should be 0x200 according to docs, but the two
> >
> > I was looking at this a bit with Stéphane. One thing we both see is that
> > the bit isn't sticking as it should. Clearly doing the write is having
> > an effect, but something is seriously wrong. Writing the bit manually
> > with IGT does make the bit stick.
> >
> > Stéphane, could you try to write the bit with IGT before and after
> > resume, and see if it helps?
> 
> It doesn't seem to stick, and so seems to have no effect.
> 
> The confusing thing is that video decode works fine before suspend,
> even though that reg is 0. And after resume, it is broken, and that
> reg is still 0. So something else is going on. Maybe this reg is
> write-once-ever?

BSpec says "This Bit is cleared by the Hardware once the Boot fetch is
complete."

-- 
Ville Syrjälä
Intel OTC