Re: [PATCH 06/10] drm/i915: check the power down well on assert_pipe()

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

On Fri, Jan 25, 2013 at 01:40:08PM -0200, Paulo Zanoni wrote:
> Hi
> 
> 2013/1/21 Ville Syrjälä <ville.syrjala@linux.intel.com>:
> > On Fri, Jan 18, 2013 at 06:29:08PM -0200, Paulo Zanoni wrote:
> >> From: Paulo Zanoni <paulo.r.zanoni@intel.com>
> >>
> >> If the power well is disabled, we should not try to read its
> >> registers, otherwise we'll get "unclaimed register" messages.
> >>
> >> Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
> >> ---
> >>  drivers/gpu/drm/i915/intel_display.c |   12 +++++++++---
> >>  1 file changed, 9 insertions(+), 3 deletions(-)
> >>
> >> diff --git a/drivers/gpu/drm/i915/intel_display.c b/drivers/gpu/drm/i915/intel_display.c
> >> index a7fb7e1..921b020 100644
> >> --- a/drivers/gpu/drm/i915/intel_display.c
> >> +++ b/drivers/gpu/drm/i915/intel_display.c
> >> @@ -1214,9 +1214,15 @@ void assert_pipe(struct drm_i915_private *dev_priv,
> >>       if (pipe == PIPE_A && dev_priv->quirks & QUIRK_PIPEA_FORCE)
> >>               state = true;
> >>
> >> -     reg = PIPECONF(cpu_transcoder);
> >> -     val = I915_READ(reg);
> >> -     cur_state = !!(val & PIPECONF_ENABLE);
> >> +     if (cpu_transcoder == TRANSCODER_EDP ||
> >> +         (I915_READ(HSW_PWR_WELL_DRIVER) & HSW_PWR_WELL_STATE)) {
> >
> > Should that also check HSW_PWR_WELL_ENABLE? KVMR might have the well
> > enabled, while the driver has it disabled. But KVMR might have already
> > disabled the well, and it might get disabled just after this check,
> > and then you would hit the unclaimed register issue again.
> 
> By reading your text it seems you think that if bit 31 is 0 then bit
> 30 will necessarily be 0. This is not true. If any entity (including
> KVMr) needs the power well enabled (and is requesting so), then bit 30
> of HSW_PWR_WELL_DRIVER will be 1, no matter the status of bit 31.

Did you verify that on real HW?

It makes little sense when you consider that the spec says not
to toggle the request bit when there's a state transition currently
happening. If all the status bits read as the same thing, then
there's no way to know when a transition is actually happening,
and hence the text in the spec is complete nonsense.

The only sensible interpretation of that text I could think of is
that you're not supposed to abort a state transition you yourself
caused. That would be achieved by polling the status bit after
toggling the request bit, assuming that the status bit is just a
delayed version of request bit.

> Another thing to mention is that no entity can force the power well to
> be "off", it can only force to be "on".
> 
> Still, your text does contain an example of a problem with the current
> code. Let me explicitly write it:
> - Our driver doesn't need it to be enabled, so bit 31 of
> HSW_PWR_WELL_DRIVER is 0
> - KVMr needs it enabled, so HSW_PWR_WELL_DRIVER bit 30 is 1
> - We do the I915_READ above
> - After the read, KVMr says it doesn't need the power well to be
> enabled anymore, so it gets disabled because no one else is requesting
> it
> - Then we touch an unclaimed register.
> 
> I think the only policy to avoid these race conditions should be "if I
> don't need the power well to be enabled, then I won't touch registers
> that require it to be enabled, no matter what the real state of the
> power well is".This should prevent any kinds of problems, because if I
> need the power well to be enabled, then I requested it to be enabled
> and it is enabled because I asked so. I'll send a second version of
> the patch with this.
> 
> >
> >> +             reg = PIPECONF(cpu_transcoder);
> >> +             val = I915_READ(reg);
> >> +             cur_state = !!(val & PIPECONF_ENABLE);
> >> +     } else {
> >> +             cur_state = false;
> >> +     }
> >> +
> >>       WARN(cur_state != state,
> >>            "pipe %c assertion failure (expected %s, current %s)\n",
> >>            pipe_name(pipe), state_string(state), state_string(cur_state));
> >> --
> >> 1.7.10.4
> >>
> >> _______________________________________________
> >> Intel-gfx mailing list
> >> Intel-gfx@lists.freedesktop.org
> >> http://lists.freedesktop.org/mailman/listinfo/intel-gfx
> >
> > --
> > Ville Syrjälä
> > Intel OTC
> 
> 
> 
> -- 
> Paulo Zanoni

-- 
Ville Syrjälä
Intel OTC