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

[Paulo Zanoni]

Hi

2013/1/25 Ville Syrjälä <ville.syrjala at linux.intel.com>:
> On Fri, Jan 25, 2013 at 01:40:08PM -0200, Paulo Zanoni wrote:
>> Hi
>>
>> 2013/1/21 Ville Syrjälä <ville.syrjala at linux.intel.com>:
>> > On Fri, Jan 18, 2013 at 06:29:08PM -0200, Paulo Zanoni wrote:
>> >> From: Paulo Zanoni <paulo.r.zanoni at 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 at 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?

Yes.

>
> 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.
>

I agree... I'm investigating this problem. That's why there is still
no new version of patch 3/10 here :)

> 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.

If we're enabling then it's fine: we requested it to enable, then at
some point it will be enabled, then we can start using the registers.
We can always avoid touching the registers when the well is still
being enabled. The only situation where the "restriction" doesn't make
sense is on the "disable" case.

OTOH, if we requested it to be disabled, the fact that we can't
predict whether it's really going to be disabled or not shouldn't
really matter that much since we're not supposed to touch the
"possibly disabled" registers, and if anybody needs to touch those
registers they will have to request the well to be enabled first...
This still doesn't explain the "Restriction", but maybe that
restriction is just for the "enable" case.

>
>> 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 at lists.freedesktop.org
>> >> http://lists.freedesktop.org/mailman/listinfo/intel-gfx
>> >
>> > --
>> > Ville Syrjälä
>> > Intel OTC
>>
>>
>>
>> --
>> Paulo Zanoni
>
> --
> Ville Syrjälä
> Intel OTC



-- 
Paulo Zanoni