[Intel-gfx] [PATCH] drm/i915/cnp: Ignore VBT request for know invalid DDC pin.

[Rodrigo Vivi]

On Wed, Jan 24, 2018 at 09:25:06AM +0000, Jani Nikula wrote:
> On Tue, 23 Jan 2018, Rodrigo Vivi <rodrigo.vivi at intel.com> wrote:
> > On Tue, Jan 23, 2018 at 05:40:50PM +0000, Rodrigo Vivi wrote:
> >> Let's ignore VBT request if the pin is clearly wrong.
> >> 
> >> Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=104139
> >> Cc: Kai Heng Feng <kai.heng.feng at canonical.com>
> >> Signed-off-by: Rodrigo Vivi <rodrigo.vivi at intel.com>
> >
> > Reviewed-by: Radhakrishna Sripada <radhakrishna.sripada at intel.com>
> > (f2f)
> > thanks
> >
> > merged to dinq.
> 
> A bit too fast IMO. The bug report doesn't include the vbt dump which
> you should *always* look at before making workarounds for vbt.

my apologies. My biggest mistake was taking so long to reply to the
bug and taking so long to post the patch here. :(

I looked to the VBT now and it is really just this ddc pin for this
particular port wrong. Or should I look to something else?

What other kind of useful information would that bring to us
normally?

> 
> Also, see below.
> 
> >> ---
> >>  drivers/gpu/drm/i915/intel_bios.c | 11 ++++++++---
> >>  1 file changed, 8 insertions(+), 3 deletions(-)
> >> 
> >> diff --git a/drivers/gpu/drm/i915/intel_bios.c b/drivers/gpu/drm/i915/intel_bios.c
> >> index b0668202dc7e..95f0b310d656 100644
> >> --- a/drivers/gpu/drm/i915/intel_bios.c
> >> +++ b/drivers/gpu/drm/i915/intel_bios.c
> >> @@ -1115,9 +1115,14 @@ static const u8 cnp_ddc_pin_map[] = {
> >>  
> >>  static u8 map_ddc_pin(struct drm_i915_private *dev_priv, u8 vbt_pin)
> >>  {
> >> -	if (HAS_PCH_CNP(dev_priv) &&
> >> -	    vbt_pin > 0 && vbt_pin < ARRAY_SIZE(cnp_ddc_pin_map))
> >> -		return cnp_ddc_pin_map[vbt_pin];
> >> +	if (HAS_PCH_CNP(dev_priv)) {
> >> +		if (vbt_pin > 0 && vbt_pin < ARRAY_SIZE(cnp_ddc_pin_map))
> >> +			return cnp_ddc_pin_map[vbt_pin];
> >> +		if (vbt_pin > GMBUS_PIN_4_CNP) {
> >> +			DRM_DEBUG_KMS("Ignoring alternate pin: VBT claims DDC pin %d, which is not valid for this platform\n", vbt_pin);
> >> +			return 0;
> >> +		}
> 
> The original code was wrong in the sense that it would only do the
> mapping if the provided vbt_pin was within bounds for CNP, and would
> return the unmapped vbt_pin otherwise. Instead, I think you should
> directly return 0 for out of bounds vbt_pin.
> 
> This change here is effectively the same, but more confusing. As if
> there's a case where the vbt_pin can be both out of bounds, within CNP
> pin limits, and should be returned unmapped. That can't happen, so
> please make the code reflect that.

Yes. That's a great idea. Thanks for the suggestion.

Honestly, at first I assumed our behavior was something
like this so I couldn't believe we had the issue even with
the original mapping there. But when I did the last fix
I avoided changing the original behavior and messed things up. Sorry.

Thanks,
Rodrigo.

> 
> BR,
> Jani.
> 
> 
> >> +	}
> >>  
> >>  	return vbt_pin;
> >>  }
> >> -- 
> >> 2.13.6
> >> 
> > _______________________________________________
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> > Intel-gfx at lists.freedesktop.org
> > https://lists.freedesktop.org/mailman/listinfo/intel-gfx
> 
> -- 
> Jani Nikula, Intel Open Source Technology Center