[Intel-gfx] [PATCH] drm/i915/dp: Poll for DDI Idle status to be 0 after enabling DDI Buf

[Ville Syrjälä]

On Tue, Jun 16, 2020 at 10:42:44PM +0300, Ville Syrjälä wrote:
> On Tue, Jun 16, 2020 at 12:30:56PM -0700, Manasi Navare wrote:
> > The Bspec sequence expects us to poll for DDI Idle status
> > to be 0 (not idle) with a timeout of 600usecs after enabling the
> > DDI BUF CTL.
> 
> It only says that for newer platforms. We need to either keep
> the fixed delay before starting to poll, or someone needs confirm 
> how the idle bit really behaves on the older platforms.

In fact it says not to use this bit at all on BXT. So even our disable
sequence is potentially borked on BXT. Unfortunately the spec doesn't
say which way the bit is broken, so not clear if that's the case or
not.

> 
> > But currently in the driver we just wait for 600usecs
> > without polling so add that.
> > 
> > Cc: Ville Syrjälä <ville.syrjala at linux.intel.com>
> > Cc: Imre Deak <imre.deak at intel.com>
> > Signed-off-by: Manasi Navare <manasi.d.navare at intel.com>
> > ---
> >  drivers/gpu/drm/i915/display/intel_ddi.c | 6 +++++-
> >  1 file changed, 5 insertions(+), 1 deletion(-)
> > 
> > diff --git a/drivers/gpu/drm/i915/display/intel_ddi.c b/drivers/gpu/drm/i915/display/intel_ddi.c
> > index ca7bb2294d2b..de7e15de0bc5 100644
> > --- a/drivers/gpu/drm/i915/display/intel_ddi.c
> > +++ b/drivers/gpu/drm/i915/display/intel_ddi.c
> > @@ -4023,7 +4023,11 @@ static void intel_ddi_prepare_link_retrain(struct intel_dp *intel_dp)
> >  	intel_de_write(dev_priv, DDI_BUF_CTL(port), intel_dp->DP);
> >  	intel_de_posting_read(dev_priv, DDI_BUF_CTL(port));
> >  
> > -	udelay(600);
> > +	if (wait_for_us(!(intel_de_read(dev_priv, DDI_BUF_CTL(port)) &
> > +			  DDI_BUF_IS_IDLE),
> > +			600))
> > +		drm_err(&dev_priv->drm, "DDI port:%c buffer idle\n",
> > +			port_name(port));
> >  }
> >  
> >  static void intel_ddi_set_link_train(struct intel_dp *intel_dp,
> > -- 
> > 2.19.1
> 
> -- 
> Ville Syrjälä
> Intel
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-- 
Ville Syrjälä
Intel