[Intel-gfx] [PATCH 2/2] drm/i915: Ignore long hpds on eDP ports

[Ville Syrjälä]

On Thu, Oct 16, 2014 at 12:38:55PM -0700, Todd Previte wrote:
> 
> On 10/16/2014 10:46 AM, ville.syrjala at linux.intel.com wrote:
> > From: Ville Syrjälä <ville.syrjala at linux.intel.com>
> >
> > Turning vdd on/off can generate a long hpd pulse on eDP ports. In order
> > to handle hpd we would need to turn on vdd to perform aux transfers.
> > This would lead to an endless cycle of
> > "vdd off -> long hpd -> vdd on -> detect -> vdd off -> ..."
> >
> > So ignore long hpd pulses on eDP ports. eDP panels should be physically
> > tied to the machine anyway so they should not actually disappear and
> > thus don't need long hpd handling. Short hpds are still needed for link
> > re-train and whatnot so we can't just turn off the hpd interrupt
> > entirely for eDP ports. Perhaps we could turn it off whenever the panel
> > is disabled, but just ignoring the long hpd seems sufficient.
> >
> > Signed-off-by: Ville Syrjälä <ville.syrjala at linux.intel.com>
> > ---
> >   drivers/gpu/drm/i915/intel_dp.c | 12 ++++++++++++
> >   1 file changed, 12 insertions(+)
> >
> > diff --git a/drivers/gpu/drm/i915/intel_dp.c b/drivers/gpu/drm/i915/intel_dp.c
> > index f07f02c..4455009 100644
> > --- a/drivers/gpu/drm/i915/intel_dp.c
> > +++ b/drivers/gpu/drm/i915/intel_dp.c
> > @@ -4611,6 +4611,18 @@ intel_dp_hpd_pulse(struct intel_digital_port *intel_dig_port, bool long_hpd)
> >   	if (intel_dig_port->base.type != INTEL_OUTPUT_EDP)
> >   		intel_dig_port->base.type = INTEL_OUTPUT_DISPLAYPORT;
> >   
> > +	if (long_hpd && intel_dig_port->base.type == INTEL_OUTPUT_EDP) {
> > +		/*
> > +		 * vdd off can generate a long pulse on eDP which
> > +		 * would require vdd on to handle it, and thus we
> > +		 * would end up in an endless cycle of
> > +		 * "vdd off -> long hpd -> vdd on -> detect -> vdd off -> ..."
> > +		 */
> > +		DRM_DEBUG_KMS("ignoring long hpd on eDP port %c\n",
> > +			      port_name(intel_dig_port->port));
> > +		return false;
> > +	}
> > +
> >   	DRM_DEBUG_KMS("got hpd irq on port %c - %s\n",
> >   		      port_name(intel_dig_port->port),
> >   		      long_hpd ? "long" : "short");
> I'm not sure that ignoring a long pulse is the best way to handle it. 
> eDP does not appear to differentiate between short and long pulses per 
> the specification (not to mention that HPD support for eDP is optional 
> in the first place). It seems to me that it would probably be better to 
> handle them as a normal (short) HPD pulse and just do the regular link 
> maintenance stuff. As I said, the spec doesn't differentiate between the 
> long and short pulses for eDP so it's a safer bet to handle them as a 
> short pulse than to ignore them entirely.

The spec does talk a lot about IRQ_HPD (which is the short pulse) and
the power sequencing diagrams explicitly show that HPD should be asserted
after the T3 delay and deasserted immediately on VDD off, so those would
be the long pulses. So based on that my patch seems to make sense.

It seems HPD is optional for the source only, in the sink it's madatory.
But that doesn't really matter anyway because if either end doesn't have
it it won't work. The spec does go on to say that we should periodically
poll the sink status if HPD_IRQ isn't available. We don't do that
currently, but it does sound like a sane idea in case the link drops out.

-- 
Ville Syrjälä
Intel OTC