[Intel-gfx] [PATCH v2] drm/i915: intel_dp_link_is_valid() should only return status of link

[Manasi Navare]

On Thu, Aug 11, 2016 at 08:18:54PM -0700, Pandiyan, Dhinakaran wrote:
> On Thu, 2016-08-11 at 15:23 -0700, Manasi Navare wrote:
> > Intel_dp_link_is_valid() function reads the Link status registers
> > and returns a boolean to indicate link is valid or not.
> > If the link has lost lock and is not valid any more, link
> > training is performed outside the function else previously trained link
> > is retained.
> > This gives us flexibility of checking whether link is valid and training
> > it independently.
> > 
> > v2:
> > * Changed the function name from intel_dp_check_link_status()
> > to intel_dp_link_is_valid()  (Lukas Wunner)
> > * Checks for CRTC and active CRTC are moved outside the
> > intel_dp_link_is_valid() function (Rodrigo Vivi)
> > 
> > Signed-off-by: Manasi Navare <manasi.d.navare at intel.com>
> > ---
> >  drivers/gpu/drm/i915/intel_dp.c | 56 +++++++++++++++++++++++++++--------------
> >  1 file changed, 37 insertions(+), 19 deletions(-)
> > 
> > diff --git a/drivers/gpu/drm/i915/intel_dp.c b/drivers/gpu/drm/i915/intel_dp.c
> > index 364db90..891147d 100644
> > --- a/drivers/gpu/drm/i915/intel_dp.c
> > +++ b/drivers/gpu/drm/i915/intel_dp.c
> > @@ -3881,36 +3881,33 @@ go_again:
> >  	return -EINVAL;
> >  }
> >  
> > -static void
> > -intel_dp_check_link_status(struct intel_dp *intel_dp)
> > +static bool
> > +intel_dp_link_is_valid(struct intel_dp *intel_dp)
> >  {
> > -	struct intel_encoder *intel_encoder = &dp_to_dig_port(intel_dp)->base;
> >  	struct drm_device *dev = intel_dp_to_dev(intel_dp);
> >  	u8 link_status[DP_LINK_STATUS_SIZE];
> >  
> >  	WARN_ON(!drm_modeset_is_locked(&dev->mode_config.connection_mutex));
> >  
> >  	if (!intel_dp_get_link_status(intel_dp, link_status)) {
> > -		DRM_ERROR("Failed to get link status\n");
> > -		return;
> > +		DRM_DEBUG_KMS("Failed to get link status\n");
> > +		return false;
> >  	}
> >  
> > -	if (!intel_encoder->base.crtc)
> > -		return;
> > +	/* Check if the link is valid by reading the bits of Link status
> > +	 * registers
> > +	 */
> > +	if (!drm_dp_channel_eq_ok(link_status, intel_dp->lane_count)) {
> > +		DRM_DEBUG_KMS("Channel EQ or CR not ok, need to retrain\n");
> drm_dp_channel_eq_ok() does not check for CR. Should we just say
> "Channel EQ not ok" to preempt ambiguity while debugging ?

Actually this macro checks for DP_CHANNEL_EQ_BITS which is defined as:
#define DP_CHANNEL_EQ_BITS (DP_LANE_CR_DONE |           \
                            DP_LANE_CHANNEL_EQ_DONE |   \
                            DP_LANE_SYMBOL_LOCKED)
So it includes checking for Channel EQ and Clock Recovery CR bits


> 
> > +		return false;
> > +	}
> >  
> > -	if (!to_intel_crtc(intel_encoder->base.crtc)->active)
> > -		return;
> > +	DRM_DEBUG_KMS("Link is good, no need to retrain\n");
> The caller does not expect us to link train anymore, I don't think we
> have to explicitly state "no need to retrain". Also, do we need debug
> messages if the link is good?

I agree , maybe this is not needed. I will remove this

> 
> > +	return true;
> >  
> > -	/* if link training is requested we should perform it always */
> > -	if ((intel_dp->compliance_test_type == DP_TEST_LINK_TRAINING) ||
> > -	    (!drm_dp_channel_eq_ok(link_status, intel_dp->lane_count))) {
> > -		DRM_DEBUG_KMS("%s: channel EQ not ok, retraining\n",
> > -			      intel_encoder->base.name);
> > -		intel_dp_start_link_train(intel_dp);
> > -		intel_dp_stop_link_train(intel_dp);
> > -	}
> >  }
> >  
> > +
> >  /*
> >   * According to DP spec
> >   * 5.1.2:
> > @@ -3928,6 +3925,8 @@ static bool
> >  intel_dp_short_pulse(struct intel_dp *intel_dp)
> >  {
> >  	struct drm_device *dev = intel_dp_to_dev(intel_dp);
> > +	struct intel_digital_port *intel_dig_port = dp_to_dig_port(intel_dp);
> > +	struct intel_encoder *intel_encoder = &intel_dig_port->base;
> >  	u8 sink_irq_vector = 0;
> >  	u8 old_sink_count = intel_dp->sink_count;
> >  	bool ret;
> > @@ -3968,8 +3967,18 @@ intel_dp_short_pulse(struct intel_dp *intel_dp)
> >  			DRM_DEBUG_DRIVER("CP or sink specific irq unhandled\n");
> >  	}
> >  
> > +	/* Do not train the link if there is no crtc */
> > +	if (!intel_encoder->base.crtc)
> > +		return true;
> > +	if (!to_intel_crtc(intel_encoder->base.crtc)->active)
> > +		return true;
> > +
> I might be completely off base here. Shouldn't we keep the link valid
> irrespective of whether there is an active crtc? I thought that is what
> the refactoring is supposed to enable. Does intel_dp_short_pulse() get
> called when there is a link loss during upfront link training? And in
> that case, shouldn't we retrain even without a crtc? 

We cannot ever retrain without a CRTC. This check is more for making sure that the clocks
are set up befofe we try to retrain else we will see AUX channel failures.
If I track this back in the kernel tree, this check was added to avoid the lock up issues on some
platforms.

> 
> Besides that, how about using just one return?
> 
> struct drm_crtc *crtc = intel_encoder->base.crtc;
> 
> if (crtc == NULL || !to_intel_crtc(crtc)->active)
> 	return true;
> 
>

The only problem with doing both these checks together is that if crtc is NULL
then we are trying to dereference a NULL pointer in the second check.
So it should be seuqential, check if crtc is active only if there is crtc available.

Manasi
 
> >  	drm_modeset_lock(&dev->mode_config.connection_mutex, NULL);
> > -	intel_dp_check_link_status(intel_dp);
> > +	if (!intel_dp_link_is_valid(intel_dp) ||
> > +	    intel_dp->compliance_test_type == DP_TEST_LINK_TRAINING) {
> > +		intel_dp_start_link_train(intel_dp);
> > +		intel_dp_stop_link_train(intel_dp);
> > +	}
> >  	drm_modeset_unlock(&dev->mode_config.connection_mutex);
> >  
> >  	return true;
> > @@ -4298,8 +4307,17 @@ intel_dp_long_pulse(struct intel_connector *intel_connector)
> >  		 * check links status, there has been known issues of
> >  		 * link loss triggerring long pulse!!!!
> >  		 */
> > +		/* Do not train the link if there is no crtc */
> > +		if (!intel_encoder->base.crtc)
> > +			goto out;
> > +		if (!to_intel_crtc(intel_encoder->base.crtc)->active)
> > +			goto out;
> > +
> >  		drm_modeset_lock(&dev->mode_config.connection_mutex, NULL);
> > -		intel_dp_check_link_status(intel_dp);
> > +		if (!intel_dp_link_is_valid(intel_dp)) {
> > +			intel_dp_start_link_train(intel_dp);
> > +			intel_dp_stop_link_train(intel_dp);
> > +		}
> >  		drm_modeset_unlock(&dev->mode_config.connection_mutex);
> >  		goto out;
> >  	}
>