[Intel-gfx] [PATCH v2 1/2] drm/i915/mst: Do not retrain new links

[Dhinakaran Pandiyan]

On Wed, 2018-07-18 at 10:45 -0700, Manasi Navare wrote:
> On Wed, Jul 18, 2018 at 10:19:42AM -0700, Dhinakaran Pandiyan wrote:
> > 
> > The short pulse handler checks if channel equalization is okay and
> > goes onto retrain a link if there are active MST links. This
> > retraining
> > path is not meant for new MST connections, but due to a bug
> > elsewhere, if
> > active_mst_links is < 0 the boolean check for active_mst_links
> > passes and
> This bug is probably around the way we track the active_mst_links and
> we are
> probably decrementing it more times than the available links

Yeah, that indeed is one aspect of the bug.

>  and since its an int
> variable it goes to negative which is not the expected behaviour.
> Why not change this active_mst_links variable to be an unsigned int
> since
> we do not expect any negative values for this anyways.
> That way we can still check against just intel_dp->active_mst_links
> as opposed checking
> for it being greater than 0 and would also not need a WARN_ON here.

I did not get this, mind sharing code diff?

-DK

> 
> Manasi
> 
> > 
> > we proceed to retrain a new link. This results in a sequence of
> > failed link
> > training attempts, most likely due to the hardware not setup for
> > link
> > training at that point i.e., missing the DDI pre_enable sequence.
> > 
> > [   80.301272] [drm:intel_dp_check_mst_status] channel EQ not ok,
> > retraining
> > [   80.301312] [drm:intel_ddi_prepare_link_retrain] *ERROR* Timeout
> > waiting for DDI BUF C idle bit
> > 
> > The above error gives us a hint something went wrong before link
> > training started.
> > 
> > Check for a positive value of active_mst_links and throw in a
> > warning for
> > invalid active_mst_links as debug aid.
> > 
> > Cc: Nathan Ciobanu <nathan.d.ciobanu at linux.intel.com>
> > Cc: Rodrigo Vivi <rodrigo.vivi at intel.com>
> > Signed-off-by: Dhinakaran Pandiyan <dhinakaran.pandiyan at intel.com>
> > ---
> >  drivers/gpu/drm/i915/intel_dp.c | 4 +++-
> >  1 file changed, 3 insertions(+), 1 deletion(-)
> > 
> > diff --git a/drivers/gpu/drm/i915/intel_dp.c
> > b/drivers/gpu/drm/i915/intel_dp.c
> > index b45b08420c1f..2d61ff01cf51 100644
> > --- a/drivers/gpu/drm/i915/intel_dp.c
> > +++ b/drivers/gpu/drm/i915/intel_dp.c
> > @@ -4213,12 +4213,14 @@ intel_dp_check_mst_status(struct intel_dp
> > *intel_dp)
> >  		int ret = 0;
> >  		int retry;
> >  		bool handled;
> > +
> > +		WARN_ON_ONCE(intel_dp->active_mst_links < 0);
> >  		bret = intel_dp_get_sink_irq_esi(intel_dp, esi);
> >  go_again:
> >  		if (bret == true) {
> >  
> >  			/* check link status - esi[10] = 0x200c */
> > -			if (intel_dp->active_mst_links &&
> > +			if (intel_dp->active_mst_links > 0 &&
> >  			    !drm_dp_channel_eq_ok(&esi[10],
> > intel_dp->lane_count)) {
> >  				DRM_DEBUG_KMS("channel EQ not ok,
> > retraining\n");
> >  				intel_dp_start_link_train(intel_dp
> > );