Re: [PATCH 02/20] drm/i915/dp_mst: Verify the link status always the same way

[Imre Deak <imre.deak@intel.com>]

On Thu, Jun 26, 2025 at 11:31:31AM +0300, Jani Nikula wrote:
> On Thu, 26 Jun 2025, Imre Deak <imre.deak@intel.com> wrote:
> > From: Imre Deak <imre.deak@gmail.com>
> >
> > The MST link status should be always verified from the same DPCD
> > registers after link training. Atm, both the legacy (0x202 - 0x205) and
> > the ESI (0x200C - 0x200F) link status registers are used. Use always the
> > latter ESI version of link status registers.
> >
> > Signed-off-by: Imre Deak <imre.deak@gmail.com>
> > ---
> >  drivers/gpu/drm/i915/display/intel_dp.c | 20 ++++++++++++++++++--
> >  1 file changed, 18 insertions(+), 2 deletions(-)
> >
> > diff --git a/drivers/gpu/drm/i915/display/intel_dp.c b/drivers/gpu/drm/i915/display/intel_dp.c
> > index 3f911c930a30b..ac7e08f485309 100644
> > --- a/drivers/gpu/drm/i915/display/intel_dp.c
> > +++ b/drivers/gpu/drm/i915/display/intel_dp.c
> > @@ -5176,6 +5176,23 @@ intel_dp_handle_hdmi_link_status_change(struct intel_dp *intel_dp)
> >  	}
> >  }
> >  
> > +static bool
> > +intel_dp_read_link_status(struct intel_dp *intel_dp, u8 link_status[DP_LINK_STATUS_SIZE])
> > +{
> > +	bool ret;
> > +
> > +	memset(link_status, 0, DP_LINK_STATUS_SIZE);
> > +
> > +	if (intel_dp_mst_active_streams(intel_dp) > 0)
> > +		ret = drm_dp_dpcd_read_data(&intel_dp->aux, DP_LANE0_1_STATUS_ESI,
> > +					    link_status, DP_LINK_STATUS_SIZE - 2) == 0;
> > +	else
> > +		ret = drm_dp_dpcd_read_phy_link_status(&intel_dp->aux, DP_PHY_DPRX,
> > +						       link_status) == 0;
> 
> Please propagate the int instead of having "== 0" at the end of the
> assignment.
> 
> I'm increasingly critical of bool returns for success/failure, because
> they don't really mix well with 0 for success and negative values for
> error.

Ok will do that.

Both ways are used all around, but I suppose propagating the error code
should be the default choice. An exception being adding a new variant
of an already existing function with a bool success/failure return type
where the new variant should do the same.

> > +
> > +	return ret;
> > +}
> > +
> >  static bool
> >  intel_dp_needs_link_retrain(struct intel_dp *intel_dp)
> >  {
> > @@ -5198,8 +5215,7 @@ intel_dp_needs_link_retrain(struct intel_dp *intel_dp)
> >  	if (intel_dp->link.force_retrain)
> >  		return true;
> >  
> > -	if (drm_dp_dpcd_read_phy_link_status(&intel_dp->aux, DP_PHY_DPRX,
> > -					     link_status) < 0)
> > +	if (!intel_dp_read_link_status(intel_dp, link_status))
> >  		return false;
> >  
> >  	/*
> 
> -- 
> Jani Nikula, Intel