Re: [Intel-gfx] [PATCH 2/6] drm/i915/dp: Ensure sink rate values are always valid

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

On Tue, Oct 19, 2021 at 10:37:33AM +0300, Jani Nikula wrote:
> On Tue, 19 Oct 2021, Imre Deak <imre.deak@intel.com> wrote:
> > On Tue, Oct 19, 2021 at 10:27:18AM +0300, Jani Nikula wrote:
> >> On Mon, 18 Oct 2021, Imre Deak <imre.deak@intel.com> wrote:
> >> > Atm, there are no sink rate values set for DP (vs. eDP) sinks until the
> >> > DPCD capabilities are successfully read from the sink. During this time
> >> > intel_dp->num_common_rates is 0 which can lead to a
> >> >
> >> > intel_dp->common_rates[-1]    (*)
> >> >
> >> > access, which is an undefined behaviour, in the following cases:
> >> >
> >> > - In intel_dp_sync_state(), if the encoder is enabled without a sink
> >> >   connected to the encoder's connector (BIOS enabled a monitor, but the
> >> >   user unplugged the monitor until the driver loaded).
> >> > - In intel_dp_sync_state() if the encoder is enabled with a sink
> >> >   connected, but for some reason the DPCD read has failed.
> >> > - In intel_dp_compute_link_config() if modesetting a connector without
> >> >   a sink connected on it.
> >> > - In intel_dp_compute_link_config() if modesetting a connector with a
> >> >   a sink connected on it, but before probing the connector first.
> >> >
> >> > To avoid the (*) access in all the above cases, make sure that the sink
> >> > rate table - and hence the common rate table - is always valid, by
> >> > setting a default minimum sink rate when registering the connector
> >> > before anything could use it.
> >> >
> >> > I also considered setting all the DP link rates by default, so that
> >> > modesetting with higher resolution modes also succeeds in the last two
> >> > cases above. However in case a sink is not connected that would stop
> >> > working after the first modeset, due to the LT fallback logic. So this
> >> > would need more work, beyond the scope of this fix.
> >> >
> >> > As I mentioned in the previous patch, I don't think the issue this patch
> >> > fixes is user visible, however it is an undefined behaviour by
> >> > definition and triggers a BUG() in CONFIG_UBSAN builds, hence CC:stable.
> >> 
> >> I think the question here, and in the following patches, is whether this
> >> papers over potential bugs elsewhere.
> >> 
> >> Would the original bug fixed by patch 1 have been detected if all the
> >> safeguards here had been in place? Point being, we shouldn't be doing
> >> any of these things before we've read the dpcd.
> >
> > Modesets are possible even without a connected sink or a read-out DPCD,
> > so the link parameters need to be valid even without those.
> 
> Modeset on a disconnected DP? How?

Yes, just do a modeset on it. It doesn't have to be disconnected either,
you can modeset a DP connector before probing it.

> 
> BR,
> Jani.
> 
> 
> >
> >> BR,
> >> Jani.
> >> 
> >> 
> >> >
> >> > Closes: https://gitlab.freedesktop.org/drm/intel/-/issues/4297
> >> > References: https://gitlab.freedesktop.org/drm/intel/-/issues/4298
> >> > Suggested-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
> >> > Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
> >> > Cc: <stable@vger.kernel.org>
> >> > Signed-off-by: Imre Deak <imre.deak@intel.com>
> >> > ---
> >> >  drivers/gpu/drm/i915/display/intel_dp.c | 8 ++++++++
> >> >  1 file changed, 8 insertions(+)
> >> >
> >> > diff --git a/drivers/gpu/drm/i915/display/intel_dp.c b/drivers/gpu/drm/i915/display/intel_dp.c
> >> > index 23de500d56b52..153ae944a354b 100644
> >> > --- a/drivers/gpu/drm/i915/display/intel_dp.c
> >> > +++ b/drivers/gpu/drm/i915/display/intel_dp.c
> >> > @@ -120,6 +120,12 @@ bool intel_dp_is_uhbr(const struct intel_crtc_state *crtc_state)
> >> >  	return crtc_state->port_clock >= 1000000;
> >> >  }
> >> >  
> >> > +static void intel_dp_set_default_sink_rates(struct intel_dp *intel_dp)
> >> > +{
> >> > +	intel_dp->sink_rates[0] = 162000;
> >> > +	intel_dp->num_sink_rates = 1;
> >> > +}
> >> > +
> >> >  /* update sink rates from dpcd */
> >> >  static void intel_dp_set_sink_rates(struct intel_dp *intel_dp)
> >> >  {
> >> > @@ -5003,6 +5009,8 @@ intel_dp_init_connector(struct intel_digital_port *dig_port,
> >> >  	}
> >> >  
> >> >  	intel_dp_set_source_rates(intel_dp);
> >> > +	intel_dp_set_default_sink_rates(intel_dp);
> >> > +	intel_dp_set_common_rates(intel_dp);
> >> >  
> >> >  	if (IS_VALLEYVIEW(dev_priv) || IS_CHERRYVIEW(dev_priv))
> >> >  		intel_dp->pps.active_pipe = vlv_active_pipe(intel_dp);
> >> 
> >> -- 
> >> Jani Nikula, Intel Open Source Graphics Center
> 
> -- 
> Jani Nikula, Intel Open Source Graphics Center