On Tue, Oct 19, 2021 at 10:37:33AM +0300, Jani Nikula wrote: > On Tue, 19 Oct 2021, Imre Deak <imre.deak@xxxxxxxxx> wrote: > > On Tue, Oct 19, 2021 at 10:27:18AM +0300, Jani Nikula wrote: > >> On Mon, 18 Oct 2021, Imre Deak <imre.deak@xxxxxxxxx> 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@xxxxxxxxxxxxxxx> > >> > Cc: Ville Syrjälä <ville.syrjala@xxxxxxxxxxxxxxx> > >> > Cc: <stable@xxxxxxxxxxxxxxx> > >> > Signed-off-by: Imre Deak <imre.deak@xxxxxxxxx> > >> > --- > >> > 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
