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? 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
