On Sun, Dec 31, 2017 at 11:34:54PM +0100, Stefan Brüns wrote: > The ACK/NACK implementation as found in e.g. the G965 has the falling > clock edge and the release of the data line after the ACK for the received > byte happen at the same time. > > This is conformant with the I2C specification, which allows a zero hold > time, see footnote [3]: "A device must internally provide a hold time of > at least 300 ns for the SDA signal (with respect to the V IH(min) of the > SCL signal) to bridge the undefined region of the falling edge of SCL." > > Some HDMI-to-VGA converters apparently fail to adhere to this requirement > and latch SDA at the falling clock edge, so instead of an ACK > sometimes a NACK is read and the slave (i.e. the EDID ROM) ends the > transfer. > > The bitbanging releases the data line for the ACK only 1/4 bit time after > the falling clock edge, so a slave will see the correct value no matter > if it samples at the rising or the falling clock edge or in the center. > > Fallback to bitbanging is already done for the CRT connector. > > Bug: https://bugs.freedesktop.org/show_bug.cgi?id=92685 > > Signed-off-by: Stefan Brüns <stefan.bruens@xxxxxxxxxxxxxx> > > --- > > Changes in v2: > - Fix/enhance commit message, no code changes Ok, found v2, merged this one instead. -Daniel > > drivers/gpu/drm/i915/intel_hdmi.c | 14 +++++++++++--- > 1 file changed, 11 insertions(+), 3 deletions(-) > > diff --git a/drivers/gpu/drm/i915/intel_hdmi.c b/drivers/gpu/drm/i915/intel_hdmi.c > index 4dea833f9d1b..847cda4c017c 100644 > --- a/drivers/gpu/drm/i915/intel_hdmi.c > +++ b/drivers/gpu/drm/i915/intel_hdmi.c > @@ -1573,12 +1573,20 @@ intel_hdmi_set_edid(struct drm_connector *connector) > struct intel_hdmi *intel_hdmi = intel_attached_hdmi(connector); > struct edid *edid; > bool connected = false; > + struct i2c_adapter *i2c; > > intel_display_power_get(dev_priv, POWER_DOMAIN_GMBUS); > > - edid = drm_get_edid(connector, > - intel_gmbus_get_adapter(dev_priv, > - intel_hdmi->ddc_bus)); > + i2c = intel_gmbus_get_adapter(dev_priv, intel_hdmi->ddc_bus); > + > + edid = drm_get_edid(connector, i2c); > + > + if (!edid && !intel_gmbus_is_forced_bit(i2c)) { > + DRM_DEBUG_KMS("HDMI GMBUS EDID read failed, retry using GPIO bit-banging\n"); > + intel_gmbus_force_bit(i2c, true); > + edid = drm_get_edid(connector, i2c); > + intel_gmbus_force_bit(i2c, false); > + } > > intel_hdmi_dp_dual_mode_detect(connector, edid != NULL); > > -- > 2.15.1 > > _______________________________________________ > dri-devel mailing list > dri-devel@xxxxxxxxxxxxxxxxxxxxx > https://lists.freedesktop.org/mailman/listinfo/dri-devel -- Daniel Vetter Software Engineer, Intel Corporation http://blog.ffwll.ch _______________________________________________ dri-devel mailing list dri-devel@xxxxxxxxxxxxxxxxxxxxx https://lists.freedesktop.org/mailman/listinfo/dri-devel
