On Sun, Dec 24, 2017 at 10:45:24PM +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 to ACK the received byte
> happen at the same time.
>
> Some HDMI-to-VGA converters apparently read the ACK not in the middle of
> the clock high phase, but at the rising 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 is 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
Applied with a Cc: stable so it gets backported.
Thanks, 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
>
> ___
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--
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch