** Description changed:
[Impact]
-
+ System hangs on black screen when reboot
[Fix]
-
+ Looks like our VBIOS/GOP generally fail to turn the DP dual mode adater
+ TMDS output buffers back on after a reboot. This leads to a black screen
+ after reboot if we turned the TMDS output buffers off prior to reboot.
+ And if i915 decides to do a fastboot the black screen will persist even
+ after i915 takes over.
+
+ Apparently this has been a problem ever since commit b2ccb822d376
("drm/i915:Enable/disable TMDS output buffers in DP++ adaptor as needed") if
one rebooted while the display was turned off. And things became worse with
commit fe0f1e3bfdfe ("drm/i915: Shut down displays gracefully on reboot") since
now we always turn the display off before a reboot.
+
+ This was reported on a RKL, but I confirmed the same behaviour on my
+ SNB as well. So looks pretty universal.
+
+ Let's fix this by explicitly turning the TMDS output buffers back on
+ in the encoder->shutdown() hook. Note that this gets called after irqs
+ have been disabled, so the i2c communication with the DP dual mode
+ adapter has to be performed via polling (which the gmbus code is
+ perfectly happy to do for us).
+
+ We also need a bit of care in handling DDI encoders which may or may
+ not be set up for HDMI output. Specifically ddc_pin will not be
+ populated for a DP only DDI encoder, in which case we don't want to
+ call intel_gmbus_get_adapter(). We can handle that by simply doing
+ the dual mode adapter type check before calling
+ intel_gmbus_get_adapter().
+ Ref: https://patchwork.freedesktop.org/series/96433/
[Test]
-
+ 1. install kernel
+ 2. reboot
+ 3. check the monitor works well.
[Regression Potential]
+ Medium, patch has sent to patchwork but may have a upgraded version in the
furtue.
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https://bugs.launchpad.net/bugs/1949321
Title:
Fix System hangs on black screen when reboot
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