"EDID checksum is invalid"

[Daniel Vetter]

On Wed, Jan 7, 2015 at 6:07 PM, Linus Torvalds
<torvalds at linux-foundation.org> wrote:
> On Tue, Jan 6, 2015 at 10:21 PM, Robert Morell <rmorell at nvidia.com> wrote:
>>
>> FWIW, I've seen that exact symptom on some monitors when the +5V pin on
>> the DVI or HDMI cable from the GPU isn't enabled (or isn't providing
>> enough current).  Some monitors power the i2c/edid/DDC logic from that
>> +5V either exclusively or when in the DPMS off state, and the i2c chip
>> will just stop responding after a few cycles if not provided sufficient
>> power.
>
> That makes a ton of sense, especially the "when in DPMS off state" case.
>
> I'll do the drm.debug=0xe thing, and maybe Daniel can make more sense
> of the details. Maybe the i2c driver ends up powering down too soon
> (or maybe  it needs to power up a bit earlier)?
>
> This is a bog-standard intel motherboard (DH87RL), but I actually
> needed to update the BIOS for it to get it to POST reliably with this
> monitor. I was blaming that on the odd 3840x2160 at 30Hz mode, but maybe
> it's related to the EDID being finicky wrt i2c power.
>
> I'm assuming even the hdmi +5V line is under sw control at least for
> power management reasons. Maybe dpms off turns it off, and shouldn't?
> My monitor actually says "No HDMI (HML) Cable" when I do "xset dpms
> force off". Maybe that's normal, but maybe that's indicative of dpms
> turning things a bit *too* off?

Not sure whether that'd be the same voltage rails, but
i915.disable_power_wells=0 disable all the runtime pm we do (which
does kick in for dpms off and shut down the entire display block and a
bunch more). Maybe we just need to detect that the chip dropped off
the bus and retry after 50ms (to give the caps some time to charge).
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch