Re: [PATCH] drm/i915/ddi: add DDI A force 4 lanes quirk for Apple MacBookPro11,5

[Theo Andersen Carton]

> I'm guessing this is v2 of [1], but I shouldn't have to guess.

You're right, it is effectively v2 of [1] -- same patch, no functional
change. Sorry for not labelling it.

[1] was pasted into a mail client by hand, which hard-wrapped the
"diff --git" lines and turned the leading tabs into spaces. It did reach
the lists, but not as an applicable patch, so I resent it with
git send-email and didn't think of it as a revision. I'll use proper vN
and changelogs from here on.

> Is there a bug report about this? Would be nice to see the logs in the
> failing case. See [2].

There wasn't one -- I went straight to the list. Filed now:

  https://gitlab.freedesktop.org/drm/i915/kernel/-/issues/17087

It has the full failing-case log with drm debug enabled, on an unpatched
i915 with the panel muxed to the iGPU. AUX and EDID are fine, and then the
panel's only mode is pruned:

  [drm:intel_dp_print_rates] sink rates: 162000, 270000
  [drm:update_display_info] [CONNECTOR:93:eDP-1] Assigning EDID-1.4
digital sink color depth as 8 bpc.
  [drm:drm_mode_prune_invalid] Rejected mode: "2880x1800": 60 337750
2880 2928 2960 3040 1800 1803 1809 1852 0x48 0x9 (CLOCK_HIGH)

because intel_ddi_max_lanes() reports a source maximum of 2 for port A.
The matching log with the quirk applied is attached too, where the same
link trains at lane count 4.

> i915_vbt from debugfs would also be interesting.

Attached to the bug as i915_vbt.bin, 6144 bytes -- a $VBT HASWELL block,
BIOS build 1215 dated 12/13/2012, on Apple firmware 427.140.8.0.0. Say the
word if you'd rather have a specific block dumped inline.

> The patch itself looks sane.

Thanks. One thing I got wrong in the commit message, which v3 fixes: I
wrote Broadwell, but the machine is Haswell Crystal Well -- i7-4980HQ,
8086:0d26, $VBT HASWELL, and i915 logs "Found haswell". Apple's ACPI DMAR
OEM string says BDW, which is what misled me. No code change; the quirk
matches on DMI model either way.

I'll send it as v3 rather than v2, to match the numbering you used
above -- neither earlier posting was labelled, and renumbering them now
would just recreate the ambiguity. It has that fix, a Closes: tag for the
bug, and a changelog spelling out what v1 and v2 were.

BR,
Theo