Regression in i915 in kernel 4.6.0-git - bisected to f21a21983ef13a031

From: Larry Finger
Date: Thu May 26 2016 - 18:11:51 EST

The latest mainline kernel (commit 3f59de0) shows a regression. The symptom is that as soon as the kernel is started, the display is blanked, and it is never turned on again. This problem was bisected to commit f21a21983ef13a031250c4c3f6018e29a549d0f1
("drm/i915: Splitting intel_dp_detect"). The adapter in question is the integrated graphics controller on a Toshiba Tecra A50 laptop. The output of "lspci -nnv" for that controller is as follows:

00:02.0 VGA compatible controller [0300]: Intel Corporation 4th Gen Core Processor Integrated Graphics Controller [8086:0416] (rev 06) (prog-if 00 [VGA controller])
Subsystem: Toshiba America Info Systems Device [1179:0002]
Flags: bus master, fast devsel, latency 0, IRQ 27
Memory at e0000000 (64-bit, non-prefetchable) [size=4M]
Memory at d0000000 (64-bit, prefetchable) [size=256M]
I/O ports at 4000 [size=64]
Expansion ROM at <unassigned> [disabled]
Capabilities: [90] MSI: Enable+ Count=1/1 Maskable- 64bit-
Capabilities: [d0] Power Management version 2
Capabilities: [a4] PCI Advanced Features
Kernel driver in use: i915
Kernel modules: i915

If the kernel is booted with "nomodeset", the display is normal.

I will be happy to test any patches, or to answer any further questions.