[Bug 105056] New: HDMI port not working on Intel® Skylake U SoC integrated Graphics

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=105056

            Bug ID: 105056
           Summary: HDMI port not working on Intel® Skylake U SoC
                    integrated Graphics
           Product: xorg
           Version: unspecified
          Hardware: Other
                OS: All
            Status: NEW
          Severity: normal
          Priority: medium
         Component: Driver/intel
          Assignee: chris at chris-wilson.co.uk
          Reporter: e.deotto at gds.com
        QA Contact: intel-gfx-bugs at lists.freedesktop.org

Created attachment 137291
  --> https://bugs.freedesktop.org/attachment.cgi?id=137291&action=edit
output of dmesg

We are running Linux on a EMX-SKLUP machine
(https://www.avalue.com.tw/Product/Industrial-PC/Industrial-Motherboard/Thin-Mini-ITX/EMX-SKLUP_2521
)
This machine mounts a graphics card: Intel® Skylake U SoC integrated Graphics
and we are using the i915 driver to drive two screens, one using the HDMI port
and the other using the DisplayPort. 
We tried different kernels and we noticed the following:

1.      Kernel 4.4  all versions work like a charm
2.      Kernel 4.9  all versions before 4.9.30 work. All versions after 4.9.40
have problem with the HDMI port. It simply turns out to be always disconnected
when issuing an xrandr. Thus, the screen is simply blank. The DisplayPort
connection works perfectly. 
We made some narrowing and the problem seems to have been introduced between
the following two commits:
•       4.9.30 commit:
https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux-stable.git/commit/?h=linux-4.9.y&id=db3fd4527ed32be44cbd8ffa6dd6a301c89d0d6d
•       4.9.40 commit:
https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux-stable.git/commit/?h=linux-4.9.y&id=efcfbfb1d8bf756d1b58fe215bf4e419d176435b
•       diff of i915 dir:
https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux-stable.git/diff/drivers/gpu/drm/i915?h=linux-4.9.y&id=db3fd4527ed32be44cbd8ffa6dd6a301c89d0d6d&id2=efcfbfb1d8bf756d1b58fe215bf4e419d176435b
3.      All following kernels  have the same problem.

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