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[bug #49814] vbe does not reset display start

From: Stas Sergeev
Subject: [bug #49814] vbe does not reset display start
Date: Fri, 9 Dec 2016 15:35:02 +0000 (UTC)
User-agent: Mozilla/5.0 (X11; Ubuntu; Linux x86_64; rv:50.0) Gecko/20100101 Firefox/50.0


                 Summary: vbe does not reset display start
                 Project: GNU GRUB
            Submitted by: stsp
            Submitted on: Fri 09 Dec 2016 03:35:00 PM GMT
                Category: None
                Severity: Major
                Priority: 5 - Normal
              Item Group: None
                  Status: None
                 Privacy: Public
             Assigned to: None
         Originator Name: stsp
        Originator Email: address@hidden
             Open/Closed: Open
         Discussion Lock: Any
                 Release: Git master
         Reproducibility: Intermittent
         Planned Release: None




grub2's vbe module uses page flipping.
When exiting and setting the old mode, it doesn't
reset the display start.
This is perfectly fine as long as the vbios does things right.
Unfortunately some vbioses forget to reset the display start
when setting new mode, and even when doing POST.
The result is that when some app sets the video mode and draws,
there is just a blank screen.
This bug has very poor reproducibility before you find out
its roots. For example it you set a too small timeout in grub,
there will be no problem, because the problem happens only when
the timeout clocks are ticking. So, after about a month of
debugging, I finally know how to reproduce it reliably: set
quite large timeout (>3sec), wait for the timer to appear and
start ticking, then press Enter. In this case grub will exit
with active page not within the first frame buffer (depending
on the luck, may need a few retries), and, if the vbios is buggy
too, you get the permanent blank screen.

The easiest work-around is so that grub to reset the display
start before restoring the original video mode.


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