[Bug 229235] vt(4) of 11.2-RELEASE is broken with hardware dependent problem.

[bugzilla-noreply at freebsd.org]

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=229235

--- Comment #3 from Yasuhiro KIMURA <yasu at utahime.org> ---
>From the output of following commands,

svn log https://svn.freebsd.org/base/releng/11.2/sys/dev/vt
svn log https://svn.freebsd.org/base/releng/11.1/sys/dev/vt
svn log https://svn.freebsd.org/base/stable/11/sys/dev/vt

Graph can be made about update of /usr/src/sys/dev/vt and branch of
releng/11.1 and releng/11.2 as following.

stable/11       releng/11.1     releng/11.2
|
r318506
|
+---------------(r320486)
|               |
r321087         |
|               |
r321199         |
|               |
r326543         |
|               |
r329283         |
|               |
r329305         |
|               |
r330641         |
|               |
r330642         |
|               |
r330897         |
|               |
r316019         |
|               |
r315480         |
|               |
r330916         |
|               |
r330917         |
|               |
r330921         |
|               |
r331722         |
|               |
r331982         |
|               |
|               r331984
|               |
r332643         
|
r332744
|
+-------------------------------(r334459)
|                               |

So I used binary search algorithm to find which commit is the cause of
vt(4) breakage and got following result.

(2)  r318506  vt(4) works fine
     r321087
     r321199
     r326543
     r329283
     r329305
     r330641
     r330642
(4)  r330897  vt(4) works fine
(3)  r316019 'make buildkernel' fails
     r315480
(6)  r330916  vt(4) works fine
(7)  r330917  vt(4) works fine
(8) (r330920) vt(4) works fine
(5)  r330921  vt(4) is broken
     r331722
     r331982
     r332643            
     r332744
(1)  r334459  vt(4) is broken

(1),(2),(3),... are order of search.

According to the result above the source of problem is base r330921.
Commit messages of this commit says as following.

----------------------------------------------------------------------
r330921 | eadler | 2018-03-14 17:27:05 +0900 (2018/03/14 (水)) | 7 lines

MFC r330834:

vt_vga: check if VGA is available from ACPI FADT table

On x86 the IA-PC Boot Flags in the FADT can signal whether VGA is
available or not.
----------------------------------------------------------------------

So I guess something is wrong with way to check availability of VGA.
Or there may be bug about IA-PC Boot Flags in the ACPI FADT table.

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