[PATCH 0/1][SRU][B/OEM-B] Fix boot regression caused by a disabled PIT

[You-Sheng Yang]

BugLink: https://bugs.launchpad.net/bugs/1856387

[Impact]
In bug 1840239, HPET is disabled on some systems for they caused TSC
being marked unstable while it is not. This caused an regression as bug
1851216 that some systems may then hang at early, so a fix cherry picked
back from v5.3-rc1. However, this fix also introduce yet another
regression that some other users may hang at boot while PIT is diabled
in the previous fix.

[Fix]
Commit 979923871f69 ("x86/timer: Don't skip PIT setup when APIC is
disabled or in legacy mode") from v5.6-rc1, also backported to v5.4.19
and v5.5.3, fixes PIT setup in this case.

[Test Case]
Simply boot a patch kernel on systems affected and it shouldn't hang.

[Regression Potential]
Low. Stable patch and trivial backport.

[Other Info]
The same fix for bug 1851216 was also backported to Disco and Eoan, but
they were then fixed with this 979923871f69 commit backported in bug
1866858 and bug 1867051, which pulls v5.4 stable patches into Disco and
Eoan correspondingly, leaving B/OEM-B the only victims so far.

Thomas Gleixner (1):
  x86/timer: Don't skip PIT setup when APIC is disabled or in legacy
    mode

 arch/x86/include/asm/apic.h     |  2 ++
 arch/x86/include/asm/x86_init.h |  2 ++
 arch/x86/kernel/apic/apic.c     | 23 ++++++++++++++++++-----
 arch/x86/kernel/time.c          | 12 ++++++++++--
 arch/x86/kernel/x86_init.c      |  1 +
 arch/x86/xen/enlighten_pv.c     |  1 +
 6 files changed, 34 insertions(+), 7 deletions(-)

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2.27.0.rc0