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SubjectV4.0.x fails to create /dev/rtc0 on Winbook TW100 when CONFIG_PINCTRL_BAYTRAIL is set, bisected to commit 7486341
On a Winbook TW100 BayTrail tablet, kernel 4.0 and later do not create /dev/rtc0 
when CONFIG_PINCTRL_BAYTRAIL is set in the configuration. Removing this option
from the config creates a real-time clock; however, it is no longer possible to
get the tablet to sleep using the power button. Only complete shutdown works.

This problem was bisected to the following commit:

commit 7486341a98f26857f383aec88ffa10950087c3a1
Author: Li, Aubrey <>
Date: Wed Mar 11 16:09:00 2015 +0800

x86/platform, acpi: Bypass legacy PIC and PIT in ACPI hardware reduced mode

On a platform in ACPI Hardware-reduced mode, the legacy PIC and
PIT may not be initialized even though they may be present in
silicon. Touching these legacy components causes unexpected
results on the system.

On the Bay Trail-T(ASUS-T100) platform, touching these legacy
components blocks platform hardware low idle power state(S0ix)
during system suspend. So we should bypass them in ACPI hardware
reduced mode.

Suggested-by: Arjan van de Ven <>
Signed-off-by: Li Aubrey <>
Cc: <>
Cc: Alan Cox <>
Cc: H. Peter Anvin <>
Cc: Len Brown <>
Cc: Rafael J. Wysocki <>
Cc: Rafael J. Wysocki <>
Signed-off-by: Ingo Molnar <>

The commit message says that this change was needed for the ASUS-T100 tablet. It
appears that not all BayTrail systems are the same.

The complete dmesg output is at and the
configuration file is attached.



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