On Wednesday 25 July 2007 14:48, Linus Torvalds wrote:
> ... ACPI now seems to select CPU hotplug. Why?
ACPI=y SMP=y systems require SUSPEND_SMP=y for system sleep support,
and that requires HOTPLUG_CPU=y.
Note that ACPI=y SMP=n systems do not need it,
and thus will not select HOTPLUG_CPU=y
> That is just *broken*. Sure, if you select STR or hibernation, we need CPU
> hotplug, but just for picking ACPI? Why?
My assumption is that if somebody selects CONFIG_ACPI,
that 99% of the time, they intend that to include support for
the ACPI hooks for system sleep states.
Conversely, supporting the 1% of people who don't want it
isn't worth messing with the 99% who do, nor is
the burden of yet another config option to maintain and
#ifdefs in the code.
On UP, they'd get ACPI system sleep support 100% of the time
by default, but on SMP this option had become problematic.
We used to have this:
if ACPI
...
config ACPI_SLEEP
bool "Sleep States"
depends on X86 && (!SMP || SUSPEND_SMP)
depends on PM
default y
So the poster-child failure was i386/defconfig itself...
It couldn't support suspend to RAM because it didn't include
CONFIG_ACPI_SLEEP. Not trivial for a user to select it
when it doesn't even appear on the menu. It doesn't appear
because CONFIG_SUSPEND_SMP isn't enabled, but that doesn't
appear either -- because CONFIG_HOTPLUG_CPU isn't selected.
Most users don't want that.
So today we have this:
menuconfig ACPI
...
select HOTPLUG_CPU if X86 && SMP
select SUSPEND_SMP if X86 && SMP
Which I think leads to fewer surprises, and less complicated code.
(even though using select itself is fraught with peril:-)
thanks,
-Len
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