revisiting tunables under Safe Mode menu option

First, I think that this proposal/discussion could have been more useful before
the 9.0.  Maybe the RE would be interested in adding another item to their
pre-release checklist: ask developers about what could be dropped and what should
be added to the Safe Mode settings in a new (.0) release.  Probably the developers
should keep the Safe Mode in mind too when adding new features or making other
drastic changes, but the reminder should be welcome.

What we have for Safe Mode now (from menu-commands.4th):
> 		\ 
> 		\ Toggle ACPI elements if necessary
> 		\ 
> 		acpipresent? if acpienabled? if
> 			menuacpi _at_ dup 0<> if
> 				toggle_menuitem ( N -- N )
> 			then
> 			drop
> 			acpi_disable
> 		then then
> 
> 		s" set hint.apic.0.disabled=1" evaluate
> 		s" set hw.ata.ata_dma=0" evaluate
> 		s" set hw.ata.atapi_dma=0" evaluate
> 		s" set hw.ata.wc=0" evaluate
> 		s" set hw.eisa_slots=0" evaluate
> 		s" set hint.kbdmux.0.disabled=1" evaluate

o Since we have a separate ACPI option and because ACPI now is almost a mandatory
thing (and not a significant source of boot troubles), maybe we could remove the
code that automatically disables ACPI in Safe Mode?

o hint.apic.0.disabled - APIC code doesn't seem to be a significant source of boot
troubles, like ACPI it has become almost a mandatory thing.  So maybe we should
remove this setting?

o hw.ata.ata_dma, hw.ata.atapi_dma - I am not sure if there have been any
significant problems with ATA DMA recently.  Maybe these could be removed?

o hw.ata.wc - I am not sure if this setting is relevant to the safe boot.  Another
candidate for removal?

o hw.eisa_slot - Looks like something from ancient times.  Probably just
irrelevant for most systems.

o hint.kbdmux.0.disabled - I do not recall any recent problems with kbdmux.  In
fact disabling it may produce a surprising behavior for a user if there are
multiple keyboards actually attached.

Just so that the Safe Mode doesn't turn into a NOP I propose to add the following
tunables:

o kern.eventtimer.periodic=1 - Use periodic timer to drive clocks just in case a
system has any problems with the default mode.  Example: PR 164457.

o kern.geom.part.check_integrity=0 - Let GPART code be more permissive, could be
useful during upgrades from earlier versions of FreeBSD or when multi-booting with
other OSes.

o More?

-- 
Andriy Gapon