Re: [PATCH] pci: increase alignment to make more space for hidden code

[Ingo Molnar <mingo@elte.hu>]

* Bjorn Helgaas <bjorn.helgaas@hp.com> wrote:

> On Sunday 11 October 2009 03:17:16 pm Yinghai Lu wrote:
> > for
> > 
> > http://bugzilla.kernel.org/show_bug.cgi?id=13940
> > 
> > some system when acpi are enabled, acpi clears some BAR for some devices without
> > reason, and kernel will need to allocate devices for them.
> 
> "ACPI clears some BARs"?  I'm dubious.  The handoff state is the same
> whether we boot with "acpi=off" or not, so the BIOS can't be clearing
> them.  I really don't think the ACPI code in Linux clears BARs.  The
> Linux PCI code might be clearing BARs, but it sure would be nice to
> know exactly why.  Did you ever figure that out?
> 
> > try to increase alignment to get more safe range for unassigned devices.
> > 
> > Signed-off-by: Yinghai Lu <yinghai@kernel.org>
> > 
> > ---
> >  arch/x86/kernel/e820.c |    4 ++--
> >  1 file changed, 2 insertions(+), 2 deletions(-)
> > 
> > Index: linux-2.6/arch/x86/kernel/e820.c
> > ===================================================================
> > --- linux-2.6.orig/arch/x86/kernel/e820.c
> > +++ linux-2.6/arch/x86/kernel/e820.c
> > @@ -1378,8 +1378,8 @@ static unsigned long ram_alignment(resou
> >  	if (mb < 16)
> >  		return 1024*1024;
> >  
> > -	/* To 32MB for anything above that */
> > -	return 32*1024*1024;
> > +	/* To 64MB for anything above that */
> > +	return 64*1024*1024;
> 
> How do we know 64MB is the correct alignment?
> 
> This feels like a hack that accidentally covers up the problem.  I 
> don't think we understand what's happening well enough.

Perhaps hidden chipset BARs getting protected by the larger granularity? 
Do we know the before/after allocation layout?

	Ingo