Re: Virtual alias cache coherency results (was: x86, ARM, PARISC, PPC, MIPS and Sparc folks please run this)

[Russell King <rmk@arm.linux.org.uk>]

On Thu, Sep 11, 2003 at 01:35:35PM +0100, Jamie Lokier wrote:
> Russell King wrote:
> > > Does your fix, which makes pages uncacheable andq disables write
> > > combining (correct?) only fix your test results which intermittently
> > > reported write buffer problems, or does it fix _all_ the ARM test
> > > results I received, including those which don't report write buffer
> > > problems?
> > 
> > It's relatively simple, and I'm not sure why its causing such
> > misunderstanding.  Let me try one more time:
> > 
> > ARM caches are VIVT.  VIVT caches have inherent aliasing issues.  The
> > kernel works around these issues by marking memory uncacheable where
> > appropriate, and will continue to do so for VIVT cached ARM CPUs.
> 
> That I understand fully.

I don't think you do.

> My question arises because I have 3 SA-110 results which report "cache
> not coherent".  They do not report "store buffer not coherent".  All 3
> are Rebel Netwinders, of different bogomips ratings.
> 
> The point is: those results _don't_ indicate write buffer problems.

Maybe those StrongARM chips don't exhibit the write buffer bug?  Remember,
I said _SOME_ StrongARM-110 chips exhibit the problem.  I did not say
_ALL_ StrongARM-110 chips exhibit the problem.

> It means that your VIVT explanation and workaround does not explain
> those results, so I cannot have confidence that your workaround fixes
> those particular ARM devices.

Well, as far as I'm concerned, I completely believe that I have explained
it entirely, and I still don't know why you're trying to make this more
difficult than it factually is.

> Now, if you can assure me that those results are _definitely_ due to
> using very old kernels which don't even mark pages uncacheable, and
> with newer kernels those Netwinders would exhibit coherent virtual
> aliases, that's great.

Well, once you collect the kernel information and forward it to me, I
can have a look.

-- 
Russell King (rmk@arm.linux.org.uk)	http://www.arm.linux.org.uk/personal/
Linux kernel maintainer of:
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