Re: [PATCH] ARM: Fix errata 751472 handling on Cortex-A9 r1p*

[Catalin Marinas <catalin.marinas@arm.com>]

On Fri, Nov 16, 2012 at 09:59:21AM +0000, Russell King - ARM Linux wrote:
> On Thu, Nov 15, 2012 at 08:31:33AM -0600, Rob Herring wrote:
> > So we should make all these work-arounds depend on !MULTI_PLATFORM then.
> 
> No, because some of the work-arounds don't require setting bits in magic
> registers.
> 
> Also realise this: we test for the revision of the CPU to which they're
> applicable before attempting to set them.  If you have a work-around
> enabled in the kernel, and it fails at boot, _that_ is an indicator not
> that the kernel is doing something wrong, but that you need to ensure
> that the work-around has been applied by the earlier stages.
> 
> It's not about "but platform X doesn't need it" - it's about platform X
> not having the earlier stages updated to fix the errata.

There is another aspect. Many CPUs in the field, even if they are a
certain rxpy revision, have ECO fixes applied for critical bugs and
don't require the workaround. Sometimes those undocumented bits have
considerable performance impact and vendors may apply an ECO (unless
they are very late in the tape-out process). But the ECO fix does not
change the CPU revision, hence the workaround becomes platform specific.

That's why I think it's better if those workarounds are just pushed to
the boot-loader for multi-platform kernels. Linux could still check the
bits and warn about them rather than failing to boot.

-- 
Catalin