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

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

On Thu, Nov 15, 2012 at 12:54:48AM +0000, Rob Herring wrote:
> On 11/14/2012 04:21 PM, Tony Lindgren wrote:
> > * Rob Herring <robherring2@gmail.com> [121114 13:59]:
> >> On 11/14/2012 02:32 PM, Tony Lindgren wrote:
> >>>
> >>> Checking for the bit already set should work in this case, I'll post
> >>> a patch for that shortly.
> >>
> >> Can you actually read the state of the diagnostic register in non-secure
> >> mode? If you can on the A9, is the same true on A8 or others?
> > 
> > Looks like it can be read on at least TI omap 4430 which is A9.
> > But it reads as zero, so the below patch is what I came up with.
> > 
> > No idea if assuming that zero value for the diagnostic register
> > is safe.. What's the default value of the diagnostic register supposed
> > to be?
> 
> RTFM. Oh, wait it's a super secret, undocumented register. We shouldn't
> even be talking about it.
> 
> It could vary by rev, but I see 0 for the reset value, so this would not
> work if the bootloader did not do any setup of the diagnostic register.
> 
> One way to determine secure mode on the A9 would be seeing if you can
> change the auxcr register. Something like this (untested):
> 
> mrc	p15, 0, r0, c1, c0, 1; Read ACTLR
> eor	r1, r0, #0x100		; Modify alloc in 1 way
> mcr	p15, 0, r1, c1, c0, 1
> mrc	p15, 0, r2, c1, c0, 1; Read ACTLR
> mcr	p15, 0, r0, c1, c0, 1	; Restore original value
> cmp	r1, r2
> bne	skip_errata

This would fail on platforms where Linux runs in non-secure mode. What
we do for some errata workarounds is to test whether the bit was already
set and avoid writing the register. But this assumes that, for a given
workaround in the kernel, there is a corresponding workaround in the
code running before the kernel (boot-loader, firmware) which sets that
bit.

Since the kernel will run more often in non-secure mode (on Cortex-A15
you need this for the virtualisation extensions) I strongly suggest that
the workaround (usually undocumented bit setting) is done before the
kernel is started and we simply remove it from Linux (or add a clear
comment that it only works if running in secure mode; if unsure say
'N').

I don't think it's worth the hassle detecting whether the kernel runs in
secure or non-secure mode, just assume the latter and get SoC vendors to
update the boot loaders or firmware (if possible) with any errata
workarounds.

Having a common SMC API for errata workarounds is not feasible since not
all registers are public, most are implementation specific and it could
have secure implications with exposing them.

-- 
Catalin