[RFC PATCH 1/2] ARM/ARM64: arch_timer: Work around QorIQ Erratum A-008585

[Scott Wood]

On Wed, 2016-04-13 at 08:22 -0500, Rob Herring wrote:
> On Sun, Apr 10, 2016 at 9:22 PM, Scott Wood <oss at buserror.net> wrote:
> > Erratum A-008585 says that the ARM generic timer "has the potential to
> > contain an erroneous value for a small number of core clock cycles
> > every time the timer value changes" and that the workaround is to
> > reread TVAL and count registers until successive reads return the same
> > value.
> > 
> > This erratum can be found on LS1021A (32-bit), LS1043A (64-bit), and
> > LS2080A (64-bit).
> > 
> > This patch is loosely based on work by Priyanka Jain and Bhupesh
> > Sharma.
> > 
> > Signed-off-by: Scott Wood <oss at buserror.net>
> > ---
> >  .../devicetree/bindings/arm/arch_timer.txt         |  4 ++
> >  arch/arm/boot/dts/ls1021a.dtsi                     |  1 +
> >  arch/arm/include/asm/arch_timer.h                  | 71
> > +++++++++++++++++++---
> >  arch/arm/include/asm/vdso_datapage.h               |  1 +
> >  arch/arm/kernel/vdso.c                             |  1 +
> >  arch/arm/vdso/vgettimeofday.c                      |  2 +-
> >  arch/arm64/boot/dts/freescale/fsl-ls1043a.dtsi     |  1 +
> >  arch/arm64/boot/dts/freescale/fsl-ls2080a.dtsi     |  1 +
> >  arch/arm64/include/asm/arch_timer.h                | 35 ++++++++---
> >  arch/arm64/include/asm/vdso_datapage.h             |  1 +
> >  arch/arm64/kernel/asm-offsets.c                    |  1 +
> >  arch/arm64/kernel/vdso.c                           |  2 +
> >  arch/arm64/kernel/vdso/gettimeofday.S              | 14 ++++-
> >  drivers/clocksource/arm_arch_timer.c               |  5 ++
> >  14 files changed, 121 insertions(+), 19 deletions(-)
> > 
> > diff --git a/Documentation/devicetree/bindings/arm/arch_timer.txt
> > b/Documentation/devicetree/bindings/arm/arch_timer.txt
> > index e774128..7117fbd 100644
> > --- a/Documentation/devicetree/bindings/arm/arch_timer.txt
> > +++ b/Documentation/devicetree/bindings/arm/arch_timer.txt
> > @@ -25,6 +25,10 @@ to deliver its interrupts via SPIs.
> >  - always-on : a boolean property. If present, the timer is powered
> > through an
> >    always-on power domain, therefore it never loses context.
> > 
> > +- fsl,erratum-a008585 : A boolean property. Indicates the presence of
> > +  QorIQ erratum A-008585, which says reading the timer is unreliable
> > +  unless the same value is returned by back-to-back reads.
> 
> Bindings need to go to the DT list.

Yes, sorry about that.

> An excellent example of how not having specific compatible strings
> will bite you latter.

I don't think that would have helped...  Even if it said "arm,cortex-a57
-timer" it would not have identified the problem, because it comes from system
logic rather than the logic being described.  Even if the SoC name were in
there, it wouldn't usually tell you if it's the revision with the problem.

Normally we'd use SVR to detect such errata but I didn't want to add that
dependency to the common code.

-Scott