Cortex A9 MP: ARM errata 754323 implementation?

[Catalin Marinas]

On Thu, Sep 03, 2015 at 10:26:49AM +0200, Dirk Behme wrote:
> On 03.09.2015 10:05, Russell King - ARM Linux wrote:
> >On Thu, Sep 03, 2015 at 09:40:21AM +0200, Dirk Behme wrote:
> >>looking through the ARM Cortex A9 errata list [1] I wonder why we don't have
> >>a workaround for
> >>
> >>(754323) Repeated Store in the same cache line might delay the visibility of
> >>the Store
> >>
> >>in the kernel? Or have I missed it?
> >
> >The policy for errata is not to implement them unless there's a requirement
> >to do so - and then the errata should be implemented in board firmware in
> >preference to the kernel where possible.
> >
> >Are you seeing a problem directly attributable to this errata?
> 
> I got a report from some internal testing that an issue they see goes away
> if they enable 754327. I rejected this because i.MX6 is > r2p0 and therefore
> can't be affected by this errata. Looking through the list of erratas I then
> found the related 754323 which seems to apply to i.MX6, but is not
> implemented.

These errata are usually harmless, in most cases it prevents the system
from making progress (like flag update not visible while being polled by
another CPU), hence the workaround makes cpu_relax() a barrier since
most polling loops should use it.

> The issue we are talking about is
> 
> Internal error: Oops - BUG: 0 [#1] PREEMPT SMP ARM
> PC is at kfree+0x10c/0x238
> LR is at release_firmware+0x5c/0x70
> 
> which is said to be triggered by this code
> 
> void kfree(const void *x)
> ...
> page = virt_to_head_page(x);
> if (unlikely(!PageSlab(page))) {
> BUG_ON(!PageCompound(page));
> ...
> 
> on a custom 3.14.x kernel. I haven't looked into this myself, but at least
> two people think that the kmalloc/kfree is correct with the
> request_firmware()/release_firmware() usage in the driver.

I don't see how the erratum above would trigger a BUG. It's possible
that there are some memory ordering issues (and A9 has some read after
read bugs) that are hidden when enabling the barrier in cpu_relax().

-- 
Catalin