Re: ERRATUM_858921 is broken on 5.15 kernel

[Mark Rutland <mark.rutland@arm.com>]

On Thu, Jan 05, 2023 at 07:03:48PM +0530, Yogesh Lal wrote:
> Hi,
> 
> We are observing issue on A73 core where ERRATUM_858921 is broken.

Do you *only* see this issue on v5.15.y, or is mainline (e.g. v6.2-rc2) also
broken?

I don't see any fix that fits your exact description below, but I do see that
we've made a bunch of changes in this area since.

> 
> On 5.15 kernel arch_timer_enable_workaround is set by reading
> arm64_858921_read_cntpct_el0 and arm64_858921_read_cntvct_el0 during timer
> register using following path.
> 
> arch_timer_enable_workaround->atomic_set(&timer_unstable_counter_workaround_in_use,
> 1);
> 
> [code snap]
> 564 static
> 565 void arch_timer_enable_workaround(const struct
> arch_timer_erratum_workaround *wa,
> 566                               bool local)
> 567 {
> 568     int i;
> 569
> 570     if (local) {
> 571 __this_cpu_write(timer_unstable_counter_workaround, wa);
> 572     } else {
> 573             for_each_possible_cpu(i)
> 574                     per_cpu(timer_unstable_counter_workaround, i) = wa;
> 575     }
> 576
> 577     if (wa->read_cntvct_el0 || wa->read_cntpct_el0)
> 578 atomic_set(&timer_unstable_counter_workaround_in_use, 1);
> 
> 
> and based on above workaround enablement , appropriate function to get
> counter is used.
> 
> 1008 static void __init arch_counter_register(unsigned type)
> 1009 {
> 1010     u64 start_count;
> 1011
> 1012     /* Register the CP15 based counter if we have one */
> 1013     if (type & ARCH_TIMER_TYPE_CP15) {
> 1014         u64 (*rd)(void);
> 1015
> 1016         if ((IS_ENABLED(CONFIG_ARM64) && !is_hyp_mode_available()) ||
> 1017             arch_timer_uses_ppi == ARCH_TIMER_VIRT_PPI) {
> 1018             if (arch_timer_counter_has_wa())
> 1019                 rd = arch_counter_get_cntvct_stable;
> 1020             else
> 1021                 rd = arch_counter_get_cntvct;
> 1022         } else {
> 1023             if (arch_timer_counter_has_wa())
> 1024                 rd = arch_counter_get_cntpct_stable;
> 1025             else
> 1026                 rd = arch_counter_get_cntpct;
> 1027         }
> [snap]
> 1043     /* 56 bits minimum, so we assume worst case rollover */
> 1044     sched_clock_register(arch_timer_read_counter, 56, arch_timer_rate);
> 
> 
> As our boot cores are not impacted by errata sched_clock_register() will
> register !arch_timer_counter_has_wa() callback.

It would be helpful to mention this fact (that the system is big.LITTLE, and
the boot cores are not Cortex-A73) earlier in the report.

> Now when errata impacted core boots up and sched_clock_register already
> register will !arch_timer_counter_has_wa() path.
> As sched_clock_register is not per_cpu bases so arch_timer_read_counter will
> always point to !arch_timer_counter_has_wa() function calls.

Hmm... yes, AFAICT this cannot work unless the affected CPUs are up before we
probe, and it doesn't make much sense for arch_counter_register() to look at
arch_timer_counter_has_wa() since it can be called before all CPUs are up.

> Looks like this bug is side effect of following patch:
> 
> commit 0ea415390cd345b7d09e8c9ebd4b68adfe873043
> Author: Marc Zyngier <marc.zyngier@arm.com>
> Date:   Mon Apr 8 16:49:07 2019 +0100
> 
>     clocksource/arm_arch_timer: Use arch_timer_read_counter to access stable
> counters
> 
>     Instead of always going via arch_counter_get_cntvct_stable to access the
>     counter workaround, let's have arch_timer_read_counter point to the
>     right method.
> 
>     For that, we need to track whether any CPU in the system has a
>     workaround for the counter. This is done by having an atomic variable
>     tracking this.
> 
>     Acked-by: Mark Rutland <mark.rutland@arm.com>
>     Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
>     Signed-off-by: Will Deacon <will.deacon@arm.com>
> 

Yeah, that does look to be broken, but I think there are futher issues anyway
(e.g. late onlining).

AFAICT we need to detect this *stupidly early* in the CPU bringup path in order
to handle this safely, which is quite painful.

What a great.

Thanks,
Mark.