From: Joe Korty joe.korty@concurrent-rt.com

The TVAL register is 32 bit signed. Thus only the lower 31 bits are available to specify when an interrupt is to occur at some time in the near future. Attempting to specify a larger interval with TVAL results in a negative time delta which means the timer fires immediately upon being programmed, rather than firing at that expected future time.

The solution is for Linux to declare that TVAL is a 31 bit register rather than give its true size of 32 bits. This prevents Linux from programming TVAL with a too-large value. Note that, prior to 5.16, this little trick was the standard way to handle TVAL in Linux, so there is nothing new happening here on that front.

The softlockup detector hides the issue, because it keeps generating short timer deadlines that are within the scope of the broken timer.

Disable it, and you start using NO_HZ with much longer timer deadlines, which turns into an interrupt flood:

11: 1124855130 949168462 758009394 76417474 104782230 30210281 310890 1734323687 GICv2 29 Level arch_timer

And "much longer" isn't that long: it takes less than 43s to underflow TVAL at 50MHz (the frequency of the counter on XGene-1).

Some comments on the v1 version of this patch by Marc Zyngier:

XGene implements CVAL (a 64bit comparator) in terms of TVAL (a countdown register) instead of the other way around. TVAL being a 32bit register, the width of the counter should equally be 32. However, TVAL is a *signed* value, and keeps counting down in the negative range once the timer fires.

It means that any TVAL value with bit 31 set will fire immediately, as it cannot be distinguished from an already expired timer. Reducing the timer range back to a paltry 31 bits papers over the issue.

Another problem cannot be fixed though, which is that the timer interrupt *must* be handled within the negative countdown period, or the interrupt will be lost (TVAL will rollover to a positive value, indicative of a new timer deadline).

Cc: stable@vger.kernel.org # 5.16+ Fixes: 012f18850452 ("clocksource/drivers/arm_arch_timer: Work around broken CVAL implementations") Signed-off-by: Joe Korty joe.korty@concurrent-rt.com Reviewed-by: Marc Zyngier maz@kernel.org [maz: revamped the commit message] Signed-off-by: Marc Zyngier maz@kernel.org Link: https://lore.kernel.org/r/20221024165422.GA51107@zipoli.concurrent-rt.com ---

Notes: v3: rejigged commit message along the lines of the v2 review.

drivers/clocksource/arm_arch_timer.c | 7 +++++-- 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/drivers/clocksource/arm_arch_timer.c b/drivers/clocksource/arm_arch_timer.c index a7ff77550e17..41323c2bc0b1 100644 --- a/drivers/clocksource/arm_arch_timer.c +++ b/drivers/clocksource/arm_arch_timer.c @@ -806,6 +806,9 @@ static u64 __arch_timer_check_delta(void) /* * XGene-1 implements CVAL in terms of TVAL, meaning * that the maximum timer range is 32bit. Shame on them. + * + * Note that TVAL is signed, thus has only 31 of its + * 32 bits to express magnitude. */ MIDR_ALL_VERSIONS(MIDR_CPU_MODEL(ARM_CPU_IMP_APM, APM_CPU_PART_POTENZA)), @@ -813,8 +816,8 @@ static u64 __arch_timer_check_delta(void) };

if (is_midr_in_range_list(read_cpuid_id(), broken_cval_midrs)) { - pr_warn_once("Broken CNTx_CVAL_EL1, limiting width to 32bits"); - return CLOCKSOURCE_MASK(32); + pr_warn_once("Broken CNTx_CVAL_EL1, using 32 bit TVAL instead.\n"); + return CLOCKSOURCE_MASK(31); } #endif return CLOCKSOURCE_MASK(arch_counter_get_width());