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.
Disabling it, it starts 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).
Fixes: 012f188504 ("clocksource/drivers/arm_arch_timer: Work around broken CVAL implementations")
Signed-off-by: Joe Korty <joe.korty@concurrent-rt.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20221024165422.GA51107@zipoli.concurrent-rt.com
Link: https://lore.kernel.org/r/20221121145343.896018-1-maz@kernel.org
[maz: revamped the commit message]
839a973988