[patch 20/48] clocksource: Update clocksource::freq_khz on registration

[Thomas Gleixner <tglx@kernel.org>]

Borislav reported a division by zero in the timekeeping code and random
hangs with the new coupled clocksource/clockevent functionality.

It turned out that the TSC clocksource is not always updating the
freq_khz field of the clocksource on registration. The coupled mode
conversion calculation requires the frequency and as it's not
initialized the resulting factor is zero or a random value. As a
consequence this causes a division by zero or random boot hangs.

Instead of chasing down all clocksources which fail to update that
member, fill it in at registration time where the caller has to supply
the frequency anyway. Except for special clocksources like jiffies which
never can have coupled mode.

To make this more robust put a check into the registration function to
validate that the caller supplied a frequency if the coupled mode
feature bit is set. If not, emit a warning and clear the feature bit.

Fixes: cd38bdb8e696 ("timekeeping: Provide infrastructure for coupled clockevents")
Reported-by: Borislav Petkov <bp@alien8.de>
Reported-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@kernel.org>
---
 kernel/time/clocksource.c |    7 +++++++
 1 file changed, 7 insertions(+)

--- a/kernel/time/clocksource.c
+++ b/kernel/time/clocksource.c
@@ -1169,6 +1169,9 @@ void __clocksource_update_freq_scale(str
 
 		clocks_calc_mult_shift(&cs->mult, &cs->shift, freq,
 				       NSEC_PER_SEC / scale, sec * scale);
+
+		/* Update cs::freq_khz */
+		cs->freq_khz = div_u64((u64)freq * scale, 1000);
 	}
 
 	/*
@@ -1241,6 +1244,10 @@ int __clocksource_register_scale(struct
 
 	if (WARN_ON_ONCE((unsigned int)cs->id >= CSID_MAX))
 		cs->id = CSID_GENERIC;
+
+	if (WARN_ON_ONCE(!freq && cs->flags & CLOCK_SOURCE_HAS_COUPLED_CLOCK_EVENT))
+		cs->flags &= ~CLOCK_SOURCE_HAS_COUPLED_CLOCK_EVENT;
+
 	if (cs->vdso_clock_mode < 0 ||
 	    cs->vdso_clock_mode >= VDSO_CLOCKMODE_MAX) {
 		pr_warn("clocksource %s registered with invalid VDSO mode %d. Disabling VDSO support.\n",