Re: [PATCH] clockevents/drivers/i8253: Do not zero timer counter in shutdown

[Thomas Gleixner <tglx@linutronix.de>]

On Thu, Aug 01 2024 at 18:49, David Woodhouse wrote:
> On Thu, 2024-08-01 at 16:21 +0200, Thomas Gleixner wrote:
>> The stop sequence is wrong:
>> 
>>     When there is a count in progress, writing a new LSB before the
>>     counter has counted down to 0 and rolled over to FFFFh, WILL stop
>>     the counter.  However, if the LSB is loaded AFTER the counter has
>>     rolled over to FFFFh, so that an MSB now exists in the counter, then
>>     the counter WILL NOT stop.
>> 
>> The original i8253 datasheet says:
>> 
>>     1) Write 1st byte stops the current counting
>>     2) Write 2nd byte starts the new count
>
> It says that for mode zero ("Interrupt on Terminal Count"), yes. But in
> that mode, shouldn't the IRQ only fire *one* more time anyway, rather
> than repeatedly? That should be OK, shouldn't it?
>
> "When terminal count is reached, the output will go high and remain
> high until the selected count register is reloaded wityh the mode or a
> new count is loaded".

I just confirmed that this is the case on KVM.

> It's OK for it to keep *counting* as long as it stops firing
> interrupts, isn't it?

Yes. So the sequence should stop KVM from trying to inject
interrupts. Maybe someone fixes it to actually stop fiddling with the
counter too :)

> Either way, this is somewhat orthogonal to the patch I posted in 
> https://lore.kernel.org/kvm/6cd62b5058e11a6262cb2e798cc85cc5daead3b1.camel@infradead.org/T/#u
> for the fact that we don't shut down the PIT at *all* if we aren't ever
> going to use it.
>
> I'm glad I decided to export a function from the clocksource driver and
> just *call* it from pit_timer_init() though. Means we can bikeshed the
> shutdown sequence in *one* place and it isn't duplicated.

Right. Though we don't have to make this conditional on hypervisor I
think.

Thanks,

        tglx