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

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Message-ID: <87le1g2hrx.ffs@tglx>
Date: Thu, 01 Aug 2024 20:57:06 +0200
From: Thomas Gleixner <tglx@...utronix.de>
To: David Woodhouse <dwmw2@...radead.org>, lirongqing@...du.com,
 seanjc@...gle.com, kys@...rosoft.com, haiyangz@...rosoft.com,
 wei.liu@...nel.org, decui@...rosoft.com, mingo@...hat.com, bp@...en8.de,
 dave.hansen@...ux.intel.com, x86@...nel.org, linux-hyperv@...r.kernel.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH] clockevents/drivers/i8253: Do not zero timer counter in
 shutdown

On Thu, Aug 01 2024 at 19:25, David Woodhouse wrote:
> On Thu, 2024-08-01 at 18:49 +0100, David Woodhouse 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 also prefixes that with "Rewriting a counter register during
> counting results in the following:".
>
> But after you write the MODE register, is it actually supposed to be
> counting? Just a little further up, under 'Counter Loading', it says:

It's not counting right out of reset. But once it started counting it's
tedious to stop :)

> "The count register is not loaded until the count value is written (one
> or two bytes, depending on the mode selected by the RL bits), followed
> by a rising edge and a falling edge of the clock. Any read of the
> counter prior to that falling clock edge may yield invalid data".
>
> OK, but what *triggers* that invalid state? Given that it explicitly
> says that a one-byte counter write ends that state, it isn't the first
> of two bytes. Surely that means that from the time the MODE register is
> written, any read of the counter may yield invalid data, until the
> counter is written?

It seems to keep ticking with the old value.

> I suspect there are as many implementations (virt and hardware) as
> there are reasonable interpretations of the spec... and then some.

Indeed.

Thanks,

        tglx