Re: [PATCHv2 4/6] sched_clock: Add support for >32 bit sched_clock

[Russell King - ARM Linux <linux@arm.linux.org.uk>]

On Sat, Jun 01, 2013 at 11:39:41PM -0700, Stephen Boyd wrote:
> The ARM architected system counter has at least 56 useable bits.
> Add support for counters with more than 32 bits to the generic
> sched_clock implementation so we can avoid the complexity of
> dealing with wrap-around on these devices while benefiting from
> the irqtime accounting and suspend/resume handling that the
> generic sched_clock code already has.

This looks like a horrid hack to me.

> +static unsigned long long notrace sched_clock_64(void)
> +{
> +	u64 cyc = read_sched_clock_64() - cd.epoch_ns;
> +	return cyc * cd.mult;

So, the use of cd.mult implies that the return value from
read_sched_clock_64() is not nanoseconds but something else.  But then
we subtract it from the nanoseconds epoch - which has to be nanoseconds
because you simply return that when suspended.

> +}
> +
> +void __init
> +setup_sched_clock_64(u64 (*read)(void), int bits, unsigned long rate)
> +{
> +	if (cd.rate > rate)
> +		return;
> +
> +	BUG_ON(bits <= 32);
> +	WARN_ON(!irqs_disabled());
> +	read_sched_clock_64 = read;
> +	sched_clock_func = sched_clock_64;
> +	cd.rate = rate;
> +	cd.mult = NSEC_PER_SEC / rate;

Here, you don't check that the (2^bits) * mult results in a wrap of the
resulting 64-bit number, which is a _basic_ requirement for sched_clock
(hence all the code for <=32bit clocks, otherwise we wouldn't need this
complexity in the first place.)

So, I think this whole approach is broken.