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

[Stephen Boyd <sboyd@codeaurora.org>]

On 06/03/13 02:39, Russell King - ARM Linux wrote:
> On Sat, Jun 01, 2013 at 11:39:41PM -0700, Stephen Boyd wrote:
>> +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.

You're right, it is confusing and broken. I was thinking we may need a
union for epoch_ns but I will try to make it always nanoseconds and see
if that makes the code clearer.

>
>> +}
>> +
>> +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.)

Ok I will use clocks_calc_mult_shift() here.

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