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SubjectRe: [PATCH v2 2/2] clocksource: Add driver for the Ingenic JZ47xx OST
Date
Paul Cercueil <paul@crapouillou.net> writes:
> +static u64 notrace ingenic_ost_clocksource_read64(struct clocksource *cs)
> +{
> + u32 val1, val2;
> + u64 count, recount;
> + s64 diff;
> +
> + /*
> + * The buffering of the upper 32 bits of the timer prevents wrong
> + * results from the bottom 32 bits overflowing due to the timer ticking
> + * along. However, it does not prevent wrong results from simultaneous
> + * reads of the timer, which could reset the buffer mid-read.
> + * Since this kind of wrong read can happen only when the bottom bits
> + * overflow, there will be minutes between wrong reads, so if we read
> + * twice in succession, at least one of the reads will be correct.
> + */
> +
> + /* Bypass the regmap here as we must return as soon as possible */

I have a hard time to understand this comment. "Bypass the regmap ..."
and then use a regmap function?

> + regmap_read(ingenic_ost->map, TCU_REG_OST_CNTL, &val1);
> + regmap_read(ingenic_ost->map, TCU_REG_OST_CNTHBUF, &val2);
> + count = (u64)val1 | (u64)val2 << 32;
> +
> + regmap_read(ingenic_ost->map, TCU_REG_OST_CNTL, &val1);
> + regmap_read(ingenic_ost->map, TCU_REG_OST_CNTHBUF, &val2);
> + recount = (u64)val1 | (u64)val2 << 32;
> +
> + /*
> + * A wrong read will produce a result that is 1<<32 too high: the bottom
> + * part from before overflow and the upper part from after overflow.
> + * Therefore, the lower value of the two reads is the correct value.
> + */
> +
> + diff = (s64)(recount - count);
> + if (unlikely(diff < 0))
> + count = recount;

Is this really the right approach here? What is the 64bit readout buying
you?

The timekeeping code can handle a 32bit counter perfectly fine and the
only advantage you get is that your maximum possible idle time will be
longer with a 64bit counter.

But is that really worth the overhead of four MMIO reads versus one in a
hotpath?

Thanks,

tglx

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