From: Thomas Gleixner <tglx@linutronix.de>
To: Paul Cercueil <paul@crapouillou.net>,
Daniel Lezcano <daniel.lezcano@linaro.org>
Cc: od@zcrc.me, linux-kernel@vger.kernel.org,
Maarten ter Huurne <maarten@treewalker.org>,
Paul Cercueil <paul@crapouillou.net>,
Mathieu Malaterre <malat@debian.org>,
Artur Rojek <contact@artur-rojek.eu>
Subject: Re: [PATCH v2 2/2] clocksource: Add driver for the Ingenic JZ47xx OST
Date: Thu, 09 Jan 2020 15:28:00 +0100 [thread overview]
Message-ID: <87a76wln67.fsf@nanos.tec.linutronix.de> (raw)
In-Reply-To: <20200107010630.954648-2-paul@crapouillou.net>
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
next prev parent reply other threads:[~2020-01-09 14:28 UTC|newest]
Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-01-07 1:06 [PATCH v2 1/2] time/sched_clock: Disable interrupts in sched_clock_register Paul Cercueil
2020-01-07 1:06 ` [PATCH v2 2/2] clocksource: Add driver for the Ingenic JZ47xx OST Paul Cercueil
2020-01-09 14:28 ` Thomas Gleixner [this message]
2020-01-09 18:26 ` Paul Cercueil
2020-01-09 11:55 ` [PATCH v2 1/2] time/sched_clock: Disable interrupts in sched_clock_register Daniel Lezcano
2020-01-09 17:33 ` [tip: timers/core] time/sched_clock: Disable interrupts in sched_clock_register() tip-bot2 for Paul Cercueil
2020-01-09 17:56 ` tip-bot2 for Paul Cercueil
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