From: shinya.kuribayashi.px@renesas•com (Shinya Kuribayashi)
To: linux-arm-kernel@lists•infradead.org
Subject: [PATCH v2 2/2] ARM: delay: allow timer-based delay implementation to be selected
Date: Tue, 17 Jul 2012 12:10:06 +0900 [thread overview]
Message-ID: <5004D78E.4050606@renesas.com> (raw)
In-Reply-To: <20120713111337.GH18079@mudshark.cambridge.arm.com>
Will, Stephen and Santosh,
On 7/13/2012 8:13 PM, Will Deacon wrote:
> I was anticipating that the platform would set the initial loops_per_jiffy
> value if it requires udelays before loop calibration and the default of 4k
> is wildly off.
I overlooked two different lpj setups were involved at hand.
First one was, the initial loops_per_jiffy value of 4k was too small for
almost all processors running Linux today, so I set up loops_per_jiffy
_early_, calculated from the CPU clock speed. I didn't mentioned this
before, sorry for confusion.
So my initial loops_per_jiffy is not 4k at this point. It's optimized
for loop-based delay with the CPU running at 1.2GHz (much bigger than
default 4k).
And later, init_current_timer_delay() got processed. Actual udelay()
behavior switched from loop-based delay to timer-based one immediately,
while my loops_per_jiffy was not changed/updated to appropriate value.
This is why my udelay()s, used after init_current_timer_delay(), were
taking considerable long time to expire. Note that my initial tests
for Will's patchset was done using a loadable module dedicated for
udelay tests, that was prepared for 2.6.35/3.0 kernels beforehand.
And this time, I confirmed that updating loops_per_jiffy at the same
time as lpj_fine, works perfectly as expected for me.
> If people need loops_per_jiffy to be updated at the same time as lpj_fine,
> I can post that as a separate patch (below) as Russell has merged v2 of these
> patches into his delay branch. That said, I'd certainly like to know if this
> is actually a real problem (and can't be solved by choosing a compromise value
> as the initial loops_per_jiffy). I think Shinya was doing some tests so
> I'll wait to see how those went.
>From my observations:
(1) loops_per_jiffy can easily be calculated from the CPU clock speed.
If your platform is capable of detecting CPU frequency at run-time,
settingi up loops_per_jiffy _early_ can allow you early use of udelay()s.
Or even if you don't need udelay() early, setting up lpj_fine (or having
calibrate_delay_is_known()) allows you to skip calibrate_delay() later.
This is useful and can be applied to both UP and SMP systems.
(2) For SMP platforms, if you need ealy use of udelay(), you have to
update loops_per_jiffy at the same time as init_current_timer_delay().
It could be done in init_current_timer_delay(), or platforms can take
care of that, that need udelay() available early. Either one should be
fine with me.
--
Shinya Kuribayashi
Renesas Electronics
next prev parent reply other threads:[~2012-07-17 3:10 UTC|newest]
Thread overview: 38+ messages / expand[flat|nested] mbox.gz Atom feed top
2012-06-29 17:33 [PATCH v2 0/2] Use architected timers for delay loop Will Deacon
2012-06-29 17:33 ` [PATCH v2 1/2] ARM: arch timer: implement read_current_timer and get_cycles Will Deacon
2012-07-02 19:14 ` Stephen Boyd
2012-07-05 12:35 ` Shinya Kuribayashi
2012-07-05 12:59 ` Will Deacon
2012-06-29 17:33 ` [PATCH v2 2/2] ARM: delay: allow timer-based delay implementation to be selected Will Deacon
2012-07-02 19:14 ` Stephen Boyd
2012-07-02 21:53 ` Will Deacon
2012-07-03 12:09 ` Shinya Kuribayashi
2012-07-04 15:36 ` Will Deacon
2012-07-05 12:12 ` Shinya Kuribayashi
2012-07-05 12:56 ` Will Deacon
2012-07-05 16:51 ` Stephen Boyd
2012-07-05 13:06 ` Shinya Kuribayashi
2012-07-05 14:15 ` Will Deacon
2012-07-12 7:33 ` Shinya Kuribayashi
2012-07-12 8:44 ` Will Deacon
2012-07-12 9:35 ` Shinya Kuribayashi
2012-07-12 16:40 ` Stephen Boyd
2012-07-13 2:16 ` Shinya Kuribayashi
2012-07-13 8:57 ` Will Deacon
2012-07-13 10:48 ` Shilimkar, Santosh
2012-07-13 11:13 ` Will Deacon
2012-07-13 12:04 ` Shilimkar, Santosh
2012-07-13 12:08 ` Will Deacon
2012-07-13 12:14 ` Shilimkar, Santosh
2012-07-13 12:23 ` Will Deacon
2012-07-13 12:28 ` Shilimkar, Santosh
2012-07-17 3:10 ` Shinya Kuribayashi [this message]
2012-07-17 6:11 ` Shilimkar, Santosh
2012-07-17 7:42 ` Shinya Kuribayashi
2012-07-17 9:05 ` Will Deacon
2012-07-19 12:43 ` Shinya Kuribayashi
2012-07-18 17:52 ` Will Deacon
2012-07-19 15:19 ` Jonathan Austin
2012-07-20 10:17 ` Will Deacon
2012-07-24 9:06 ` Shinya Kuribayashi
2012-07-24 9:15 ` Will Deacon
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