linux-kernel - Re: [PATCH v2] clk: si570: Add a driver for SI570 oscillators

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Message-ID: <20130919002308.GA11266@roeck-us.net>
Date:	Wed, 18 Sep 2013 17:23:08 -0700
From:	Guenter Roeck <linux@...ck-us.net>
To:	Sören Brinkmann <soren.brinkmann@...inx.com>
Cc:	Joe Perches <joe@...ches.com>,
	Rob Herring <rob.herring@...xeda.com>,
	Pawel Moll <pawel.moll@....com>,
	Mark Rutland <mark.rutland@....com>,
	Stephen Warren <swarren@...dotorg.org>,
	Ian Campbell <ijc+devicetree@...lion.org.uk>,
	Rob Landley <rob@...dley.net>,
	Mike Turquette <mturquette@...aro.org>,
	Grant Likely <grant.likely@...aro.org>,
	Sebastian Hesselbarth <sebastian.hesselbarth@...il.com>,
	devicetree@...r.kernel.org, linux-doc@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	Hyun Kwon <hyunk@...inx.com>
Subject: Re: [PATCH v2] clk: si570: Add a driver for SI570 oscillators

On Wed, Sep 18, 2013 at 04:32:59PM -0700, Sören Brinkmann wrote:
> On Wed, Sep 18, 2013 at 04:18:56PM -0700, Joe Perches wrote:
> > On Wed, 2013-09-18 at 16:09 -0700, Sören Brinkmann wrote:
> > > On Wed, Sep 18, 2013 at 04:02:41PM -0700, Joe Perches wrote:
> > > > On Wed, 2013-09-18 at 15:43 -0700, Soren Brinkmann wrote:
> > > > > Add a driver for SILabs 570, 571, 598, 599 programmable oscillators.
> > > > > The devices generate low-jitter clock signals and are reprogrammable via
> > > > > an I2C interface.
> > > > []
> > > > > v2:
> > > > []
> > > > >  - use 10000 as MIN and MAX value in usleep_range
> > > > []
> > > > > diff --git a/drivers/clk/clk-si570.c b/drivers/clk/clk-si570.c
> > > > []
> > > > > +static int si570_set_frequency(struct clk_si570 *data, unsigned long frequency)
> > > > > +{
> > > > []
> > > > > +	/* Applying a new frequency can take up to 10ms */
> > > > > +	usleep_range(10000, 10000);
> > > > 
> > > > Generally it's nicer to have an actual range for usleep_range.
> > > Well, as I said in the discussion with Guenther. I'm flexible and nobody
> > > objected when I said to make both equal. A real range doesn't make sense
> > > here though, but I don't know what's common practice for cases like
> > > this.
> > 
> > udelay is normal, but I guess you don't need atomic context.
> After checkpatch correcting me a few times I went with what
> Documentation/timers/timers-howto.txt suggests. But yes, then we have
> this situation, that I want to sleep 10ms, but not longer using a
> *_range function. I guess it is very application specific whether a
> longer delay here is acceptable or not.
> 
You really want to sleep and not call udelay for 10ms. The idea behind usleep_range
is that you give the kernel some slack. In this case, you could for example make it
10-12 ms. That doesn't make much difference for the driver, but it might save a
timer interrupt in the kernel because it might be able to coalesce more than one
event. After all, it doesn't have to be _exactly_ 10 ms, which is what you are
claiming with the fixed number. Prior to usleep_range, you would have happily
called msleep(10) without realizing that it might sleep up to 20 ms on you.
Keep that in mind ...

> You're right. I'll add a delay there as well. The 'rang' question
> applies here as well.
> 
Same thing, really. You could make it 100-200uS. That doesn't make much
difference for this driver, but it might make a difference for overall
performance, especially if everyone is playing nicely.

Guenter
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