Date: Thu, 19 Sep 2013 07:48:47 -0700
From: =?utf-8?B?U8O2cmVu?= Brinkmann <soren.brinkmann@xilinx.com>
To: Guenter Roeck <linux@roeck-us.net>
CC: Joe Perches <joe@perches.com>, Rob Herring <rob.herring@calxeda.com>,
        Pawel Moll <pawel.moll@arm.com>, Mark Rutland <mark.rutland@arm.com>,
        Stephen Warren <swarren@wwwdotorg.org>,
        Ian Campbell <ijc+devicetree@hellion.org.uk>,
        Rob Landley <rob@landley.net>, Mike Turquette <mturquette@linaro.org>,
        Grant Likely <grant.likely@linaro.org>,
        Sebastian Hesselbarth <sebastian.hesselbarth@gmail.com>,
        <devicetree@vger.kernel.org>, <linux-doc@vger.kernel.org>,
        <linux-kernel@vger.kernel.org>, <linux-arm-kernel@lists.infradead.org>,
        Hyun Kwon <hyunk@xilinx.com>
Subject: Re: [PATCH v2] clk: si570: Add a driver for SI570 oscillators
References: <1379544219-23579-1-git-send-email-soren.brinkmann@xilinx.com>
 <1379545361.1787.87.camel@joe-AO722>
 <0da25c5c-660c-4836-8179-6e7c51fd0d9f@CO9EHSMHS005.ehs.local>
 <1379546336.1787.95.camel@joe-AO722>
 <1119f60e-99dd-45f8-8d91-2b0fa8b7f03b@DB8EHSMHS027.ehs.local>
 <20130919002308.GA11266@roeck-us.net>
MIME-Version: 1.0
Content-Type: text/plain; charset="utf-8"
Content-Disposition: inline
In-Reply-To: <20130919002308.GA11266@roeck-us.net>
User-Agent: Mutt/1.5.21 (2010-09-15)
Message-ID: <9037e962-30eb-4762-84c4-14bb266a504a@TX2EHSMHS014.ehs.local>
Content-Transfer-Encoding: 8BIT
Sender: linux-kernel-owner@vger.kernel.org
Content-Length: 3089
Lines: 64

On Wed, Sep 18, 2013 at 05:23:08PM -0700, Guenter Roeck wrote:
> 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.
> 

Okay, so I'll use a real range. 10 - 12 ms for big frequency changes and
100 - 200 us for small ones, as Guenther suggests. Does that sound okay?

	Thanks,
	Sören


--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/