linux-kernel - Re: [RFC+PATCH] RTC calibration

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Date:	Wed, 31 Oct 2007 12:03:22 +0100
From:	Pavel Machek <pavel@....cz>
To:	Dag-Erling Sm?rgrav <des@...pro.no>
Cc:	Clemens Koller <clemens.koller@...gramm.de>,
	linux-kernel@...r.kernel.org
Subject: Re: [RFC+PATCH] RTC calibration

On Tue 2007-09-11 18:04:06, Dag-Erling Sm?rgrav wrote:
> Clemens Koller <clemens.koller@...gramm.de> writes:
> > Dag-Erling Sm?rgrav <des@...pro.no> writes:
> > > Without knowing exacly which chip is present, there is no way for the
> > > userland calibration tool to know how big a difference a calibration
> > > step makes.
> > I am not talking about the calibration algorithm and it's quality.
> 
> Neither am I.
> 
> > I am talking about _how_ the calibration register is addressed from
> > userspace. It's a simple register, some bits at address 7 and I would
> > expect to read/modify/write registers to do all the things you want
> > to do. Register access in userspace doesn't put any limitation
> > to applications.
> 
> It requires the application to know the hardware intimately.
> 
> Calibration of the M41T11 is implemented using the lower 6 bits of
> register 7; this is not necessarily the case for other existing or
> future chips.

The driver should know the hardware.

> Let's say I normalize this to [-128;127]; an application that tried to
> speed up the clock would waste several hours increasing the
> calibration value from 0 to 1, 2, 3 before seeing an effect after
> increasing it to 4.  And how do I normalize the assymetric range of
> the M41T11?

So you normalize it to -32;32 range, and tell application (using
ioctl) that range is -32;32? Or you just -EINVAL if it goes out of
range?
								Pavel

-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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