linux-kernel - Re: [RFC,PATCH 2/3] clk: Generic support for fixed-rate clocks

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Message-Id: <201102151418.37780.jeremy.kerr@canonical.com>
Date:	Tue, 15 Feb 2011 14:18:37 +0800
From:	Jeremy Kerr <jeremy.kerr@...onical.com>
To:	Saravana Kannan <skannan@...eaurora.org>
Cc:	linux-arm-kernel@...ts.infradead.org,
	Nicolas Pitre <nicolas.pitre@...aro.org>,
	Lorenzo Pieralisi <Lorenzo.Pieralisi@....com>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	linux-sh@...r.kernel.org,
	Ben Herrenschmidt <benh@...nel.crashing.org>,
	Sascha Hauer <s.hauer@...gutronix.de>,
	Paul Mundt <lethal@...ux-sh.org>, linux-kernel@...r.kernel.org,
	Ryan Mallon <ryan@...ewatersys.com>,
	Dima Zavin <dmitriyz@...gle.com>,
	Ben Dooks <ben-linux@...ff.org>,
	"Uwe Kleine-König" 
	<u.kleine-koenig@...gutronix.de>,
	Russell King <linux@....linux.org.uk>
Subject: Re: [RFC,PATCH 2/3] clk: Generic support for fixed-rate clocks

Hi Saravana,

> >> A fixed clock may still have other operations such as enable/disable.
> > 
> > Then it's not a fixed clock; I'd prefer this to be a separate type, as
> > it's now hardware dependent.
> 
> I'm confused. If a clock's rate can't be changed and it can't be enabled
> or disabled, then what's the point of representing that clock
> signal/line as a clock in the driver.

Because the drivers using this clock don't know that it's a fixed clock.

For example, a uart needs to know the rate of its clock source, so that it can 
set its internal divisors to get a valid baud rate. The uart driver will query  
the input rate using clk_get_rate(). The driver still needs to call 
clk_enable/clk_prepare/etc, because on some systems it may have a switchable 
clock.

Cheers,


Jeremy

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