linux-kernel - Re: Locking in the clk API

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Message-ID: <4D41D5EC.6030405@codeaurora.org>
Date:	Thu, 27 Jan 2011 12:30:36 -0800
From:	Saravana Kannan <skannan@...eaurora.org>
To:	Russell King - ARM Linux <linux@....linux.org.uk>
CC:	Lorenzo Pieralisi <Lorenzo.Pieralisi@....com>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	linux-sh <linux-sh@...r.kernel.org>,
	Ben Herrenschmidt <benh@...nel.crashing.org>,
	Sascha Hauer <s.hauer@...gutronix.de>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Colin Cross <ccross@...gle.com>,
	Ben Dooks <ben-linux@...ff.org>,
	Uwe Kleine-König 
	<u.kleine-koenig@...gutronix.de>,
	Jeremy Kerr <jeremy.kerr@...onical.com>,
	linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
	Richard Zhao <linuxzsc@...il.com>
Subject: Re: Locking in the clk API

On 01/27/2011 12:54 AM, Russell King - ARM Linux wrote:
> On Wed, Jan 26, 2011 at 08:34:20PM -0800, Saravana Kannan wrote:
>> I'm not too familiar with serial/tty, does anyone know if the
>> .set_termios needs to be atmoic? If not, we could just change
>> cpm_uart/cpm_uart_core.c to use mutex instead of spinlock.
>
> The locking is there to protect against the interrupt handler accessing
> the port->* stuff (which seems to have been forgotten by the cpm driver).
>
> I don't see any reason why clk_set_rate() needs to be under the spinlock
> there - we need the reprogramming of the baud rate within the spinlock
> on 8250 because of DLAB bit hiding the data registers.  It's also a good
> idea that it _is_ within the spinlock along with uart_update_timeout()
> to ensure timeouts and the baud rate are updated together.

For internal tree purposes, does .set_termios need to be atomic? Can it 
grab mutexes instead of spinlock?

Going back to the topic, how about CPU freq drivers possibly using 
clk_set_rate() to change freq? Do you think that's not the case or a 
concern?

All,

Do any one of your mach's control CPU freq using clk_set_rate() and does 
it need to be atomic? CPUfreq doesn't need it to be atomic. So, you will 
need clk_set_rate() to be atomic only if you try to use it to lower CPU 
freq very late during idle/suspend.

-Saravana

-- 
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The Qualcomm Innovation Center, Inc. is a member of the Code Aurora Forum.
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