linux-kernel - Re: Locking in the clk API

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Message-ID: <20110127204354.GB1597@n2100.arm.linux.org.uk>
Date:	Thu, 27 Jan 2011 20:43:54 +0000
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Saravana Kannan <skannan@...eaurora.org>
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 Thu, Jan 27, 2011 at 12:30:36PM -0800, Saravana Kannan wrote:
> 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?

I think I already answered that question above where I said "protect
against the interrupt handler accessing the port->* stuff".

> 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?

CPUfreq drivers probably should busy-wait until the CPU PLL has locked
if the CPU is allowed to continue running while the PLL relocks.  Some
implementations will halt the CPU while the PLL is transitioning and
that's really not unreasonable to do.

I think some even require the code to be running out of SRAM and SDRAM
remain untouched while the PLL is transitioning (omap maybe?)
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