linux-kernel - Re: Locking in the clk API

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Message-ID: <4D42379D.5030503@codeaurora.org>
Date:	Thu, 27 Jan 2011 19:27:25 -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 12:43 PM, Russell King - ARM Linux wrote:
> 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?)

Looks like you are confident to consider clk_set_rate() as sleepable. 
Can we add a comment to clk.h that says so?

Otherwise, there is no point in the suggesting the 
clk_prepare/unprepare() APIs.

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