linux-kernel - Re: Locking in the clk API

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Date:	Fri, 21 Jan 2011 18:22:34 -0800
From:	Colin Cross <ccross@...gle.com>
To:	Saravana Kannan <skannan@...eaurora.org>
Cc:	Russell King - ARM Linux <linux@....linux.org.uk>,
	Nicolas Pitre <nicolas.pitre@...aro.org>,
	Dima Zavin <dmitriyz@...gle.com>,
	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,
	Ben Dooks <ben-linux@...ff.org>,
	"Uwe Kleine-K??nig" <u.kleine-koenig@...gutronix.de>,
	Jeremy Kerr <jeremy.kerr@...onical.com>,
	linux-arm-kernel@...ts.infradead.org
Subject: Re: Locking in the clk API

On Fri, Jan 21, 2011 at 5:35 PM, Saravana Kannan <skannan@...eaurora.org> wrote:
> On 01/21/2011 02:28 PM, Colin Cross wrote:
>>
>> On Fri, Jan 21, 2011 at 2:02 PM, Russell King - ARM Linux
>> <linux@....linux.org.uk>  wrote:
>>>
>>> On Fri, Jan 21, 2011 at 04:53:44PM -0500, Nicolas Pitre wrote:
>>>>
>>>> So I think that the API must be augmented with more methods, such as:
>>>>
>>>> clk_slow_enable():
>>>>   - may sleep
>>>>   - may be a no-op if the clk_fast_enable() is supported
>>>>
>>>> clk_fast_enable():
>>>>   - may not sleep, used in atomic context
>>>>   - may be a no-op if controlling the clock takes time, in which case
>>>>     clk_slow_enable() must have set the clock up entirely
>>>>
>>>> ... and similar for clk_slow_disable() and clk_fast_disable().
>>>
>>> Isn't this along the same lines as my clk_prepare() vs clk_enable()
>>> suggestion?
>>>
>>> I suggested that clk_prepare() be callable only from non-atomic contexts,
>>> and do whatever's required to ensure that the clock is available.  That
>>> may end up enabling the clock as a result.
>>>
>>> clk_enable() callable from atomic contexts, and turns the clock on if
>>> the hardware supports such an operation.
>>>
>>> So, if you have something like:
>>>
>>> Xtal--->PLL--->Routing/Masking--->Device
>>>
>>> clk = clk_get() returns the clock for the device.
>>>
>>> clk_prepare(clk) would walk up the clock tree, selecting the routing and
>>> preparing each clock.  Clocks prior to _and_ including the PLL would need
>>> to be enabled.
>>>
>>> clk_enable(clk) would walk up the tree if the clock isn't already
>>> enabled,
>>> calling clk_enable() on the parent clock.  As we require prepared clocks
>>> to already be enabled, this automatically stops at the PLL.
>>>
>>> To encourage correct usage, we just need to make sure that clk_prepare()
>>> has a might_sleep() thing, and clk_enable() throws a fit if it's used
>>> on a clk without prepare being used first.  The second point is not easy
>>> to do in a foolproof manner though, but doing _something_ is better than
>>> nothing.
>>
>> I like this proposal, and I prefer the clk_prepare naming over
>> clk_slow_enable - too many people would call clk_slow_enable instead
>> of, and not as well as, clk_fast_enable.
>>
>> On Tegra, I currently use the ugly conditional mutex or spinlock
>> method to deal with voltage scaling based on clock frequency.
>
> Colin,
>
> MSM is in a similar situation, so thought I should bring this up to you
> attention -- do you have no use case for changing the rate in atomic
> context? If you do, the clk_prepare/unprepare() approach won't work.
>
> Do you have no such requirement?

I can't think of any case that requires clk_set_rate in atomic
context.  We usually only scale frequencies to save power, so we can
always scale up early in sleepable context for a minor power penalty.
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