Locking in the clk API

[Saravana Kannan]

On 01/21/2011 02:28 PM, Colin Cross wrote:
> On Fri, Jan 21, 2011 at 2:02 PM, Russell King - ARM Linux
> <linux at arm.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?

-Saravana

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