[PATCH 01/10] Add a common struct clk

[skannan@codeaurora.org (Saravana Kannan)]

On 05/02/2011 03:36 PM, Russell King - ARM Linux wrote:
> On Mon, May 02, 2011 at 11:30:10AM -0500, Rob Herring wrote:
>> On 05/01/2011 10:40 PM, Jeremy Kerr wrote:
>>> Hi Rob,
>>>
>>>> I think you will find many examples in the kernel where that is not done
>>>> by drivers.
>>>
>>> Drivers should be checking the return value of clk_get - if they don't,
>>> it's a bug. This is the logical place to check, rather than before all
>>> clock API calls.
>>
>> Maybe so, but it's common practice. Why not allow it, but add a warning?
>> Or allow NULL, but not an error value.
>
> If drivers don't check the return value of clk_get() then they'll suffer
> kernel oopses.  What's to say that if the driver doesn't check the return
> value from clk_get() that they'll bother checking the return value from
> any of the other clk_* API functions.  Nothing.
>
> So, checking for ERR values inside the clk_* functions is silly, and just
> means that buggy drivers continue with undiscovered bugs because they
> happen not to fail.
>
>>> For cases where there is no clock provided for the device (but is a
>>> valid clock on some machines), the platform code should return a no-op
>>> clock from the clk_get call. This 'noop clock' would be a good contender
>>> for inclusion into the kernel-wide infrastructure, like clk_fixed.
>>
>> There could be cases where the driver wants to know if there is no
>> clock.
>
> How exactly would a driver know that the clock it has is a no-op clock
> with it being a pointer to some random data structure?  A better question
> would be in the case of a cpufreq driver, how would such a driver know
> that it's pointless trying to set the rate on a fixed rate clock?
>
> There's soo many combinations here where you can say "if a clock has
> property X, then maybe the driver doesn't want to do Y".  So, instead
> of trying to cover all the possibilities, take a step back and look at
> the bigger picture.
>
> 1. in a cpufreq driver, if it gets a clock, one would hope that the
> clock it has been provided with is adjustable - in other words,
> clk_set_rate() stands a chance of succeeding.  If it fails, then it
> has the wrong clock.
>
> 2. if the clock it has does not support adjustments in this way, then
> there is a bug in the data involved in selecting that clock, and that's
> what's at fault.  You find this out when you try to change its rate
> via clk_set_rate() which should fail.
>
> 3. (to head off something that I can see being proposed) no, I don't
> think we need a system of properties.
>
> A cpufreq driver which obtains clks should be no different from any
> other driver.  It should get a clk via clk_get(), and report errors if
> that fails.  If clk_set_rate() fails, then it should report that error
> too.  There's really nothing "special" here.
>
> This does bring us to an interesting question though: should clk_set_rate()
> succeed or fail with a NULL clk?  There is no clock to control, so my
> feeling is that it should fail, just like clk_get_rate() should return
> zero because the rate is meaningless.  There is no rate to get and no
> rate to set.

I think having NULL clocks return success on set_rate() would be more 
useful. The most common use case for NULL clocks is when a clocks is 
present in some soc/arch/mach but is not present in another. Instead of 
having the consumers having an "if" around every clk_* calls on that 
clk, they would get a NULL clk. If NULL clk starts failing for 
clk_set_rate(), I think the point of a NULL clk will get defeated.

-Saravana

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