Thanks Ayan.
My answers are inline below -
On Fri, Aug 15, 2014 at 6:41 PM, AYAN KUMAR HALDER <ayankumarh@xxxxxxxxx> wrote:
> On Fri, Aug 15, 2014 at 3:50 PM, Mohammad Merajul Islam Molla
> <meraj.enigma@xxxxxxxxx> wrote:
>> Hello,
>>
>> I was looking into the code of drivers/cpuidle/driver.c. I have some
>> doubts regarding the implementation of __cpuidle_set_driver function
>> when CONFIG_CPU_IDLE_MULTIPLE_DRIVERS is defined.
>>
>> If CONFIG_CPU_IDLE_MULTIPLE_DRIVERS is defined, the code for
>> __cpuidle_set_driver/__cpuidle_unset_driver looks as -
>>
>> 39 * __cpuidle_unset_driver - unset per CPU driver variables.
>> 40 * @drv: a valid pointer to a struct cpuidle_driver
>> 41 *
>> 42 * For each CPU in the driver's CPU mask, unset the registered
>> driver per CPU
>> 43 * variable. If @drv is different from the registered driver, the
>> corresponding
>> 44 * variable is not cleared.
>> 45 */
>> 46 static inline void __cpuidle_unset_driver(struct cpuidle_driver *drv)
>> 47 {
>> 48 int cpu;
>> 49
>> 50 for_each_cpu(cpu, drv->cpumask) {
>> 51
>> 52 if (drv != __cpuidle_get_cpu_driver(cpu))
>> 53 continue;
>> 54
>> 55 per_cpu(cpuidle_drivers, cpu) = NULL;
>> 56 }
>> 57 }
>> 58
>> 59 /**
>> 60 * __cpuidle_set_driver - set per CPU driver variables for the given driver.
>> 61 * @drv: a valid pointer to a struct cpuidle_driver
>> 62 *
>> 63 * For each CPU in the driver's cpumask, unset the registered driver per CPU
>> 64 * to @drv.
>> 65 *
>> 66 * Returns 0 on success, -EBUSY if the CPUs have driver(s) already.
>> 67 */
>> 68 static inline int __cpuidle_set_driver(struct cpuidle_driver *drv)
>> 69 {
>> 70 int cpu;
>> 71
>> 72 for_each_cpu(cpu, drv->cpumask) {
>> 73
>> 74 if (__cpuidle_get_cpu_driver(cpu)) {
>> 75 __cpuidle_unset_driver(drv);
>> 76 return -EBUSY;
>> 77 }
>> 78
>> 79 per_cpu(cpuidle_drivers, cpu) = drv;
>> 80 }
>> 81
>> 82 return 0;
>> 83 }
>>
>> Apparently, the comment should be - "set/register the driver per CPU
>> to @drv" instead of "unset the registered driver per CPU to @drv" in
>> case of __cpuidle_set_driver.
>
> I would like to slightly differ here. As per my understanding, the
> comments given
> for the function is correct. The function __cpuidle_set_driver(drv)
> would do the following:-
>
> 1. If the cpu has any registered idle driver which is same as @drv,
> the registered
> driver would be unset. ie "unset the registered driver per CPU to @drv"
> 2. If the cpu has any registered idle driver which is different from
> @drv, do nothing
> 3. If the cpu has no registered idle driver, set the idle driver to @drv
>
>> However, regarding the logic, I have a few doubts -
>>
>> 1. for each cpu in drv->cpumask, if there is already a driver
>> registered, its calling __cpuidle_unset_driver which loops over for
>> each cpu in drv->cpumask again. Isn't it unnecessary to do this nested
>> calls?
>
> __cpuidle_unset_driver :- This function gets called from
> "__cpuidle_unregister_driver()" too.
> So it needs to loop over each cpu to see if its registered driver is
> same as @drv. What
Yes, to unset the driver __cpuidle_unregister_driver calls
__cpuidle_unset_driver(),
which has to loop over the cpus. But __cpuidle_set_driver() can be
implemented itself and therefore,
I don't see any point doing the loop twice.
> you might be trying to convey here is that instead of calling
> __cpuidle_unset_driver, we could
> have done the following:-
> static inline int __cpuidle_set_driver(struct cpuidle_driver *drv)
> {
> int cpu;
> struct cpuidle_driver *tmp = NULL;
> for_each_cpu(cpu, drv->cpumask) {
>
> tmp = __cpuidle_get_cpu_driver(cpu); /* This would
> prevent nesting of loops */
> if ( tmp != NULL ) {
> if ( tmp == drv )
> per_cpu(cpuidle_drivers, cpu) = NULL;
> return -EBUSY;
> }
>
> per_cpu(cpuidle_drivers, cpu) = drv;
> }
>
> return 0;
> }
>
>
>
>> 2. after calling __cpuidle_unset_driver, if drv equals already
>> registered driver, it sets per_cpu driver to null? Isn't it wrong when
>> we are trying to set to a new driver? Why do we need to unset and make
>> the driver null when we are returning EBUSY from __cpuidle_set_driver?
>
> My understanding is that if there is a previously registered cpuidle
> driver, returning
> EBUSY is fine. But I do share the same doubt as you have that if the previous
> registered cpuidle driver is same as the new one, then why should it
> be unset and NULLed.
Right, I don't understand this either.
>
>> Would it be correct and cleaner if the code is written as below -
>>
>> static inline int __cpuidle_set_driver(struct cpuidle_driver *drv)
>> {
>> int ret = -EBUSY;
>> int cpu;
>>
>> for_each_cpu(cpu, drv->cpumask) {
>> if (drv == __cpuidle_get_cpu_driver(cpu)) [if drv
>> is already the registered driver, do nothing]
>> continue;
>>
>> per_cpu(cpuidle_drivers, cpu) = drv; [if only drv !=
>> already registered driver, set per_cpu driver to drv and set ret 0]
>> ret = 0;
>> }
>>
>> return ret; [only if all cpus already had drv as
>> registered driver, return -EBUSY. Otherwise return 0]
>> }
>>
> The difference that might cause some trouble is that the timer
> broadcast notification is
> not sent while changing the cpuide drivers.
>
> Regards,
> Ayan Kumar Halder
Anyone else looking at this? Please clarify.
Thanks,
-Meraj
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