Re: [Xen-devel] [PATCH v3 04/11] x86/intel_pstate: relocate the driver register function

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 23.06.15 at 10:24, <wei.w.wang@xxxxxxxxx> wrote:
> On 23/06/2015 16:11, Jan Beulich wrote:
>> >>> On 23.06.15 at 10:01, <wei.w.wang@xxxxxxxxx> wrote:
>> > On 23/06/2015 15:31, Jan Beulich wrote:
>> >> >>> On 23.06.15 at 05:40, <wei.w.wang@xxxxxxxxx> wrote:
>> >> > On 18/06/2015 22:30, Jan Beulich wrote:
>> >> >> >>> On 11.06.15 at 10:27, <wei.w.wang@xxxxxxxxx> wrote:
>> >> >> > -static int __init cpufreq_presmp_init(void)
>> >> >> > +int cpufreq_register_driver(struct cpufreq_driver *driver_data)
>> >> >> >  {
>> >> >> >      void *cpu = (void *)(long)smp_processor_id();
>> >> >> >      cpu_callback(&cpu_nfb, CPU_ONLINE, cpu);
>> >> >> > +    if (!driver_data || !driver_data->init
>> >> >> > +        || !driver_data->verify || !driver_data->exit
>> >> >> > +        || (!driver_data->target == !driver_data->setpolicy))
>> >> >>
>> >> >> Do you really want/need to enforce this policy (target set if and
>> >> >> only if setpolicy is not set) here? And if that's to uniformly
>> >> >> hold, the two could be put into a union...
>> >> >
>> >> > driver_data->target() is used by a driver which relies on the old
>> >> > Governor framework.
>> >> > driver_data->setpolicy() is used by a driver which implements its
>> >> > internal governor.
>> >> > So, the driver either uses the old Governor framework or has its
>> >> > own private internal governor.
>> >> > We shouldn't change to use union, because in many places, we
>> >> > distinguish the two by checking if it's using "->target" or 
>> >> > "->setpolicy".
>> >>
>> >> The distinction between the two driver modes shouldn't be based on
>> >> arbitrary accessors they may or may not implement. There should be a
>> >> dedicated flag or alike.
>> >
>> > This is not arbitrary - "->target()" is dedicated to the Governor
>> > framework, and "->setpolicy" is dedicated to the internal governor
>> > implementation. The Linux kernel also takes advantage of this method.
>> > I think we don't need to add another new functionally equivalent flag to do
>> so.
>> > Shall we keep using it?
>> 
>> If this distinction is being made clear by comments accompanying the
>> definitions of the target and setpolicy fields, I'm fine with keeping it 
>> that way.
>> Without making this explicit it would continue to look arbitrary (and prone 
>> to
>> break, should another driver elect to also implement the setpolicy hook [for
>> whatever purpose]).
> 
> OK, I will add some explanation to the related commit message.

The commit message will not (normally) be read by people wanting
to make further adjustments to the code. Such an explanation
belongs - as I said - alongside the field declarations.

Jan


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