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Re: [PATCH] sched_clock: add data pointer argument to read callback

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Russell King - ARM Linux <linux@arm.linux.org.uk> writes:

> On Fri, Oct 09, 2015 at 10:57:35PM +0100, Mans Rullgard wrote:
>> This passes a data pointer specified in the sched_clock_register()
>> call to the read callback allowing simpler implementations thereof.
>> 
>> In this patch, existing uses of this interface are simply updated
>> with a null pointer.
>
> This is a bad description.  It tells us what the patch is doing,
> (which we can see by reading the patch) but not _why_.  Please include
> information on why the change is necessary - describe what you are
> trying to achieve.

Currently most of the callbacks use a global variable to store the
address of a counter register.  This has several downsides:

- Loading the address of a global variable can be more expensive than
  keeping a pointer next to the function pointer.

- It makes it impossible to have multiple instances of a driver call
  sched_clock_register() since the caller can't know which clock will
  win in the end.

- Many of the existing callbacks are practically identical and could be
  replaced with a common generic function if it had a pointer argument.

If I've missed something that makes this a stupid idea, please tell.

-- 
Måns Rullgård
mans@mansr.com
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Re: [PATCH] sched_clock: add data pointer argument to read callback Russell King - ARM Linux <linux@arm.linux.org.uk> - 2015-10-10 01:30 +0200
  Re: [PATCH] sched_clock: add data pointer argument to read callback Måns Rullgård <mans@mansr.com> - 2015-10-10 01:50 +0200
    Re: [PATCH] sched_clock: add data pointer argument to read callback Russell King - ARM Linux <linux@arm.linux.org.uk> - 2015-10-10 02:00 +0200
      Re: [PATCH] sched_clock: add data pointer argument to read callback Måns Rullgård <mans@mansr.com> - 2015-10-10 02:50 +0200
        Re: [PATCH] sched_clock: add data pointer argument to read callback Russell King - ARM Linux <linux@arm.linux.org.uk> - 2015-10-10 17:40 +0200

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