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

From: Russell King - ARM Linux
Date: Fri Oct 09 2015 - 19:54:56 EST


On Sat, Oct 10, 2015 at 12:48:22AM +0100, Måns Rullgård wrote:
> Russell King - ARM Linux <linux@xxxxxxxxxxxxxxxx> 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.

So my next question is whether you intend to pass an iomem pointer
through this, or a some kind of structure, or both. It matters,
because iomem pointers have a __iomem attribute to keep sparse
happy. Having to force that attribute on and off pointers is frowned
upon, as it defeats the purpose of the sparse static checker.

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