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

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Date:	Sat, 10 Oct 2015 16:33:36 +0100
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Måns Rullgård <mans@...sr.com>
Cc:	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	linux-omap@...r.kernel.org, linux-cris-kernel@...s.com,
	linux-mips@...ux-mips.org, linux-xtensa@...ux-xtensa.org,
	kernel@...inux.com, linux-rpi-kernel@...ts.infradead.org,
	linux-samsung-soc@...r.kernel.org, linux-tegra@...r.kernel.org
Subject: Re: [PATCH] sched_clock: add data pointer argument to read callback

On Sat, Oct 10, 2015 at 01:42:47AM +0100, Måns Rullgård wrote:
> Russell King - ARM Linux <linux@....linux.org.uk> writes:
> 
> > On Sat, Oct 10, 2015 at 12:48:22AM +0100, Måns Rullgård wrote:
> >> Russell King - ARM Linux <linux@....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.
> >
> > 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.
> 
> So this is an instance where tools like sparse get in the way of doing
> the simplest, most efficient, and obviously correct thing.  Who wins in
> such cases?

In that case, NAK on the patch.  I don't have time for your stupid games.

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