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

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Message-ID: <yw1x4mhzioxl.fsf@unicorn.mansr.com>
Date:	Sat, 10 Oct 2015 00:48:22 +0100
From:	Måns Rullgård <mans@...sr.com>
To:	Russell King - ARM Linux <linux@....linux.org.uk>
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

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.

-- 
Måns Rullgård
mans@...sr.com
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