linux-kernel - Re: [PATCH v2 1/2] devicetree: add binding for generic mmio clocksource

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Date:	Fri, 9 Oct 2015 11:46:32 -0700
From:	Stephen Boyd <sboyd@...eaurora.org>
To:	Rob Herring <robherring2@...il.com>
Cc:	Måns Rullgård <mans@...sr.com>,
	Mark Rutland <mark.rutland@....com>,
	Pawel Moll <pawel.moll@....com>,
	Ian Campbell <ijc+devicetree@...lion.org.uk>,
	Kumar Gala <galak@...eaurora.org>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 1/2] devicetree: add binding for generic mmio
 clocksource

On 10/09, Rob Herring wrote:
> +Stephen who has worked on this code.
> 
> On Fri, Oct 9, 2015 at 11:19 AM, Måns Rullgård <mans@...sr.com> wrote:
> > Måns Rullgård <mans@...sr.com> writes:
> >
> >> Rob Herring <robherring2@...il.com> writes:
> >>
> >>> On Wed, Oct 7, 2015 at 11:47 AM, Måns Rullgård <mans@...sr.com> wrote:
> >>>> What would be a proper way to select a sched_clock source?  I realise
> >>>> it's a Linux-specific thing and DT is supposed to be generic, but the
> >>>> information must be provided somehow.
> >>>
> >>> The kernel already has some logic to do this. Most number of bits
> >>> followed by highest frequency will be the winning sched_clock. You
> >>> might also want to look at things like always on or not.
> >>
> >> The problem is that sched_clock_register() doesn't take a pointer to be
> >> passed back to the read_sched_clock callback like most interfaces of
> >> this type do.  This means the callback must use global variables set up
> >> before the register call, but at that time there's no way of knowing
> >> which one will be used.  If there were a way of getting a pointer to the
> >> callback, it would be a simple matter of registering all instances and
> >> letting the kernel choose which to use.
> >
> > Anyone got a comment on this?  Do I have to send a patch adding this
> > before anyone will tell me why it's a bad idea?  (That method almost
> > always works.)
> 
> Adding a ptr to the callback seems fine to me.
> 

Does that mean a flag day? Urgh. Pain. I'm not opposed to adding
a pointer, in fact it might be better for performance so that we
don't take a cache miss in read() functions that need to load
some pointer. We were talking about that problem a few months
ago, but nothing came of it.

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