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

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Message-ID: <yw1xio6fitzd.fsf@unicorn.mansr.com>
Date:	Fri, 09 Oct 2015 22:59:18 +0100
From:	Måns Rullgård <mans@...sr.com>
To:	Stephen Boyd <sboyd@...eaurora.org>
Cc:	Rob Herring <robherring2@...il.com>,
	Mark Rutland <mark.rutland@....com>,
	Pawel Moll <pawel.moll@....com>,
	Ian Campbell <ijc+devicetree@...lion.org.uk>,
	Kumar Gala <galak@...eaurora.org>,
	"devicetree\@vger.kernel.org" <devicetree@...r.kernel.org>,
	"linux-kernel\@vger.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 1/2] devicetree: add binding for generic mmio clocksource

Stephen Boyd <sboyd@...eaurora.org> writes:

> 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.

I've sent a patch.  Let the flames begin.

-- 
Måns Rullgård
mans@...sr.com
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