linux-kernel - Re: [GIT PULL] Greybus driver subsystem for 4.9-rc1

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Date:   Thu, 15 Sep 2016 16:47:36 +0100
From:   Mark Rutland <mark.rutland@....com>
To:     Bryan O'Donoghue <pure.logic@...us-software.ie>
Cc:     Greg KH <gregkh@...uxfoundation.org>,
        Arnd Bergmann <arnd@...db.de>, linux-kernel@...r.kernel.org,
        Johan Hovold <johan@...oldconsulting.com>,
        Rui Miguel Silva <rmfrfs@...il.com>,
        Laurent Pinchart <laurent.pinchart@...asonboard.com>,
        Sandeep Patil <sspatil@...gle.com>,
        Matt Porter <mporter@...nel.crashing.org>,
        John Stultz <john.stultz@...aro.org>,
        Rob Herring <robh@...nel.org>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        Alex Elder <elder@...aro.org>, David Lin <dtwlin@...gle.com>,
        Vaibhav Agarwal <vaibhav.agarwal@...aro.org>,
        Mark Greer <mgreer@...malcreek.com>, marc.zyngier@....com,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [GIT PULL] Greybus driver subsystem for 4.9-rc1

On Thu, Sep 15, 2016 at 04:40:00PM +0100, Bryan O'Donoghue wrote:
> On Thu, 2016-09-15 at 13:46 +0100, Mark Rutland wrote:
> > On Thu, Sep 15, 2016 at 12:48:08PM +0100, Bryan O'Donoghue wrote:
> > > On Thu, 2016-09-15 at 12:20 +0100, Mark Rutland wrote:
> > > > For example, you have absolutely no guarantee as to what backs
> > > > get_cycles(). Despite this, the code assumes that get_cycles() is
> > > > backed by something running at the frequency described in a
> > > > "google,greybus-frame-time-counter" node.
> > > > 
> > > > Even if this *happens* to match what some piece of arch code
> > > > provides
> > > > today on some platform, it is in no way *guaranteed*.
> > > That's the point though, if you declare "google,greybus-frame-time-
> > > counter" in your platform code - then you can use 'get_cycles()' in
> > > this manner
> > To be clear, *some* properties (and perhaps additional nodes) may
> > need
> > to be in the DT, in order to capture the hardware property or
> > relationship that you are reliant upon.
> 
> Sure but on the relevant platform we know 
> 
> 1. get_cycles() is derived from an architectured MMIO timer
> 2. What is clocking that timer
> 
> So any platform that declares that property must be aware of what its
> doing.

I can't say this any more explicitly:

****************************************************
* The DT *cannot* know anything about get_cycles() *
****************************************************

It's no more complex than that.

Thanks,
Mark.