linux-kernel - Re: [PATCH RFC v4 2/2] clk: Add handling of clk parent and rate assigned from DT

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Message-ID: <20140402053715.GV17250@pengutronix.de>
Date:	Wed, 2 Apr 2014 07:37:15 +0200
From:	Sascha Hauer <s.hauer@...gutronix.de>
To:	Greg KH <gregkh@...uxfoundation.org>
Cc:	Sylwester Nawrocki <s.nawrocki@...sung.com>,
	Ben Dooks <ben.dooks@...ethink.co.uk>,
	devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org, mturquette@...aro.org,
	linux@....linux.org.uk, robh+dt@...nel.org,
	grant.likely@...aro.org, mark.rutland@....com,
	galak@...eaurora.org, kyungmin.park@...sung.com,
	sw0312.kim@...sung.com, m.szyprowski@...sung.com,
	t.figa@...sung.com, laurent.pinchart@...asonboard.com
Subject: Re: [PATCH RFC v4 2/2] clk: Add handling of clk parent and rate
 assigned from DT

On Tue, Apr 01, 2014 at 09:37:44AM -0700, Greg KH wrote:
> On Tue, Apr 01, 2014 at 04:23:12PM +0200, Sylwester Nawrocki wrote:
> > On 01/04/14 15:19, Ben Dooks wrote:
> > > On 31/03/14 21:06, Greg KH wrote:
> > >> > On Mon, Mar 31, 2014 at 06:41:56PM +0200, Sylwester Nawrocki wrote:
> > [...]
> > >> > I don't understand why you need the driver core to initialize this one
> > >> > type of thing?  That should be in a driver, or in a class, or at worse
> > >> > case, the platform code.
> > >> >
> > >> > What makes clocks so "unique" here?
> > 
> > The reason I put it in the driver core was mainly to avoid having many
> > drivers doing same call to this initialization function.
> > I was considering moving it to the bus code, still there are several
> > buses for which it would need to be repeated.
> 
> "several" is how many?  2?  3?  10?
> 
> Please fix it "correctly" and don't put it in the driver core just
> because it seems easier that way.
> 
> > Maybe really_probe() is not a best place to put this, nonetheless
> > the requirements I could list were:
> > 
> >  1. not involving individual drivers,
> 
> Why not?
> 
> >  2. have such an initialization call done for all devices, irrespective
> >     of Linux bus or class type,
> 
> Why?  Do _all_ devices that Linux supports have this issue to be
> resolved?
> 
> >  3. Handle errors properly, e.g. defer driver probing if a clock for
> >    a device is not yet available.
> 
> Then do it in the bus that controls that device, as it knows to defer
> probing at that point in time.

The issue this patch tries to solve is not about single devices, but
about the way these devices are connected with each other.

Clocks for different (sometimes completely unrelated) devices influence
each other. You can't control the clock from one device without
influencing other devices. Knowledge about these constraints can't be
encoded in the drivers, because the constraints differ per SoC,
sometimes even per board. Many SoCs share the same devices, but the
clock topology they are surrounded with is always different. With this I
don't mean the direct clock inputs, these are well abstracted with the
current clk_* API.  What I mean is situations like: "On this board use
the clock controller to output this particular clock on that pin,
because it happens to be the master clock of some audio codec connected
externally; also make the same clock input to the internal Audio system
to make sure both are in sync". These situations can be completely
different on the next board or on the next SoC which has the same
devices, but a different clock routing.

That said, I also think the driver core doesn't have to be bothered with
the clock setup. Putting the clock setup into the devicenode providing
the clocks (and thus parsing it from the clock controller driver) should
be sufficient.

Sascha

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