Date: Wed, 18 Feb 2015 15:06:02 +0000
From: Lee Jones <lee.jones@linaro.org>
To: Sebastian Hesselbarth <sebastian.hesselbarth@gmail.com>
Cc: Antoine Tenart <antoine.tenart@free-electrons.com>, sameo@linux.intel.com,
        jszhang@marvell.com, zmxu@marvell.com,
        linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org,
        arnd@arndb.de
Subject: Re: [PATCH 01/11] mfd: add the Berlin controller driver
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On Wed, 18 Feb 2015, Sebastian Hesselbarth wrote:

> On 02/18/2015 12:58 PM, Lee Jones wrote:
> >I do agree that using 'simple-bus' to describe only this IP would be
> >an abuse.  However, my foundation thought/argument is unchanged.  This
> >'driver' is a hack.  It has no functional use besides to work around a
> >problem of semantics and as such has no place in MFD.
> 
> Lee,
> 
> sorry I don't get it. Here you say that using simple-bus is an abuse...
> 
> >Back onto the simple-bus theme, as this is a syscon device it is a bus
> >of sorts.  Have you thought about making it a child of your its syscon
> >node, then using simple-bus to get the OF framework to register the
> >child devices?
> 
> ... and here you suggest to use simple-bus to register the child
> devices?

Nope, that's not what I said:

  "I do agree that using 'simple-bus' to describe *ONLY THIS IP* would
  be an abuse."

... although I believe there is a need to treat syscon devices as
simple buses.  There are examples of devices doing this already:

git grep -El 'syscon.*simple-bus' next/master
  next/master:arch/arm/boot/dts/imx6qdl.dtsi
  next/master:arch/arm/boot/dts/imx6sl.dtsi
  next/master:arch/arm/boot/dts/imx6sx.dtsi
  next/master:arch/arm/boot/dts/zynq-7000.dtsi

> I fundamentally disagree that either this registers or syscon in general
> should in any way be seen as a bus. The chip control registers is an
> highly unsorted bunch of bits that we try to match with cleanly
> separated subsystems. This makes it a resource but no bus of any sort.

This is where my comment about semantics comes into play.  syscon may
not be a bus is the truest sense; however, this is clearly a
requirement for sub devices to be probed in the same way a simple-bus
is currently.  So we're just missing a framework somewhere.  We can
fix that.

> The problem that we try to solve here is not a DT problem but solely
> driven by the fact that we need something to register platform_devices
> for pinctrl and reset. The unit we describe in DT is a pinctrl-clock-
> power-reset-unit - or short chip control.

I agree with the last part, but this is a DT problem.  It lacks the
functionality to be able to cleanly register these types of
(sub-)devices.  Devices which, in my opinion should be described
inside the parent syscon node.

> If you argue that mfd is not the right place for this "driver" we'll
> have to find a different place for it. I remember Mike has no problem
> with extending early probed clock drivers to register additional
> platform_devices - so I guess we end up putting it in there ignoring
> mfd's ability to do it for us.

My argument is not that this fake driver doesn't belong in MFD, it's
that it doesn't belong.  That includes shoving it in drivers/clk.  I
will be only too happy to have a chat with Mike and provide him with
my reasons why.

What I think we should do however, it write some framework code which
can neatly handle these use-cases, which may just be a case of:

  s/of_platform_bus_probe/of_platform_subdevice_probe/

... obviously I'm oversimplifying by quite some margin, but I'm sure
you catch my drift.

-- 
Lee Jones
Linaro STMicroelectronics Landing Team Lead
Linaro.org │ Open source software for ARM SoCs
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