Re: [PATCH 1/5] MFD/OF: document MFD devices and handle simple-mfd

[Lee Jones <lee.jones-QSEj5FYQhm4dnm+yROfE0A@public.gmane.org>]

On Tue, 03 Mar 2015, Rob Herring wrote:

> On Tue, Mar 3, 2015 at 3:32 AM, Linus Walleij <linus.walleij@linaro.org> wrote:
> > This defines a new compatible option for MFD devices "simple-mfd" that will
> > make the OF core spawn child devices for all subnodes of that MFD device.
> > It is optional but handy for things like syscon and possibly other
> > simpler MFD devices.
> >
> > Since there was no file to put the documentation in, I took this opportunity
> > to make a small writeup on MFD devices and add the compatible definition
> > there.
> >
> > Suggested-by: Lee Jones <lee.jones-QSEj5FYQhm4dnm+yROfE0A@public.gmane.org>
> > Cc: Arnd Bergmann <arnd-r2nGTMty4D4@public.gmane.org>
> > Cc: Devicetree <devicetree-u79uwXL29TY76Z2rM5mHXA@public.gmane.org>
> > Cc: Rob Herring <robh+dt-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org>
> > Cc: Benjamin Herrenschmidt <benh-XVmvHMARGAS8U2dJNN8I7kB+6BGkLq7r@public.gmane.org>
> > Cc: Grant Likely <grant.likely-QSEj5FYQhm4dnm+yROfE0A@public.gmane.org>
> > Cc: Pawel Moll <pawel.moll-5wv7dgnIgG8@public.gmane.org>
> > Cc: Mark Rutland <mark.rutland-5wv7dgnIgG8@public.gmane.org>
> > Cc: Ian Campbell <ijc+devicetree-KcIKpvwj1kUDXYZnReoRVg@public.gmane.org>
> > Cc: Kumar Gala <galak-sgV2jX0FEOL9JmXXK+q4OQ@public.gmane.org>
> > Signed-off-by: Linus Walleij <linus.walleij-QSEj5FYQhm4dnm+yROfE0A@public.gmane.org>
> > ---
> > I make the patch to the OF core in this one, it makes much more sense since
> > it's a oneliner
> >
> > Lee: this is a (tested!) implementation of your suggestion for simple-mfd.
> > If you can eventually ACK this from the MFD side, I think it should be
> > funneled through the ARM SoC tree.
> >
> > Grant/Rob: if either of you can ACK the change to the OF core likewise it can
> > be taken through ARM SoC.
> >
> > DT binings maintainers: there is some background discussion on this here:
> > http://marc.info/?l=linux-arm-kernel&m=142486676603889&w=2
> > http://marc.info/?l=devicetree&m=142166313621469&w=2
> > ---
> >  Documentation/devicetree/bindings/mfd/mfd.txt | 40 +++++++++++++++++++++++++++
> >  drivers/of/platform.c                         |  1 +
> >  2 files changed, 41 insertions(+)
> >  create mode 100644 Documentation/devicetree/bindings/mfd/mfd.txt
> >
> > diff --git a/Documentation/devicetree/bindings/mfd/mfd.txt b/Documentation/devicetree/bindings/mfd/mfd.txt
> > new file mode 100644
> > index 000000000000..cc057438abe8
> > --- /dev/null
> > +++ b/Documentation/devicetree/bindings/mfd/mfd.txt
> > @@ -0,0 +1,40 @@
> > +Multi-Function Devices (MFD)
> > +
> > +These devices comprise a nexus for heterogeneous hardware blocks spawning
> > +multiple child devices.
> > +
> > +A typical MFD can be:
> > +
> > +- A mixed signal ASIC on an external bus, sometimes a PMIC (power management
> > +  integrated circuit) that is manufactured in a lower technology node (rough
> > +  silicon) that handles analog drivers for things like audio amplifiers, LED
> > +  drivers, level shifters, PHY (physical interfaces to things like USB or
> > +  ethernet), regulators etc.
> > +
> > +- A range of memory registers containing "miscellaneous system registers" also
> > +  known as a system controller "syscon" or any other memory range containing a
> > +  mix of unrelated registers.
> > +
> > +Optional properties:
> > +
> > +- compatible : "simple-mfd" - this signifies that the operating system should
> > +  spawn child devices for all the subnodes of the MFD device akin to how
> > +  "simple-bus" inidicates when to spawn children for a simple memory-mapped
> > +  bus. For more complex devices, when the nexus driver has to probe registers
> > +  to figure out what children exist etc, this should not be used. In the latter
> > +  case the child devices will be instantiated by the operating system.
> 
> If you point of_platform_populate to the parent node, it should
> instantiate all children regardless of "simple-bus" or any other
> match. The match is to probe the grandchildren. I could be mistaken,
> but that's how it works at the root level. You don't have a driver
> probe for the parent, so you want the top level of_platform_populate
> call to instantiate these devices?
> 
> Why does simple-bus not work for you? After all, it is not
> "simple-memory-map-bus."

simple-bus does 'work'.  There are even some (3 I think) platforms
actively making use of it.  However, there was some controversy over
using it, as syscon isn't really a bus.

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