linux-kernel - Re: [PATCH 1/5] capemgr: Beaglebone DT overlay based cape manager

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Date:	Tue, 8 Jan 2013 10:48:32 +0000
From:	Lee Jones <lee.jones@...aro.org>
To:	Pantelis Antoniou <panto@...oniou-consulting.com>
Cc:	Arnd Bergmann <arnd@...db.de>, Tony Lindgren <tony@...mide.com>,
	Grant Likely <grant.likely@...retlab.ca>,
	Rob Herring <rob.herring@...xeda.com>,
	Rob Landley <rob@...dley.net>, Jon Loeliger <jdl@....com>,
	Stephen Warren <swarren@...dotorg.org>,
	David Gibson <david@...son.dropbear.id.au>,
	Benoit Cousson <b-cousson@...com>,
	Mitch Bradley <wmb@...mworks.com>,
	Alan Tull <atull@...era.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	linux-omap@...r.kernel.org, devicetree-discuss@...ts.ozlabs.org,
	linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
	Matt Porter <mporter@...com>, Russ Dill <Russ.Dill@...com>,
	Koen Kooi <koen@...inion.thruhere.net>,
	Joel A Fernandes <agnel.joel@...il.com>,
	Rob Clark <robdclark@...il.com>,
	Jason Kridner <jkridner@...gleboard.org>,
	Matt Ranostay <mranostay@...il.com>,
	Sascha Hauer <s.hauer@...gutronix.de>,
	Linus Walleij <linus.walleij@...ricsson.com>,
	Guennadi Liakhovetski <g.liakhovetski@....de>
Subject: Re: [PATCH 1/5] capemgr: Beaglebone DT overlay based cape manager

On Tue, 08 Jan 2013, Pantelis Antoniou wrote:

> Hi Lee,
> 
> On Jan 8, 2013, at 12:00 PM, Lee Jones wrote:
> 
> >>>>> At the end of the line, some kind of hardware glue is going to be needed.
> >>>>> 
> >>>>> I just feel that drawing from a sample size of 1 (maybe 2 if I get to throw
> >>>>> in the beagleboard), it is a bit premature to think about making it overly
> >>>>> general, besides the part that are obviously part of the infrastructure 
> >>>>> (like the DT overlay stuff).
> >>>>> 
> >>>>> What I'm getting at, is that we need some user experience about this, before
> >>>>> going away and creating structure out of possible misconception about the uses. 
> >>>> 
> >>>> IMHO stuff like this will be needed by many SoCs. Some examples of similar
> >>>> things for omaps that have eventually become generic frameworks have been
> >>>> the clock framework, USB OTG support, runtime PM, pinmux framework and
> >>>> so on.
> >>>> 
> >>>> So I suggest a minimal generic API from the start as that will make things
> >>>> a lot easier in the long run.
> >>> 
> >>> I agree. The ux500 platform already has the concept of "user interface boards",
> >>> which currently is not well integrated into devicetree. I believe Sascha
> >>> mentioned that Pengutronix had been shipping some other systems with add-on
> >>> boards and generating device tree binaries from source for each combination.
> >>> 
> >>> Ideally, both of the above should be able to use the same DT overlay logic
> >>> as BeagleBone, and I'm sure there are more of those.
> >> 
> >> Hmm, I see. 
> >> 
> >> I will need some more information about the interface of the 'user interface boards'.
> >> I.e. how is the board identified, what is typically present on those boards, etc.
> > 
> > User Interface Boards are mearly removable PCBs which are interchangeable
> > amongst various hardware platforms. They are connected via numerous
> > connectors which carry all sorts of different data links; i2c, spi, rs232,
> > etc. The UIB I'm looking at right now has a touchscreen, speakers, a key
> > pad, leds, jumpers, switches and a bunch of sensors.
> > 
> > You can find a small example of how we interface to these by viewing
> > 'arch/arm/boot/dts/stuib.dtsi'. To add a UIB to a particular build, we
> > currently include it as a *.dtsi from a platform's dts file.
> 
> I see. What I'm asking about is whether there's a method where you can read
> an EEPROM, or some GPIO code combination where I can find out what kind of board
> is plugged each time.
> 
> If there is not, there is no way to automatically load the overlays; you can always
> use the kernel command line, or have the a user space application to request the loading
> of a specific board's overlay.

Unfortunately, there is no way to probe the UIBs. :(

> >> Can we get some input by the owner of other similar hardware? I know the FPGA
> >> people have similar requirements for example. There are other people that are hitting
> >> problems getting DT to work with their systems, like the V4L people with the order
> >> of initialization; see http://lwn.net/Articles/531068/. I think the V4L problem is
> >> cleanly solved by the overlay being contained in the V4L device node and applied just before
> >> the device is probed.
> >> 
> >> In the meantime it would be better to wait until we have some ack from the maintainers
> >> of the core subsystems about what they think.

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