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

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Message-Id: <21E7B891-269B-422D-9038-381FAA15B59B@antoniou-consulting.com>
Date:	Tue, 8 Jan 2013 12:10:20 +0200
From:	Pantelis Antoniou <panto@...oniou-consulting.com>
To:	Lee Jones <lee.jones@...aro.org>
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

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.

Regards

-- Pantelis

> 
>> 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.
>> 
>> Regards
>> 
>> -- Pantelis
>> 
> 
> -- 
> Lee Jones
> Linaro ST-Ericsson Landing Team Lead
> Linaro.org │ Open source software for ARM SoCs
> Follow Linaro: Facebook | Twitter | Blog

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