Re: KConfig and DTS files

On Wed, May 07, 2014 at 10:55:27AM -0700, Olof Johansson wrote:

On Wed, May 7, 2014 at 10:47 AM, Eric Nelson
[off-list ref] wrote:

quoted

Hi Olof,


On 05/07/2014 10:24 AM, Olof Johansson wrote:

quoted

Hi,

On Wed, May 7, 2014 at 10:12 AM, Eric Nelson
[off-list ref] wrote:

quoted

Hi all,

I suspect that this has been discussed previously, but I'm a
N00b to DT and Google hasn't helped identify a discussion
on the list.

While getting my feet wet with DTS, I was quite surprised to
see that there's no support for making parts of DTS files
conditional on the kernel configuration.

We often cost-optimize BOMs for our standard boards and omit
bits and pieces not needed for a particular build, and
it would be nice to surround optional components with
conditionals:

     #ifdef CONFIG_BLAH
     #endif

Please advise,

The DTS is independent on what drivers the kernel is actually
enabling. It should focus on what is actually there on hardware, and
not what is enabled in software.

I think I know where you're coming from on this -- on some board-file
setups people used to not register the platform_device if the driver
wasn't configured. That was also not really a good way to do it, but
it made a bit more sense there, since you'd save the few bytes used by
the platform_device/platform_data structures.

It's not a question of space, but functionality.

We had this come up recently when optimizing a BOM for use
without Wi-Fi. If we omit the device and associated buffers,
the SDIO lines are left floating, so the detection code will
complain.

It seems reasonable to tell the kernel to skip this if the
WiFi driver isn't configured into the kernel (i.e. there's
no point in enumerating), so I was hoping to surround this
USDHC2 block with #ifdefs:


https://github.com/boundarydevices/linux-imx6/blob/boundary-imx_3.10.17_1.0.0_ga/arch/arm/boot/dts/imx6qdl-nitrogen6x.dtsi#L640

The alternative aren't very nice:
        populate extra parts to drive these signals, or
        copy the DTB and strip out the unused parts

The first is wasteful (and environmentally un-friendly), and
the second doesn't scale very well and pollutes the Git tree.

Prior to DT, we surrounded the device startup calls with #ifdefs
like this:

https://github.com/boundarydevices/linux-imx6/blob/boundary-jb4.3_1.0.0-ga/arch/arm/mach-mx6/board-mx6_nitrogen6x.c#L1409

It is really annoying that you can't include a DTS in another DTS
(only DTSIs). This is a major shortcoming of the tools, as far as I am
concerned.

Um.. so I'm trying to work out exactly what you're getting at here.
dtc includes, just like CPP includes are dumb textual inclusion,
there's no theoretical difference between a "dts" and a "dtsi".  There
may be practical differences, though - like the presence of the
/dts-v1/ tag and possibly paths.

So, the best way we've found to do this is to create a shared dtsi
that has practically everything in it, then a skeleton DTS for the
"full" product, and a separate DTS for the cost-reduced version. The
cost-reduced DTS would insert a status = "disabled"; for the sdio
controller. That's a solution that scales very poorly as well, though.

Best would be if a DTS could just include another DTS for minor
updates, such as disabling the sdio controller in this case. I'm sure
there were some language-puritan reasons for not allowing that though.
Someone care to fill in the history?

So, I'm not sure quite what you're envisaging and how it differs from
what we have now.  Using the "overlay" functionality, you can right
now do

/include/ "thatotherdts.dts"

&{/somewhere/in/that/other/tree} {
	adjustable-property = "new value";
};

The only oddity here is that if thatotherdts.dts includes the /dts-v1/
tag, you can't have it in the outer file as well.

If it helps we could make that tag optionally appear any number of
times.

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson