Re: KConfig and DTS files

[Eric Nelson <eric.nelson-Q5RJGjKts06CY9SHAMCTRUEOCMrvLtNR@public.gmane.org>]

Hi Olof,

On 05/07/2014 10:24 AM, Olof Johansson wrote:
> Hi,
>
> On Wed, May 7, 2014 at 10:12 AM, Eric Nelson
> <eric.nelson-Q5RJGjKts06CY9SHAMCTRUEOCMrvLtNR@public.gmane.org> wrote:
>> 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

Regards,


Eric
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