Making ARM multiplatform kernels DT-only?

[Deepak Saxena]

On 3 May 2012 07:04, Russell King - ARM Linux <linux at arm.linux.org.uk> wrote:
> On Thu, May 03, 2012 at 01:50:35PM +0000, Arnd Bergmann wrote:
>> Hi everyone,
>>
>> I've been discussing multiplatform kernels with a few people recently,
>> and we will have a lot of discussion sessions about this at Linaro
>> Connect in Hong Kong.
>>
>> One question that came up repeatedly is whether we should support all
>> possible board files for each platform in a multiplatform kernel,
>> or just the ones that are already using DT probing. I would like
>> to get a quick poll of opinions on that and I've tried to put those
>> people on Cc that would be most impacted by this, i.e. the maintainers
>> for platforms that have both DT and non-DT board files at the moment.
>>
>> My feeling is that we should just mandate DT booting for multiplatform
>> kernels, because it significantly reduces the combinatorial space
>> at compile time, avoids a lot of legacy board files that we cannot
>> test anyway, reduces the total kernel size and gives an incentive
>> for people to move forward to DT with their existing boards.
>>
>> The counterargument is that we won't be able to support all the
>> boards we currently do when the user switches on multiplatform,
>> but I think that is acceptable.
>> Note that I would still want to allow users to build platforms
>> separately in order to enable the ATAG style board files, even
>> for platforms that are not multiplatform capable.
>
> I'm basing my comments off mach-zynq.
>
> How about we take the following steps towards it?
>
> 1. create arch/arm/include/mach/ which contains standardized headers
>   for DT based implementations.  This must include all headers included
>   by asm/ or linux/ includes.  This will also be the only mach/ header
>   directory included for code outside of arch/arm/mach-*.  This also
>   acts as the 'default' set of mach/* includes for stuff like timex.h
>   and the empty hardware.h
>
> 2. DT based mach-* directories do not have an include directory; their
>   include files must be located in the main include/ heirarchy if shared
>   with other parts of the kernel, otherwise they must be in the mach-*
>   directory.

What do we do about all the mach-specific driver related headers that are
currently littered all over arch/arm/mach*? Things like <mach/mx3fb.h> and
<mach/ohci.h>? Long term I think we should move everything that is driver
specific out of arch/arm but that is fairly large task and I think
there's a lot of
intertwining between stuff that's driver specific and stuff that
should actually live
in <mach/*>.  Arnd (or maybe Nicolas?) suggested something along the lines
of scripting something to figure out the constants required for a
specific driver
and pulling them out of <mach/*.h> and into that driver as a starting point.

> 3. Allow build multiple mach-* directories (which we already do... see
>   the samsung stuff.)
>
> We still have irqs.h being SoC dependent, and we still haven't taken
> debug-macros.S far enough along to get rid of that.  Then there's also
> the problem of uncompress.h.  The last piece of the puzzle is the common
> clock stuff.

There's also some stuff outside of arch/arm that needs cleanup
including USB driver
refactoring and some other bits that I can't recall atm. Right now we
can build one and
only one host controller   inteface, even as modules. Not too
difficult of a problem to
solve and not critical to get the SOCs booting, but needed to be
usable on real devices.

We'll probably find more stuff as we start actually building things together.

~Deepak