On 3 May 2012 07:04, Russell King - ARM Linux linux@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?
- 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
- 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.
- 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