GENERIC kernel issues

[Tim Kientzle]

On Mar 3, 2013, at 6:39 PM, Warner Losh wrote:

> 
> On Mar 3, 2013, at 12:43 PM, Tim Kientzle wrote:
> 
>> I spent some time yesterday putting together a kernel
>> configuration for a GENERIC ARM kernel that would
>> support both RaspberryPi and BeagleBone.
>> 
>> Just to see how far I could get.
>> 
>> Here's a list of the problems I've found so far:
>> 
>> ** Multiple MMU support.  If you put these two lines into an
>> ARM kernel config, the build will fail in the MMU code:
>> 
>> 	cpu 		CPU_ARM1176
>> 	cpu		CPU_CORTEXA
>> 
>> Basically, this turns on the support for multiple MMUs but the
>> ARMv6/ARMv7 MMU definitions don't play nicely with run-time
>> MMU selection.
> 
> Having looked at the defines, it could be done with variables, but I fear that will slow things down to do a simple #define -> variable. We may need two sets of code for performance…

The MMU management I really know nothing about.

If you do … ;-)


>> ** PHYSADDR/KERNPHYSADDR hardwiring.  Ian has made a
>> lot of progress and I'm working on some related changes to
>> address this.  I think we understand how to eliminate these
>> constants and replace them with run-time detection of the
>> load address.  I'm still not sure what changes might be needed
>> to the loader to make this work.
>> 
>> ** PIPT vs. VIVT cache management.  This is currently set at compile
>> time; we'll need to have a way to set this at run time based on the
>> CPU.  (I have some skeletal code to select CPU at the top of
>> initarm by inspecting the FDT.  I presume this switch will be routine
>> once a robust version of that is in place.)
> 
> Generally we should be doing this, both for the Core and the SoC. I don't think we do this generally, and we should. It is one of the big advantages of FDT: It tells you what's going on so you don't have to guess....
> 
>> ** TI processor detection.   This is currently hardwired at build time,
>> so we cannot currently build a kernel that supports both AM335x
>> and OMAP4, for example.
> 
> Can't we do this with compat field in the FDT?

I have prototype code that does exactly this, but it needs
a lot of cleanup before it can be committed.

Tim