GENERIC kernel issues

[Warner Losh]

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...

> ** 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?

> Question:  should we create a /projects/arm-generic/ branch
> to hold this work while it's in flux?

Works for me.

Warner