arm: CONFIG_ARM_PATCH_PHYS_VIRT=y is broken on kirkwood

Hi Russel!

I've noticed sheevaplug is not able to boot with
kirkwood_defconfig anymore. After dumb bisection
I've figured out that
CONFIG_ARM_PATCH_PHYS_VIRT=y
was the problem.

The commit in question is :

In order to make the box boot I had to set:
CONFIG_EMBEDDED=y
# CONFIG_ARM_PATCH_PHYS_VIRT is not set
CONFIG_PHYS_OFFSET=0x0

Do you have an idea why it does not boot anymore by default?

Thanks!

  Sergei

There is a u-boot commit which fixes this issue:

commit 679530278d5a79d34e356ad2d452f4400953bfc2

Author: Michael Walle <[[email protected]]>
Date:   Mon Feb 6 22:42:10 2012 +0530

arm, arm-kirkwood: disable l2c before linux boot

The decompressor expects the L2 cache to be disabled. This fixes booting some kernels, which have CONFIG_ARM_PATCH_PHYS_VIRT enabled.

Ian.

Ian Campbell


If I can have honesty, it's easier to overlook mistakes.
		-- Kirk, "Space Seed", stardate 3141.9

Thank you, Ian. Updating u-boot helped!

Sorry for the noise.

  Sergei

Hi everyone,

I guess if updating u-boot is not an option, there's no other way around it than disabling CONFIG_ARM_PATCH_PHYS_VIRT altogether.

Could someone (Russell?) simply state what the consequences are of disabling such option? (Or, conversely, what its advantages are?) Only reference I found is the original commit, but I couldn't really understand what it does (or to what purpose), see at the bottom.

Thank you very much!
Gerlando

commit dc21af99fadcfa0ae65b52fd0895f85824f0c288

Author: Russell King <[email protected]>
Date:   Tue Jan 4 19:09:43 2011 +0000

ARM: P2V: introduce phys_to_virt/virt_to_phys runtime patching

This idea came from Nicolas, Eric Miao produced an initial version, which was then rewritten into this.

Patch the physical to virtual translations at runtime. As we modify the code, this makes it incompatible with XIP kernels, but allows us to achieve this with minimal loss of performance.

As many translations are of the form:

physical = virtual + (PHYS_OFFSET - PAGE_OFFSET) virtual = physical - (PHYS_OFFSET - PAGE_OFFSET)

we generate an 'add' instruction for __virt_to_phys(), and a 'sub' instruction for __phys_to_virt(). We calculate at run time
(PHYS_OFFSET
- PAGE_OFFSET) by comparing the address prior to MMU initialization
with
where it should be once the MMU has been initialized, and place this constant into the above add/sub instructions.

Once we have (PHYS_OFFSET - PAGE_OFFSET), we can calculate the real PHYS_OFFSET as PAGE_OFFSET is a build-time constant, and save this for the C-mode PHYS_OFFSET variable definition to use.

At present, we are unable to support Realview with Sparsemem enabled as this uses a complex mapping function, and MSM as this requires a constant which will not fit in our math instruction.

Add a module version magic string for this feature to prevent incompatible modules being loaded.

Disabling this option will not do you any harm. It will prevent your kernel binary from being usable on another platform as the physical RAM address will then be hardcoded in the compiled kernel, but there are still other things that prevents such sharing in this case anyway.

One thing that this option allowed is to remove all the physical RAM offset information for most platform out of the source tree. So to not have this option might mean that you might be prompted to provide it at kernel configuration time.

Nicolas