[PATCH 1/2] powerpc/85xx: fix problem that prevents PHYS_64BIT from configurable

[Kumar Gala]

On Feb 16, 2012, at 6:10 AM, Li Yang wrote:

> Fix the problem that large physical address support cannot be
> disabled when some platforms which only provides 36-bit support
> are selected.  According to the philosophy of kernel config
> enabling a platform support doesn't mean the kernel is only
> running on that platform.  Remove the auto selection of PHYS_64BIT
> option for these platforms.  They will need to use a 36bit default
> config that selects PHYS_64BIT explicitly.
> 
> The reason why we need to keep PHYS_64BIT option configurable is
> that enabling it cause negative performance impact on various
> aspects like TLB miss and physical address manipulating.  We should
> not enable it unless really needed, e.g. use large memory of 4GB
> or bigger.
> 
> Signed-off-by: Li Yang <leoli at freescale.com>
> ---
> arch/powerpc/platforms/85xx/Kconfig |    6 ------
> 1 files changed, 0 insertions(+), 6 deletions(-)

Nak, this isn't correct.

For some of these platforms like P2041RDB, P3041DS, P3060QDS, P4080DS, & P5020DS only a 36-bit physical address map is supported by u-boot and the device tree.  This was a decision that was made to NOT support 32-bit address map for these boards and accept the performance implication of it to reduce the # of builds, etc.

Additionally, outside of maybe P2041RDB I believe the majority of these boards ship with 4G of DDR (but that off the top of my head) and thus require the 36-bit / PHYS_64BIT support to be enabled.

- k