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

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.
=20
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.
=20
Signed-off-by: Li Yang <redacted>
---
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