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

Subject: 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:

quoted

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 <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.

Ok.  Although personally I don't think # of builds matters that much.

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.

I know that current support of DPAA boards requires 36-bit.  But I don't th=
ink they need to select the PHYS_64BIT option directly and make it not conf=
igurable.  It's conflicting with the logic that enabling a platform support=
 doesn't mean the kernel is only running on that platform.  Btw: what's you=
r recommendations on solving this?

- Leo