x86: Don't clear olpc_ofw_header when sentinel is detected

x86: Don't clear olpc_ofw_header when sentinel is detected

OpenFirmware wasn't quite following the protocol described in boot.txt
and the kernel has detected this through use of the sentinel value
in boot_params. OFW does zero out almost all of the stuff that it should
do, but not the sentinel.

This causes the kernel to clear olpc_ofw_header, which breaks x86 OLPC
support.

OpenFirmware has now been fixed. However, it would be nice if we could
maintain Linux compatibility with old firmware versions. To do that, we just
have to avoid zeroing out olpc_ofw_header.

OFW does not write to any other parts of the header that are being zapped
by the sentinel-detection code, and all users of olpc_ofw_header are
somewhat protected through checking for the OLPC_OFW_SIG magic value
before using it. So this should not cause any problems for anyone.

Signed-off-by: Daniel Drake <dsd@laptop.org>
Link: http://lkml.kernel.org/r/20130809221420.618E6FAB03@dev.laptop.org
Acked-by: Yinghai Lu <yinghai@kernel.org>
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>
Cc: <stable@vger.kernel.org> # v3.9+