Commit e17b1af9

  "ARM: 8857/1: efi: enable CP15 DMB instructions before cleaning the cache"

added some explicit handling of the CP15BEN bit in the SCTLR system
register, to ensure that CP15 barrier instructions are enabled, even
if we enter the decompressor via the EFI stub.

However, as it turns out, there are other ways in which we may end up
using CP15 barrier instructions without them being enabled. I.e., when
the decompressor startup code skips the cache_on() initially, we end
up calling cache_clean_flush() with the caches and MMU off, in which
case the CP15BEN bit in SCTLR may not be programmed either. And in
fact, cache_on() itself issues CP15 barrier instructions before actually
enabling them by programming the new SCTLR value (and issuing an ISB)

Since these routines are shared between v7 CPUs and older ones that
implement the CPUID extension as well, using the ordinary v7 barrier
instructions in this code is not possible, and so we should enable the
CP15 ones explicitly before issuing them. Note that a v7 ISB is still
required between programming the SCTLR register and using the CP15 barrier
instructions, and we should take care to branch over it if the CP15BEN
bit is already set, given that in that case, the CPU may not support it.
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>