The cnt32_to_63 algorithm relies on proper counter data evaluation
ordering to work properly. This was missing from the provided
documentation.

Let's augment the documentation with the missing usage constraint and
fix the only instance that got it wrong.
Signed-off-by: NNicolas Pitre <nico@fluxnic.net>
Acked-by: NDavid Howells <dhowells@redhat.com>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

Currently, all existing users of cnt32_to_63() are fine since the CPU
architectures where it is used don't do read access reordering, and user
mode preemption is disabled already.  It is nevertheless a good idea to
better elaborate usage requirements wrt preemption, and use an explicit
memory barrier on SMP to avoid different CPUs accessing the counter
value in the wrong order.  On UP a simple compiler barrier is
sufficient.
Signed-off-by: NNicolas Pitre <nico@marvell.com>
Acked-by: NMathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

Move asm-arm/cnt32_to_63.h to include/linux/ so that MN10300 can make
use of it too.
Signed-off-by: NDavid Howells <dhowells@redhat.com>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

Move platform independent header files to arch/arm/include/asm, leaving
those in asm/arch* and asm/plat* alone.
Signed-off-by: NRussell King <rmk+kernel@arm.linux.org.uk>

This is done in a completely lockless fashion. Bits 0 to 31 of the count
are provided by the hardware while bits 32 to 62 are stored in memory.
The top bit in memory is used to synchronize with the hardware count
half-period.  When the top bit of both counters (hardware and in memory)
differ then the memory is updated with a new value, incrementing it when
the hardware counter wraps around.  Because a word store in memory is
atomic then the incremented value will always be in synch with the top
bit indicating to any potential concurrent reader if the value in memory
is up to date or not wrt the needed increment.  And any race in updating
the value in memory is harmless as the same value would be stored more
than once.
Signed-off-by: NNicolas Pitre <nico@cam.org>
Signed-off-by: NRussell King <rmk+kernel@arm.linux.org.uk>