linux/arch/arm64
Alexander Duyck 1077fa36f2 arch: Add lightweight memory barriers dma_rmb() and dma_wmb()
There are a number of situations where the mandatory barriers rmb() and
wmb() are used to order memory/memory operations in the device drivers
and those barriers are much heavier than they actually need to be.  For
example in the case of PowerPC wmb() calls the heavy-weight sync
instruction when for coherent memory operations all that is really needed
is an lsync or eieio instruction.

This commit adds a coherent only version of the mandatory memory barriers
rmb() and wmb().  In most cases this should result in the barrier being the
same as the SMP barriers for the SMP case, however in some cases we use a
barrier that is somewhere in between rmb() and smp_rmb().  For example on
ARM the rmb barriers break down as follows:

  Barrier   Call     Explanation
  --------- -------- ----------------------------------
  rmb()     dsb()    Data synchronization barrier - system
  dma_rmb() dmb(osh) data memory barrier - outer sharable
  smp_rmb() dmb(ish) data memory barrier - inner sharable

These new barriers are not as safe as the standard rmb() and wmb().
Specifically they do not guarantee ordering between coherent and incoherent
memories.  The primary use case for these would be to enforce ordering of
reads and writes when accessing coherent memory that is shared between the
CPU and a device.

It may also be noted that there is no dma_mb().  Most architectures don't
provide a good mechanism for performing a coherent only full barrier without
resorting to the same mechanism used in mb().  As such there isn't much to
be gained in trying to define such a function.

Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
Cc: Michael Ellerman <michael@ellerman.id.au>
Cc: Michael Neuling <mikey@neuling.org>
Cc: Russell King <linux@arm.linux.org.uk>
Cc: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: Heiko Carstens <heiko.carstens@de.ibm.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Martin Schwidefsky <schwidefsky@de.ibm.com>
Cc: Tony Luck <tony.luck@intel.com>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: David Miller <davem@davemloft.net>
Acked-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Acked-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Alexander Duyck <alexander.h.duyck@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
2014-12-11 21:15:06 -05:00
..
boot ARM64: SoC changes for 3.19 2014-12-09 17:17:47 -08:00
configs arm64: defconfig: update defconfig for 3.18 2014-11-05 17:07:58 +00:00
crypto arm64/crypto: use crypto instructions to generate AES key schedule 2014-11-06 17:25:28 +00:00
include arch: Add lightweight memory barriers dma_rmb() and dma_wmb() 2014-12-11 21:15:06 -05:00
kernel arm64 updates for 3.19 2014-12-09 13:12:47 -08:00
kvm arm64 updates for 3.19 2014-12-09 13:12:47 -08:00
lib arm64: __clear_user: handle exceptions on strb 2014-11-13 15:21:26 +00:00
mm arm64: remove the unnecessary arm64_swiotlb_init() 2014-12-05 12:19:52 +00:00
net arm64: bpf: lift restriction on last instruction 2014-12-03 18:04:09 +00:00
xen arm: xen: implement multicall hypercall support. 2014-04-24 13:09:46 +01:00
Kconfig asm-generic: asm/io.h rewrite 2014-12-09 17:25:00 -08:00
Kconfig.debug arm64: add support to dump the kernel page tables 2014-11-26 17:19:18 +00:00
Makefile dts, arm/arm64: Remove dtbs build rules in sub-makes 2014-10-21 18:06:58 +02:00