Hi all, This is version 2 of the series I originally posted here: https://lkml.org/lkml/2014/4/17/269 Changes since v1 include: - Added relevant acks from arch maintainers - Fixed potential compiler re-ordering issue for x86 definitions I'd *really* appreciate some feedback on the proposed semantics here, but acks are still good :) The original cover letter is duplicated below. Cheers, Will --->8 This RFC series attempts to define a portable (i.e. cross-architecture) definition of the {readX,writeX}_relaxed MMIO accessor functions. These functions are already in widespread use amongst drivers (mainly those supporting devices embedded in ARM SoCs), but lack any well-defined semantics and, subsequently, any portable definitions to allow these drivers to be compiled for other architectures. The two main motivations for this series are: (1) To promote use of the _relaxed MMIO accessors on weakly-ordered architectures, where they can bring significant performance improvements over their non-relaxed counterparts. (2) To allow COMPILE_TEST to build drivers using the relaxed accessors across all architectures. The proposed semantics largely match exactly those provided by the ARM implementation (i.e. no weaker), with one exception (see below). Informally: - Relaxed accesses to the same device are ordered with respect to each other. - Relaxed accesses are *not* guaranteed to be ordered with respect to normal memory accesses (e.g. DMA buffers -- this is what gives us the performance boost over the non-relaxed versions). - Relaxed accesses are not guaranteed to be ordered with respect to LOCK/UNLOCK operations. In actual fact, the relaxed accessors *are* ordered with respect to LOCK/UNLOCK operations on ARM[64], but I have added this constraint for the benefit of PowerPC, which has expensive I/O barriers in the spin_unlock path for the non-relaxed accessors. A corollary to this is that mmiowb() probably needs rethinking. As it currently stands, an mmiowb() is required to order MMIO writes to a device from multiple CPUs, even if that device is protected by a lock. However, this isn't often used in practice, leading to PowerPC implementing both mmiowb() *and* synchronising I/O in spin_unlock. I would propose making the non-relaxed I/O accessors ordered with respect to LOCK/UNLOCK, leaving mmiowb() to be used with the relaxed accessors, if required, but would welcome thoughts/suggestions on this topic. Will Deacon (18): asm-generic: io: implement relaxed accessor macros as conditional wrappers microblaze: io: remove dummy relaxed accessor macros s390: io: remove dummy relaxed accessor macros for reads xtensa: io: remove dummy relaxed accessor macros for reads alpha: io: implement relaxed accessor macros for writes frv: io: implement dummy relaxed accessor macros for writes cris: io: implement dummy relaxed accessor macros for writes ia64: io: implement dummy relaxed accessor macros for writes m32r: io: implement dummy relaxed accessor macros for writes m68k: io: implement dummy relaxed accessor macros for writes mn10300: io: implement dummy relaxed accessor macros for writes parisc: io: implement dummy relaxed accessor macros for writes powerpc: io: implement dummy relaxed accessor macros for writes sparc: io: implement dummy relaxed accessor macros for writes tile: io: implement dummy relaxed accessor macros for writes x86: io: implement dummy relaxed accessor macros for writes documentation: memory-barriers: clarify relaxed io accessor semantics asm-generic: io: define relaxed accessor macros unconditionally Documentation/memory-barriers.txt | 13 +++++++++---- arch/alpha/include/asm/io.h | 12 ++++++++---- arch/cris/include/asm/io.h | 3 +++ arch/frv/include/asm/io.h | 3 +++ arch/ia64/include/asm/io.h | 4 ++++ arch/m32r/include/asm/io.h | 3 +++ arch/m68k/include/asm/io.h | 8 ++++++++ arch/m68k/include/asm/io_no.h | 4 ---- arch/microblaze/include/asm/io.h | 8 -------- arch/mn10300/include/asm/io.h | 4 ++++ arch/parisc/include/asm/io.h | 12 ++++++++---- arch/powerpc/include/asm/io.h | 12 ++++++++---- arch/s390/include/asm/io.h | 5 ----- arch/sparc/include/asm/io.h | 9 +++++++++ arch/sparc/include/asm/io_32.h | 3 --- arch/sparc/include/asm/io_64.h | 22 ++++++++++------------ arch/tile/include/asm/io.h | 4 ++++ arch/x86/include/asm/io.h | 10 +++++++--- arch/xtensa/include/asm/io.h | 7 ------- include/asm-generic/io.h | 10 ++++++++++ 20 files changed, 98 insertions(+), 58 deletions(-) -- 1.9.2 -- To unsubscribe from this list: send the line "unsubscribe linux-arch" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
