On Mon, 23 May 2022 at 13:21, Kefeng Wang <wangkefeng.wang@xxxxxxxxxx> wrote:
>
> The memory barrier dma_mb() is introduced by commit a76a37777f2c
> ("iommu/arm-smmu-v3: Ensure queue is read after updating prod pointer"),
> which is used to ensure that prior (both reads and writes) accesses
> to memory by a CPU are ordered w.r.t. a subsequent MMIO write.
>
> Reviewed-by: Arnd Bergmann <arnd@xxxxxxxx> # for asm-generic
> Signed-off-by: Kefeng Wang <wangkefeng.wang@xxxxxxxxxx>
Reviewed-by: Marco Elver <elver@xxxxxxxxxx>
> ---
> Documentation/memory-barriers.txt | 11 ++++++-----
> include/asm-generic/barrier.h | 8 ++++++++
> 2 files changed, 14 insertions(+), 5 deletions(-)
>
> diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt
> index b12df9137e1c..832b5d36e279 100644
> --- a/Documentation/memory-barriers.txt
> +++ b/Documentation/memory-barriers.txt
> @@ -1894,6 +1894,7 @@ There are some more advanced barrier functions:
>
> (*) dma_wmb();
> (*) dma_rmb();
> + (*) dma_mb();
>
> These are for use with consistent memory to guarantee the ordering
> of writes or reads of shared memory accessible to both the CPU and a
> @@ -1925,11 +1926,11 @@ There are some more advanced barrier functions:
> The dma_rmb() allows us guarantee the device has released ownership
> before we read the data from the descriptor, and the dma_wmb() allows
> us to guarantee the data is written to the descriptor before the device
> - can see it now has ownership. Note that, when using writel(), a prior
> - wmb() is not needed to guarantee that the cache coherent memory writes
> - have completed before writing to the MMIO region. The cheaper
> - writel_relaxed() does not provide this guarantee and must not be used
> - here.
> + can see it now has ownership. The dma_mb() implies both a dma_rmb() and
> + a dma_wmb(). Note that, when using writel(), a prior wmb() is not needed
> + to guarantee that the cache coherent memory writes have completed before
> + writing to the MMIO region. The cheaper writel_relaxed() does not provide
> + this guarantee and must not be used here.
>
> See the subsection "Kernel I/O barrier effects" for more information on
> relaxed I/O accessors and the Documentation/core-api/dma-api.rst file for
> diff --git a/include/asm-generic/barrier.h b/include/asm-generic/barrier.h
> index fd7e8fbaeef1..961f4d88f9ef 100644
> --- a/include/asm-generic/barrier.h
> +++ b/include/asm-generic/barrier.h
> @@ -38,6 +38,10 @@
> #define wmb() do { kcsan_wmb(); __wmb(); } while (0)
> #endif
>
> +#ifdef __dma_mb
> +#define dma_mb() do { kcsan_mb(); __dma_mb(); } while (0)
> +#endif
> +
> #ifdef __dma_rmb
> #define dma_rmb() do { kcsan_rmb(); __dma_rmb(); } while (0)
> #endif
> @@ -65,6 +69,10 @@
> #define wmb() mb()
> #endif
>
> +#ifndef dma_mb
> +#define dma_mb() mb()
> +#endif
> +
> #ifndef dma_rmb
> #define dma_rmb() rmb()
> #endif
> --
> 2.35.3
>
