Subject: Re: [PATCH v5 2/4] arch: Add lightweight memory barriers dma_rmb() and dma_wmb()

From: Will Deacon <[email-masked]>
Date: Tue, 25 Nov 2014 14:01:30 +0000

On Wed, Nov 19, 2014 at 01:24:02AM +0000, Alexander Duyck wrote:
> diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt
> index 22a969c..a1c589b 100644
> --- a/Documentation/memory-barriers.txt
> +++ b/Documentation/memory-barriers.txt
> @@ -1615,6 +1615,47 @@ There are some more advanced barrier functions:
>       operations" subsection for information on where to use these.
> 
> 
> + (*) dma_wmb();
> + (*) dma_rmb();
> +
> +     These are for use with memory based device I/O to guarantee the ordering
> +     of cache coherent writes or reads with respect to other writes or reads
> +     to cache coherent DMA memory.

Can you please make it crystal clear that "memory based device I/O" != MMIO?
If people get these barriers wrong, then debugging will be a nightmare.

> diff --git a/arch/arm/include/asm/barrier.h b/arch/arm/include/asm/barrier.h
> index c6a3e73..d2f81e6 100644
> --- a/arch/arm/include/asm/barrier.h
> +++ b/arch/arm/include/asm/barrier.h
> @@ -43,10 +43,14 @@
>  #define mb()           do { dsb(); outer_sync(); } while (0)
>  #define rmb()          dsb()
>  #define wmb()          do { dsb(st); outer_sync(); } while (0)
> +#define dma_rmb()      dmb(osh)
> +#define dma_wmb()      dmb(oshst)
>  #else
>  #define mb()           barrier()
>  #define rmb()          barrier()
>  #define wmb()          barrier()
> +#define dma_rmb()      barrier()
> +#define dma_wmb()      barrier()
>  #endif
> 
>  #ifndef CONFIG_SMP
> diff --git a/arch/arm64/include/asm/barrier.h b/arch/arm64/include/asm/barrier.h
> index 6389d60..a5abb00 100644
> --- a/arch/arm64/include/asm/barrier.h
> +++ b/arch/arm64/include/asm/barrier.h
> @@ -32,6 +32,9 @@
>  #define rmb()          dsb(ld)
>  #define wmb()          dsb(st)
> 
> +#define dma_rmb()      dmb(oshld)
> +#define dma_wmb()      dmb(oshst)
> +
>  #ifndef CONFIG_SMP
>  #define smp_mb()       barrier()
>  #define smp_rmb()      barrier()

The arm/arm64 bits look fine to me.

  Acked-by: Will Deacon <[email-masked]>

If we ever see platforms using Linux/dma_alloc_coherent with devices
mastering from a different outer-shareable domain that the one containing
the CPUs, then we'll need to revisit this.

Will