Re: [PATCH] docs/memory-barriers.txt: Fix broken DMA vs MMIO ordering example

[Paul E. McKenney]

On Tue, Mar 27, 2018 at 02:11:27PM +0100, Will Deacon wrote:
> The section of memory-barriers.txt that describes the dma_Xmb() barriers
> has an incorrect example claiming that a wmb() is required after writing
> to coherent memory in order for those writes to be visible to a device
> before a subsequent MMIO access using writel() can reach the device.
> 
> In fact, this ordering guarantee is provided (at significant cost on some
> architectures such as arm and power) by writel, so the wmb() is not
> necessary. writel_relaxed exists for cases where this ordering is not
> required.
> 
> Fix the example and update the text to make this clearer.
> 
> Cc: Benjamin Herrenschmidt <benh@xxxxxxxxxxxxxxxxxxx>
> Cc: Arnd Bergmann <arnd@xxxxxxxx>
> Cc: Jason Gunthorpe <jgg@xxxxxxxx>
> Cc: "Paul E. McKenney" <paulmck@xxxxxxxxxxxxxxxxxx>
> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> Cc: Jonathan Corbet <corbet@xxxxxxx>
> Reported-by: Sinan Kaya <okaya@xxxxxxxxxxxxxx>
> Signed-off-by: Will Deacon <will.deacon@xxxxxxx>

Good catch, queued on my lkmm branch, thank you!

							Thanx, Paul

> ---
>  Documentation/memory-barriers.txt | 17 +++++++++--------
>  1 file changed, 9 insertions(+), 8 deletions(-)
> 
> diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt
> index a863009849a3..3247547d1c36 100644
> --- a/Documentation/memory-barriers.txt
> +++ b/Documentation/memory-barriers.txt
> @@ -1909,9 +1909,6 @@ There are some more advanced barrier functions:
>  		/* assign ownership */
>  		desc->status = DEVICE_OWN;
> 
> -		/* force memory to sync before notifying device via MMIO */
> -		wmb();
> -
>  		/* notify device of new descriptors */
>  		writel(DESC_NOTIFY, doorbell);
>  	}
> @@ -1919,11 +1916,15 @@ 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.  The wmb() is needed to guarantee that the
> -     cache coherent memory writes have completed before attempting a write to
> -     the cache incoherent MMIO region.
> -
> -     See Documentation/DMA-API.txt for more information on consistent memory.
> +     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.
> +
> +     See the subsection "Kernel I/O barrier effects" for more information on
> +     relaxed I/O accessors and the Documentation/DMA-API.txt file for more
> +     information on consistent memory.
> 
> 
>  MMIO WRITE BARRIER
> -- 
> 2.1.4
>