Re: [PATCH v2 1/2] Documentation/barriers: Add memory barrier dma_mb()

[Marco Elver <elver@google.com>]

On Fri, May 20, 2022 at 11:15AM +0800, Kefeng Wang 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.
> 
> Signed-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>
> ---
>  Documentation/memory-barriers.txt | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt
> index b12df9137e1c..1eabcc0e4eca 100644
> --- a/Documentation/memory-barriers.txt
> +++ b/Documentation/memory-barriers.txt
> @@ -1894,10 +1894,13 @@ 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
> -     DMA capable device.
> +     DMA capable device, in the case of ensure the prior (both reads and
> +     writes) accesses to memory by a CPU are ordered w.r.t. a subsequent
> +     MMIO write, dma_mb().
>  

I think this is out of place; this explanation here is not yet
elaborating on either. Elaboration on dma_mb() should go where
dma_rmb() and dma_wmb() are explained. 

Something like this:

------ >8 ------

diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt
index b12df9137e1c..fb322b6cce70 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

------ >8 ------

Also, now that you're making dma_mb() part of the official API, it might
need a generic definition in include/asm-generic/barrier.h, because
as-is it's only available in arm64 builds.

Thoughts?

Thanks,
-- Marco