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

[Kefeng Wang]

On 2022/5/20 18:08, Marco Elver wrote:
> 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@xxxxxxxxxx>
> > ---
> >   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 ------
Thanks， will use above explanation.
> 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.

Ok, it's good to add the dma_mb() and __dma_mb  definition with a 
separate patch

into include/asm-generic/barrier.h.

> Thoughts?
>
> Thanks,
> -- Marco
> .