RFC on writel and writel_relaxed

[Arnd Bergmann]

On Tue, Mar 27, 2018 at 11:57 AM, Will Deacon <will.deacon at arm.com> wrote:

>
> From db0daeaf94f0f6232f8206fc07a74211324b11d9 Mon Sep 17 00:00:00 2001
> From: Will Deacon <will.deacon at arm.com>
> Date: Tue, 27 Mar 2018 10:49:58 +0100
> Subject: [PATCH] docs/memory-barriers.txt: Fix broken DMA vs MMIO ordering
>  example
>
> 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 at kernel.crashing.org>
> Cc: Arnd Bergmann <arnd at arndb.de>
> Cc: Jason Gunthorpe <jgg at ziepe.ca>
> Cc: "Paul E. McKenney" <paulmck at linux.vnet.ibm.com>
> Cc: Peter Zijlstra <peterz at infradead.org>
> Cc: Ingo Molnar <mingo at redhat.com>
> Cc: Jonathan Corbet <corbet at lwn.net>
> Reported-by: Sinan Kaya <okaya at codeaurora.org>
> Signed-off-by: Will Deacon <will.deacon at arm.com>
> ---
>  Documentation/memory-barriers.txt | 18 ++++++++++--------
>  1 file changed, 10 insertions(+), 8 deletions(-)
>
> diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt
> index a863009849a3..2556b4b0e6f9 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 +1916,16 @@ 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 cache incoherent MMIO region.
> +     If this ordering between incoherent MMIO and coherent memory regions
> +     is not required, writel_relaxed() can be used instead and is significantly
> +     cheaper on some weakly-ordered architectures.

I think that's a great improvement, but I'm a bit worried about recommending
writel_relaxed() too much: I've seen a lot of drivers that just always use
writel_relaxed() over write(), and some of them get that wrong when they
don't understand the difference but end up using DMA without explicit
barriers anyway.

Also, having an architecture-independent driver use wmb()+writel_relaxed()
ends up being more expensive than just using write(). Not sure how to
best phrase it though.

      Arnd