RFC on writel and writel_relaxed
benh at kernel.crashing.org
Tue Mar 27 22:25:44 AEDT 2018
On Tue, 2018-03-27 at 12:02 +0100, Will Deacon wrote:
> 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.
> > The cheaper writel_relaxed() does not guarantee the DMA to be visible
> > to the device and must not be used here.
> Fair enough. I'd rather people used _relaxed by default, but I have to admit
> that it will probably just result in them getting things wrong. Just a tiny
> bit of wordsmithing brings this to:
I prefer people using writel() by default for the simple reason that
99% of writels out there are configuration stuff for which the
performance difference doesn't matter, and people will just get it
Let's focus on the rare fast path for optimisation.
> 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
> If you're happy with that, I'll send it as a proper patch.
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