On Mon, 2018-03-26 at 10:54 -0600, Jason Gunthorpe wrote:

On Mon, Mar 26, 2018 at 11:08:45AM +0000, David Laight wrote:

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This is a super performance critical operation for most drivers and
directly impacts network performance.

Perhaps there ought to be writel_nobarrier() (etc) that never contain
any barriers at all.
This might mean that they are always just the memory operation,
but it would make it more obvious what the driver was doing.

I think that is what writel_relaxed is supposed to be.

The only restriction it has is that the writes to a single device
using UC memory must be kept in program order..

Which requires barriers on some architectures :-)

Also we don't have a clear definition of what happens on WC memory.

Cheers,
Ben.