Subject: Re: RFC on writel and writel_relaxed

From: Benjamin Herrenschmidt <[email-masked]>
Date: Tue, 27 Mar 2018 08:26:58 +1100

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:
> > > > 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.