Subject: Re: RFC on writel and writel_relaxed

From: Benjamin Herrenschmidt <[email-masked]>
Date: Wed, 28 Mar 2018 10:43:49 +1100

On Tue, 2018-03-27 at 14:54 -0700, Alexander Duyck wrote:
> On Tue, Mar 27, 2018 at 2:35 PM, Benjamin Herrenschmidt
> <[email-masked]> wrote:
> > On Tue, 2018-03-27 at 10:46 -0400, Sinan Kaya wrote:
> > >  combined buffers.
> > > 
> > > Alex:
> > > "Don't bother. I can tell you right now that for x86 you have to have a
> > > wmb() before the writel().
> > 
> > No, this isn't the semantics of writel. You shouldn't need it unless
> > something changed and we need to revisit our complete understanding of
> > *all* MMIO accessor semantics.
> 
> The issue seems to be that there have been two different ways of
> dealing with this. There has historically been a number of different
> drivers that have been carrying this wmb() workaround since something
> like 2002. I get that the semantics for writel might have changed
> since then, but those of us who already have the wmb() in our drivers
> will be very wary of anyone wanting to go through and remove them
> since writel is supposed to be "good enough". I would much rather err
> on the side of caution here.
> 
> I view the wmb() + writel_relaxed() as more of a driver owning and
> handling this itself. Besides in the Intel Ethernet driver case it is
> better performance as our wmb() placement for us also provides a
> secondary barrier so we don't need to add a separate smp_wmb() to deal
> with a potential race we have with the Tx cleanup.
> 
> > At least for UC space, it has always been accepted (and enforced) that
> > writel would not require any other barrier to order vs. previous stores
> > to memory.
> 
> So the one thing I would question here is if this is UC vs UC or if
> this extends to other types as well? So for x86 we could find
> references to Write Combining being flushed by a write to UC memory,
> however I have yet to find a clear explanation of what a write to UC
> does to WB. 

Well, this is the standard write memory + trigger DMA case, the one
specific case for which Linus was adamant we don't need another barrier
 back then ...

> My personal inclination would be to err on the side of
> caution.

Which means writel_relaxed is now pointless ?

We need clear semantics here. In this case the "side of caution" means
we are randomly doing things not understanding what really happens and
that makes me *more* nervous.

>  I just don't want us going through and removing the wmb()
> calls because it "should" work. I would want to know for certain it
> will work.

We need to know for certain anyway. Otherwise, all the drivers that do
not have wmb's are potentially broken.

So I dont agree with the status quo.

We need to establish precisely what x86 does, decide what we want the
semantic of writel to be, and implement things accordingly.

Ben.