Re: [PATCH v4 4/6] infiniband: cxgb4: Eliminate duplicate barriers on weakly-ordered archs

[Casey Leedom]

Yes, but ...

For instance, I see that the x86 writel() has "memory" in its asm(), which
prevents GCC from reordering generated instructions.  And it ~looks like~
arm64 ~sort of~ gets that with the inclusion of __iowmb() (which translates
to wmb() then dsb(st) which finally holds the GCC "memory" barrier).  Is
this part of the documented semantic of the writel_relaxed()?  The PowerPC
stuff simply defines writel_relaxed() as writel() and I can't find the
bottom of that Rabbit Hole ...

I'm guessing~ that this line in the documentation ~may~ imply the GCC
ordering:

     ... Note that relaxed accesses to
     the same peripheral are guaranteed to be ordered with respect to each
     other. ...

In any case, we really only have a few places where we (the various Chelsio
drivers) need to worry about this: the "Fast Paths" where we have a lot of
I/O to the device.  I think we should leave everything else alone.

Casey