linux-kernel - Re: [patches] Re: [PATCH 13/17] RISC-V: Add include subdirectory

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Date:   Sat, 24 Jun 2017 22:01:29 -0500
From:   Benjamin Herrenschmidt <benh@...nel.crashing.org>
To:     Palmer Dabbelt <palmer@...belt.com>
Cc:     Arnd Bergmann <arnd@...db.de>, linux-arch@...r.kernel.org,
        linux-kernel@...r.kernel.org, Olof Johansson <olof@...om.net>,
        albert@...ive.com, patches@...ups.riscv.org
Subject: Re: [patches] Re: [PATCH 13/17] RISC-V: Add include subdirectory

On Sat, 2017-06-24 at 14:32 -0700, Palmer Dabbelt wrote:
> On Sat, 24 Jun 2017 08:42:05 PDT (-0700), benh@...nel.crashing.org wrote:
> > On Fri, 2017-06-23 at 19:01 -0700, Palmer Dabbelt wrote:
> > > > > +#define mmiowb()       __asm__ __volatile__ ("fence io,io" : : : "memory");
> > 
> > I forgot if we already mentioned that but mmiowb is primarily intended
> > to order MMIO stores vs. a subsequent spin_unlock.
> > 
> > I'm not sure an IO only fence is sufficient here.
> > 
> > Note that I've never trusted drivers to get that right, it's a rather
> > bad abstraction to begin with, so on powerpc, instead, I just set a
> > per-cpu flag on every non-relaxed MMIO write and test it in spin_unlock
> > in order to "beef up" the barrier in there if necessary.
> 
> Sorry about that -- I thought I'd included a note somewhere that the atomics
> and barriers weren't ready to go yet, as we'd found a bunch of problems with
> them in the first review and I needed to go through them all.  Arnd suggested
> copying the PowerPC approach to mmiowb and I like that better, so we're going
> to use it.

Ah yes, I did see your note, I just wasn't sure we had clarified the
mmiowb case and thought it was worth mentioning.

Cheers,
Ben,