linux-kernel - Re: [PATCH v3 2/3] soc: rockchip: power-domain: Replace dsb() with smb()

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Date:   Wed, 27 Apr 2022 14:47:55 -0700
From:   Brian Norris <briannorris@...omium.org>
To:     Robin Murphy <robin.murphy@....com>
Cc:     Peter Geis <pgwipeout@...il.com>, Heiko Stuebner <heiko@...ech.de>,
        Arnd Bergmann <arnd@...db.de>,
        "open list:ARM/Rockchip SoC..." <linux-rockchip@...ts.infradead.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Guenter Roeck <linux@...ck-us.net>
Subject: Re: [PATCH v3 2/3] soc: rockchip: power-domain: Replace dsb() with smb()

On Tue, Apr 26, 2022 at 5:25 PM Robin Murphy <robin.murphy@....com> wrote:
> On 2022-04-27 00:55, Peter Geis wrote:
> > On Mon, Apr 25, 2022 at 9:46 PM Brian Norris <briannorris@...omium.org> wrote:
> >>
> >> It's unclear if these are really needed at all, but seemingly their
> >> purpose is only as a write barrier. Use the general macro instead of the
> >> ARM-specific one.
...
> >> -       dsb(sy);
> >> +       wmb();
> >
> > Just curious, shouldn't this be mb() instead of wmb()?
> >  From the arm64 barrier.h:
> >
> > #define mb() dsb(sy)
> > #define wmb() dsb(st)
>
> As I mentioned on v2, that would be the literal translation, however
> there's no concurrency since this is happening under a mutex, so there's
> no other agent against whose accesses loads would need to be
> synchronised, therefore the only logical reason those DSBs were ever
> there at all must be to ensure that the prior store(s) have been issued
> to their destination before proceeding. The history implies that this
> dates all the way back to RK3288, where Armv7's argument-less DSB lacked
> that distinction anyway.

Thanks Robin. I already tried to capture part of this in the commit message:

"It's unclear if these are really needed at all, but seemingly their
purpose is only as a write barrier."

i.e., it's intentional that I'm making a change, not a literal translation.

I ran through a few tests on Rockchip RK3399, FWIW, although I suppose
some nasty memory ordering bugs are not exactly the kind of thing that
would fall out in smoke tests.

Brian