linux-kernel - Re: [PATCH 10/20] mips/mmiowb: Add unconditional mmiowb() to arch_spin

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Message-ID: <20190301221616.uhpjjepmxjmahkjt@pburton-laptop>
Date:   Fri, 1 Mar 2019 22:16:18 +0000
From:   Paul Burton <paul.burton@...s.com>
To:     Will Deacon <will.deacon@....com>
CC:     "linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "Paul E. McKenney" <paulmck@...ux.ibm.com>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Arnd Bergmann <arnd@...db.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Andrea Parri <andrea.parri@...rulasolutions.com>,
        Palmer Dabbelt <palmer@...ive.com>,
        Daniel Lustig <dlustig@...dia.com>,
        David Howells <dhowells@...hat.com>,
        Alan Stern <stern@...land.harvard.edu>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        "Maciej W. Rozycki" <macro@...ux-mips.org>,
        Ingo Molnar <mingo@...nel.org>,
        Yoshinori Sato <ysato@...rs.sourceforge.jp>,
        Rich Felker <dalias@...c.org>, Tony Luck <tony.luck@...el.com>
Subject: Re: [PATCH 10/20] mips/mmiowb: Add unconditional mmiowb() to
 arch_spin_unlock()

Hi Will,

On Fri, Mar 01, 2019 at 02:03:38PM +0000, Will Deacon wrote:
> The mmiowb() macro is horribly difficult to use and drivers will continue
> to work most of the time if they omit a call when it is required.
> 
> Rather than rely on driver authors getting this right, push mmiowb() into
> arch_spin_unlock() for mips. If this is deemed to be a performance issue,
> a subsequent optimisation could make use of ARCH_HAS_MMIOWB to elide
> the barrier in cases where no I/O writes were performed inside the
> critical section.
> 
> Signed-off-by: Will Deacon <will.deacon@....com>

Cleaning up our I/O functions has been on my to-do list for a while, so
I'll aim to get to that soon & get the calls to mmiowb_set_pending() in
place as part of it so that we can look at that optimization & drop the
custom queued_spin_unlock().

Meanwhile this looks sane & I don't want to hold it up so:

    Acked-by: Paul Burton <paul.burton@...s.com>

Thanks,
    Paul