linux-kernel - Re: [PATCH 4/4] irqchip: bcm2836: Use a more generic memory barrier call

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Date:	Mon, 11 Apr 2016 09:52:27 -0600
From:	Stephen Warren <swarren@...dotorg.org>
To:	Eric Anholt <eric@...olt.net>
Cc:	linux-rpi-kernel@...ts.infradead.org,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	Lee Jones <lee@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Jason Cooper <jason@...edaemon.net>,
	Marc Zyngier <marc.zyngier@....com>
Subject: Re: [PATCH 4/4] irqchip: bcm2836: Use a more generic memory barrier
 call

On 04/10/2016 12:32 PM, Eric Anholt wrote:
> Stephen Warren <swarren@...dotorg.org> writes:
>
>> On 04/08/2016 12:20 PM, Eric Anholt wrote:
>>> Stephen Warren <swarren@...dotorg.org> writes:
>>>
>>>> On 04/04/2016 09:44 PM, Eric Anholt wrote:
>>>>> dsb() requires an argument on arm64, so we needed to add "sy".
>>>>> Instead, take this opportunity to switch to the same smp_wmb() call
>>>>> that gic uses for its IPIs.  This is a less strong barrier than we
>>>>> were doing before (dmb(ishst) compared to dsb(sy)), but it seems to be
>>>>> the correct one.
>>>>
>>>> I assume all MMIO is part of the ish domain?
>>>>
>>>> If so, the series,
>>>> Acked-by: Stephen Warren <swarren@...dotorg.org>
>>>
>>> I don't know if this barrier implies ordering all the way out to AXI on
>>> this HW, but I don't think that's a requirement of this function.
>>
>> My understanding was that the barrier was explicitly to work around a
>> bug in the bus fabric of the SoC, and hence the barrier very much does
>> have to affect the transaction all the way out to AXI. Re-reading
>> BCM2835-ARM-Peripherals.pdf section 1.3 "Peripheral access precautions
>> for correct memory ordering" seems to confirm this.
>
> My understanding of the explicit barrier here, which was copied from
> other irqchips, is "Make sure that normal memory writes before our IPI
> on this CPU appear on the other CPUs before they get the IPI" (like the
> comment says).  This barrier was not put in to deal with the
> 283x-specific weird AXI behavior.
>
> Note that we had previously decided that the weird AXI ordering
> behavior, which is about repeated reads or repeated writes from the same
> CPU across different peripherals, is already covered by the barriers
> present in readl() and writel().  The writel() barrier happens to be a
> dsb() as well, so this explicit barrier is actually redundant.

Ah OK. In that case, the change seems fine.