linux-kernel - Re: [PATCH v2 2/3] mailbox: Add support for Hi3660 mailbox

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Date:   Mon, 30 Oct 2017 19:13:13 +0800
From:   Leo Yan <leo.yan@...aro.org>
To:     Mark Rutland <mark.rutland@....com>
Cc:     Kaihua Zhong <zhongkaihua@...wei.com>, robh+dt@...nel.org,
        xuwei5@...ilicon.com, catalin.marinas@....com, will.deacon@....com,
        jassisinghbrar@...il.com, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        guodong.xu@...aro.org, haojian.zhuang@...aro.org,
        suzhuangluan@...ilicon.com, xuezhiliang@...ilicon.com,
        kevin.wangtao@...ilicon.com
Subject: Re: [PATCH v2 2/3] mailbox: Add support for Hi3660 mailbox

Hi Mark,

On Mon, Oct 30, 2017 at 10:19:40AM +0000, Mark Rutland wrote:
> Hi,
> 
> On Mon, Oct 30, 2017 at 12:45:06PM +0800, Leo Yan wrote:
> > On Fri, Oct 27, 2017 at 11:46:00AM +0100, Mark Rutland wrote:
> > > On Fri, Oct 27, 2017 at 02:15:03PM +0800, Kaihua Zhong wrote:
> > > > +static int hi3660_mbox_check_state(struct mbox_chan *chan)
> > > > +{
> 
> > > > +	/* Ensure channel is released */
> > > > +	writel_relaxed(0xffffffff, base + MBOX_IMASK_REG);
> > > > +	writel_relaxed(BIT(mdev->ack_irq), base + MBOX_SRC_REG);
> > > > +	__asm__ volatile ("sev");
> > > > +	return 0;
> > > > +}
> > > 
> > > Drivers really shouldn't be using SEV directly (even if via the
> > > sev() macro)...
> > > 
> > > This SEV isn't ordered w.r.t. anything, and it's unclear what
> > > ordering you need, so this simply does not work.
> > 
> > I will leave your questions for Hisilicon colleagues, essentially your
> > questions are related with mailbox mechanism.
> > 
> > But I'd like to firstly get clear your question for "This SEV isn't
> > ordered w.r.t. anything". From my understanding, ARMv8 architecture
> > natually adds DMB before SEV so all previous register writing
> > opreations should be ensured to endpoint before SEV?
> 
> This is not the case; SEV does not add any implicit memory barrier, and
> is not ordered w.r.t. memory accesses.
> 
> See ARM DDI 0487B.b, page D1-1905, "The Send Event instructions":
> 
>     The PE is not required to guarantee the ordering of this event with
>     respect to the completion of memory accesses by instructions before
>     the SEV instruction. Therefore, ARM recommends that software
>     includes a DSB instruction before any SEV instruction.

My fault and thanks for explanation.

> Note that a DMB is not sufficient, as SEV is not a memory access.

Understood now, so below code should be safe?

wmb();  -> dsb(st);
sev();

Thanks,
Leo Yan