linux-kernel - Re: [PATCH v2 2/2] irqchip: Add Qualcomm MPM controller driver

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Message-ID: <87ilwam3du.wl-maz@kernel.org>
Date:   Tue, 30 Nov 2021 08:52:45 +0000
From:   Marc Zyngier <maz@...nel.org>
To:     Shawn Guo <shawn.guo@...aro.org>
Cc:     Maulik Shah <quic_mkshah@...cinc.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Bjorn Andersson <bjorn.andersson@...aro.org>,
        Rob Herring <robh+dt@...nel.org>,
        Loic Poulain <loic.poulain@...aro.org>,
        devicetree@...r.kernel.org, linux-arm-msm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 2/2] irqchip: Add Qualcomm MPM controller driver

On Tue, 30 Nov 2021 08:31:44 +0000,
Shawn Guo <shawn.guo@...aro.org> wrote:
> 
> On Tue, Nov 30, 2021 at 01:19:48PM +0530, Maulik Shah wrote:
> >    Hi Shawn,
> > 
> >    On 11/30/2021 8:01 AM, Shawn Guo wrote:
> > 
> > +       do {
> > +               r_val = readl(priv->base + offset);
> > +               udelay(5);
> > +       } while (r_val != val);
> > 
> > What? Is this waiting for a bit to clear? Why isn't this one of the
> > read*_poll_timeout*() function instead? Surely you can't wait forever
> > here.
> > 
> > This is taken from downstream, and it seems to double check the written
> > value by reading it back.  But to be honest, I'm not really this is
> > necessary.  I will do some testing with the read-back check dropped.
> > 
> > How about asking for specs instead? There are QC people on Cc, and
> > many more reading the list. Hopefully they can explain what this is
> > all about.
> > 
> > Maulik,
> > 
> > If you have some information about this, that would be great.
> > 
> >    This can be converted to read poll_timeout(). This was introduced in
> >    place of wmb() to make sure writes are completed.
> 
> Hmm, in this case, writel() will just do the right thing, as it wraps
> wmb() there.  Or am I missing something?

writel() places the wmb() *before* the MMIO access. This is use for
ordering with RAM access if the device is DMA capable, for example. I
seriously doubt this is the case.

My understanding of Maulik's comment is that there is a requirement
for the MMIO access to complete. And for that, a barrier *after* the
write is the right tool for the job.

	M.

-- 
Without deviation from the norm, progress is not possible.