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

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Date:   Tue, 30 Nov 2021 08:47:52 +0000
From:   Marc Zyngier <maz@...nel.org>
To:     Maulik Shah <quic_mkshah@...cinc.com>
Cc:     Shawn Guo <shawn.guo@...aro.org>,
        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 07:49:48 +0000,
Maulik Shah <quic_mkshah@...cinc.com> wrote:
> 
> [1  <text/plain; UTF-8 (7bit)>]
> 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.

A string of reads isn't equivalent to a dsb(st). If there is a
requirement for the write to complete, then use the required barrier.

	M.

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