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

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Date:   Tue, 30 Nov 2021 16:31:44 +0800
From:   Shawn Guo <shawn.guo@...aro.org>
To:     Maulik Shah <quic_mkshah@...cinc.com>
Cc:     Marc Zyngier <maz@...nel.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, 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?

Shawn