linux-kernel - Re: [PATCH v3 3/3] irqchip: Add Qualcomm MPM controller driver

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Message-ID: <20211207094835.GO10105@dragon>
Date:   Tue, 7 Dec 2021 17:48:36 +0800
From:   Shawn Guo <shawn.guo@...aro.org>
To:     Marc Zyngier <maz@...nel.org>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Maulik Shah <quic_mkshah@...cinc.com>,
        Bjorn Andersson <bjorn.andersson@...aro.org>,
        Loic Poulain <loic.poulain@...aro.org>,
        linux-arm-msm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 3/3] irqchip: Add Qualcomm MPM controller driver

On Mon, Dec 06, 2021 at 01:48:12PM +0000, Marc Zyngier wrote:
> > > > +static int qcom_mpm_enter_sleep(struct qcom_mpm_priv *priv)
> > > > +{
> > > > +	int i, ret;
> > > > +
> > > > +	for (i = 0; i < priv->reg_stride; i++)
> > > > +		qcom_mpm_write(priv, MPM_REG_STATUS, i, 0);
> > > > +
> > > > +	/* Notify RPM to write vMPM into HW */
> > > 
> > > What do you mean by 'into HW'? We just did that, right? or are these
> > > registers just fake and most of the stuff is in the RPM?
> > 
> > I have a note about this in commit log.
> > 
> > - All the register settings are done by APSS on an internal memory
> >   region called vMPM, and RPM will flush them into hardware after it
> >   receives a mailbox/IPC notification from APSS.
> > 
> > So yes, these registers are fake/virtual in memory, and RPM will
> > actually flush the values into the MPM hardware block.
> 
> Then why are you using MMIO accessors all over the place if this is
> just RAM? Who *owns* this memory? Is it normal DRAM? Or some flops
> exposed by a device? Why isn't the state simply communicated over the
> mailbox instead?

It's a piece of internal memory (SRAM) which can be access by AP and
RPM.  The communication mechanism is defined by SoC/RPM design, and we
can do nothing but following the procedure.

Shawn