linux-kernel - Re: [EXT] Re: [PATCH v9 2/4] irqchip: Add IMX MU MSI controller driver

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Date:   Thu, 08 Sep 2022 15:51:30 +0100
From:   Marc Zyngier <maz@...nel.org>
To:     Frank Li <frank.li@....com>
Cc:     "tglx@...utronix.de" <tglx@...utronix.de>,
        "robh+dt@...nel.org" <robh+dt@...nel.org>,
        "krzysztof.kozlowski+dt@...aro.org" 
        <krzysztof.kozlowski+dt@...aro.org>,
        "shawnguo@...nel.org" <shawnguo@...nel.org>,
        "s.hauer@...gutronix.de" <s.hauer@...gutronix.de>,
        "kw@...ux.com" <kw@...ux.com>,
        "bhelgaas@...gle.com" <bhelgaas@...gle.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
        Peng Fan <peng.fan@....com>,
        Aisheng Dong <aisheng.dong@....com>,
        "jdmason@...zu.us" <jdmason@...zu.us>,
        "kernel@...gutronix.de" <kernel@...gutronix.de>,
        "festevam@...il.com" <festevam@...il.com>,
        dl-linux-imx <linux-imx@....com>,
        "kishon@...com" <kishon@...com>,
        "lorenzo.pieralisi@....com" <lorenzo.pieralisi@....com>,
        "ntb@...ts.linux.dev" <ntb@...ts.linux.dev>,
        "lznuaa@...il.com" <lznuaa@...il.com>,
        "imx@...ts.linux.dev" <imx@...ts.linux.dev>,
        "manivannan.sadhasivam@...aro.org" <manivannan.sadhasivam@...aro.org>
Subject: Re: [EXT] Re: [PATCH v9 2/4] irqchip: Add IMX MU MSI controller driver

On Thu, 08 Sep 2022 15:23:53 +0100,
Frank Li <frank.li@....com> wrote:
> 
> >
> > On Wed, 07 Sep 2022 04:48:54 +0100,
> > Frank Li <Frank.Li@....com> wrote:
> > >
> > > The MU block found in a number of Freescale/NXP SoCs supports
> > generating
> > > IRQs by writing data to a register
> > >
> > > This enables the MU block to be used as a MSI controller, by leveraging
> > > the platform-MSI API
> > 
> > Missing full stop after each sentence.
> 
> [Frank Li] Do you means missed "."?

Yes.

> > > diff --git a/drivers/irqchip/Kconfig b/drivers/irqchip/Kconfig
> > > index 5e4e50122777d..e04c6521dce55 100644
> > > --- a/drivers/irqchip/Kconfig
> > > +++ b/drivers/irqchip/Kconfig
> > > @@ -470,6 +470,15 @@ config IMX_INTMUX
> > >       help
> > >         Support for the i.MX INTMUX interrupt multiplexer.
> > >
> > > +config IMX_MU_MSI
> > > +     bool "i.MX MU work as MSI controller"
> > 
> > Why bool? Doesn't it also work as a module?
> 
> [Frank Li] I remember you said that irq-chip can't be removed. 
> So I am not sure why need build as module.

Not being removed doesn't mean it cannot be built as a module and
loaded on demand. Why should I be forced to have this driver built-in
if my kernel is used on a variety of systems, only one of them having
this device?

> > > +
> > > +struct imx_mu_msi {
> > > +     spinlock_t                      lock;
> > > +     raw_spinlock_t                  reglock;
> > 
> > Why two locks? Isn't one enough to protect both MSI allocation (which
> > happens once in a blue moon) and register access?
> 
> [Frank Li] Previously your comment, ask me to use raw_spinlock for
> read\write register access.  I don't think raw_spinlock is good for
> MSI allocation.

Why wouldn't it be good enough? I'd really like to know.

> 
> > 
> > Also, where are these locks initialised?
> > 
> 
> [Frank Li] struct imx_mu_msi is fill zero when allocated.
> Does it still need additional initialization for spinlock?

Have you heard of lockdep? Or CONFIG_DEBUG_SPINLOCK?  Maybe you should
try it.

> > > +     if (!pdev)
> > > +             return -ENODEV;
> > 
> > How can that happen?
> > 
> [Frank Li] Not sure, many driver check as it. 

And? Just because someone does something pointless, you have to
imitate them?

	M.

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