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

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Date:   Fri, 9 Sep 2022 14:59:01 +0000
From:   Frank Li <frank.li@....com>
To:     Marc Zyngier <maz@...nel.org>
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



> -----Original Message-----
> From: Marc Zyngier <maz@...nel.org>
> Sent: Friday, September 9, 2022 7:08 AM
> To: Frank Li <frank.li@....com>
> Cc: tglx@...utronix.de; robh+dt@...nel.org;
> krzysztof.kozlowski+dt@...aro.org; shawnguo@...nel.org;
> s.hauer@...gutronix.de; kw@...ux.com; bhelgaas@...gle.com; linux-
> kernel@...r.kernel.org; devicetree@...r.kernel.org; linux-arm-
> kernel@...ts.infradead.org; linux-pci@...r.kernel.org; Peng Fan
> <peng.fan@....com>; Aisheng Dong <aisheng.dong@....com>;
> jdmason@...zu.us; kernel@...gutronix.de; festevam@...il.com; dl-linux-
> imx <linux-imx@....com>; kishon@...com; lorenzo.pieralisi@....com;
> ntb@...ts.linux.dev; lznuaa@...il.com; imx@...ts.linux.dev;
> manivannan.sadhasivam@...aro.org
> Subject: Re: [EXT] Re: [PATCH v9 2/4] irqchip: Add IMX MU MSI controller
> driver
> 
> Caution: EXT Email
> 
> On Thu, 08 Sep 2022 16:35:20 +0100,
> Frank Li <frank.li@....com> wrote:
> 
> > > > > > +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.[Frank Li] '
> >
> > [Frank Li] According to my understand, raw_spinlock skip some lockdep
> > /debug feature to get better performance, which should be used when
> > Frequently call, such as irq handle\polling thread.
> 
> I'm afraid you are terribly misguided. They both have the same debug
> features because they are both using the same core implementation, and
> the only difference is whether this is preemptible for RT purposes or
> not.
> 
> > Spinlock have DEBUG feature to check wrong use lock.  Allocate MSI
> generally
> > only is call once when driver probe.
> 
> Again, you should really read the code and the documentation and stop
> making things up.

[Frank Li] Thanks. You give me the correct direction. Some stackoverflow's
Doc was misleaded.  I double checked spin_lock implementation.  PREEMPT_RT
Kernel map spin_lock to rt_mutex.

I am curious  why exist spin_lock_irqsave and raw_spin_lock_irqsave before
PREEMTP_RT merge into kernel tree. 

> 
> >
> > The basic principle,  lock should be used only when necessary.  Access reg
> and
> > Allocate msi is totally independence events.
> 
> Independent events that do not occur simultaneously. So no harm in
> sharing the same lock.
> 
>         M.
> 
> --
> Without deviation from the norm, progress is not possible.