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

[Frank Li]

> -----Original Message-----
> From: Marc Zyngier <maz@xxxxxxxxxx>
> Sent: Thursday, September 8, 2022 9:52 AM
> To: Frank Li <frank.li@xxxxxxx>
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> Subject: Re: [EXT] Re: [PATCH v9 2/4] irqchip: Add IMX MU MSI controller
> driver
> 
> Caution: EXT Email
> 
> On Thu, 08 Sep 2022 15:23:53 +0100,
> Frank Li <frank.li@xxxxxxx> wrote:
> >
> > >
> > > On Wed, 07 Sep 2022 04:48:54 +0100,
> > > Frank Li <Frank.Li@xxxxxxx> 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?

[Frank Li]  A problem,  platform_msi_create_irq_domain have NOT export to let module
Call it.   https://elixir.bootlin.com/linux/latest/source/drivers/base/platform-msi.c#L122

Do you want to me add EXPORT_SYMBOL_GPL for it  OR keep "bool" here? 
	
> 
> > > > +
> > > > +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.