RE: [PATCH V2 1/4] gpio: mxc: Support module build

[Anson Huang <anson.huang@nxp.com>]

Hi, Arnd


> Subject: Re: [PATCH V2 1/4] gpio: mxc: Support module build
> 
> On Mon, Jul 27, 2020 at 10:18 AM Anson Huang <anson.huang@nxp.com>
> wrote:
> > > Subject: Re: [PATCH V2 1/4] gpio: mxc: Support module build
> > >
> > > On Wed, Jul 22, 2020 at 3:50 AM Anson Huang <Anson.Huang@nxp.com>
> > > wrote:
> > > >
> > > > Change config to tristate, add module device table, module author,
> > > > description and license to support module build for i.MX GPIO driver.
> > > >
> > > > As this is a SoC GPIO module, it provides common functions for
> > > > most of the peripheral devices, such as GPIO pins control,
> > > > secondary interrupt controller for GPIO pins IRQ etc., without
> > > > GPIO driver, most of the peripheral devices will NOT work
> > > > properly, so GPIO module is similar with clock, pinctrl driver
> > > > that should be loaded ONCE and never unloaded.
> > > >
> > > > Since MXC GPIO driver needs to have init function to register
> > > > syscore ops once, here still use subsys_initcall(), NOT
> module_platform_driver().
> > >
> > > I'm not following this explanation.
> > >
> > > Why is this driver using syscore_ops rather than
> > > SIMPLE_DEV_PM_OPS() or similar?
> >
> > Below is the original patch of using syscore_ops, it has explanation:
> >
> > commit 1a5287a3dbc34cd0c02c8f64c9131bd23cdfe2bb
> > Author: Anson Huang <anson.huang@nxp.com>
> > Date:   Fri Nov 9 04:56:56 2018 +0000
> >
> >     gpio: mxc: move gpio noirq suspend/resume to syscore phase
> >
> >     During noirq suspend/resume phase, GPIO irq could arrive
> >     and its registers like IMR will be changed by irq handle
> >     process, to make the GPIO registers exactly when it is
> >     powered ON after resume, move the GPIO noirq suspend/resume
> >     callback to syscore suspend/resume phase, local irq is
> >     disabled at this phase so GPIO registers are atomic.
> 
> The description makes sense, but this must be a problem that other
> gpio/pinctrl irqchip drivers have as well.
> 
> Linus, could you have a look? I see these other pinctrl drivers using
> SIMPLE_DEV_PM_OPS:
> 
> drivers/pinctrl/nomadik/pinctrl-nomadik.c:static
> SIMPLE_DEV_PM_OPS(nmk_pinctrl_pm_ops,
> drivers/pinctrl/pinctrl-rockchip.c:static
> SIMPLE_DEV_PM_OPS(rockchip_pinctrl_dev_pm_ops,
> rockchip_pinctrl_suspend,
> drivers/pinctrl/pinctrl-stmfx.c:static
> SIMPLE_DEV_PM_OPS(stmfx_pinctrl_dev_pm_ops,
> drivers/pinctrl/qcom/pinctrl-msm.c:SIMPLE_DEV_PM_OPS(msm_pinctrl_dev_
> pm_ops,
> msm_pinctrl_suspend,
> drivers/pinctrl/spear/pinctrl-plgpio.c:static
> SIMPLE_DEV_PM_OPS(plgpio_dev_pm_ops, plgpio_suspend, plgpio_resume);
> 
> Linus, can you have a look and see if that same problem applies to all of the
> above?
> 
> It seems that some drivers use SET_NOIRQ_SYSTEM_SLEEP_PM_OPS() instead,
> which looks like it is meant to address the same problem, but as I have not
> used that myself, I may be misunderstanding the problem or what this one
> does.
> 
> > > Why do you need subsys_initcall() rather than a device_initcall()?
> >
> > The subsys_initcal() is done by below commit, the commit log has detail
> explanation.
> >
> >
> > commit e188cbf7564fba80e8339b9406e8740f3e495c63
> > Author: Vladimir Zapolskiy <vladimir_zapolskiy@mentor.com>
> > Date:   Thu Sep 8 04:48:15 2016 +0300
> >
> >     gpio: mxc: shift gpio_mxc_init() to subsys_initcall level
> 
> That commit made the initialization later not earlier, as it originally was a
> postcore_initcall(). In the loadable module case, you make it even later than
> that, possibly as the last module loaded when booting up the system (followed
> by a storm of deferred probes).
>

Yes, loadable module will make it even later, the assumption is userspace can load it
before any users depend on GPIO driver. Given that we have to support loadable module
for all SoC specific module, do you have any other suggestion of how to proceed this
requirement for SoC GPIO driver?

 
> > > If the subsys_initcall() is indeed required here instead of
> > > device_initcall(), how can it work if the driver is a loadable module?
> >
> > My understanding is: there are two scenarios, one for built-in case,
> > the other is for loadable module, the subsys_initcall() is for
> > built-in case according to the upper commit, for loadable module, the user
> needs to handle the sequence of modules loaded.
> 
> I don't think we can rely on user space to coordinate module load order.
> The modules are generally loaded in an arbitrary order during the coldplug
> phase of the boot when user space looks at the available devices and loads a
> module for each one of them in the order it finds them in sysfs.
> 
> This means all drivers that rely on gpio, pinctrl or irqchip interfaces exported
> from this driver have to be able to deal with them not being there. This can
> also happen when the pinctrl driver is the only one that is a loadable module,
> while everything else is built-in. While that is not a configuration that users
> would likely choose intentionally, I don't see a reason why it shouldn't work.
> 
> Using module_init() or builtin_platform_driver() here would make give similar
> behavior for the built-in and modular cases and be somewhat more consistent,
> so you don't run into bugs only when the driver is a loadable module but make
> them obvious even to existing users with a builtin driver.
> 

My original idea of adding loadable module support for SoC specific module is, try
to keep it exactly same when the driver is built-in, but for GKI support, first, we need
to support GPIO driver built as module, and we definitely need to think about the module
load sequence to handle these dependency, but thinking about the common module widely
used by devices, such as pinctrl, clock and GPIO, maybe other modules need some patches
to handle the dependency, but that will be done later when we are working for those modules.

So, could you please help advise how to proceed it for this GPIO driver to support loadable module?

Thanks,
Anson