linux-kernel - Re: [PATCH 3/9] watchdog: s3c2410_wdt: update to use new exynos_pmu

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Date: Wed, 24 Jan 2024 13:27:01 -0800
From: Saravana Kannan <saravanak@...gle.com>
To: Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>
Cc: Peter Griffin <peter.griffin@...aro.org>, arnd@...db.de, robh+dt@...nel.org, 
	krzysztof.kozlowski+dt@...aro.org, linux@...ck-us.net, wim@...ux-watchdog.org, 
	conor+dt@...nel.org, alim.akhtar@...sung.com, jaewon02.kim@...sung.com, 
	chanho61.park@...sung.com, semen.protsenko@...aro.org, 
	kernel-team@...roid.com, tudor.ambarus@...aro.org, andre.draszik@...aro.org, 
	willmcvicker@...gle.com, linux-fsd@...la.com, linux-watchdog@...r.kernel.org, 
	devicetree@...r.kernel.org, linux-kernel@...r.kernel.org, 
	linux-arm-kernel@...ts.infradead.org, linux-samsung-soc@...r.kernel.org, 
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: Re: [PATCH 3/9] watchdog: s3c2410_wdt: update to use new
 exynos_pmu_*() apis

On Tue, Jan 23, 2024 at 10:27 PM Krzysztof Kozlowski
<krzysztof.kozlowski@...aro.org> wrote:
>
> On 24/01/2024 04:37, Saravana Kannan wrote:
> > On Tue, Jan 23, 2024 at 10:12 AM Krzysztof Kozlowski
> > <krzysztof.kozlowski@...aro.org> wrote:
> >>
> >> On 23/01/2024 18:30, Peter Griffin wrote:
> >>>>>               dev_warn(wdt->dev, "Couldn't get RST_STAT register\n");
> >>>>>       else if (rst_stat & BIT(wdt->drv_data->rst_stat_bit))
> >>>>> @@ -698,14 +699,6 @@ static int s3c2410wdt_probe(struct platform_device *pdev)
> >>>>>       if (ret)
> >>>>>               return ret;
> >>>>>
> >>>>> -     if (wdt->drv_data->quirks & QUIRKS_HAVE_PMUREG) {
> >>>>> -             wdt->pmureg = syscon_regmap_lookup_by_phandle(dev->of_node,
> >>>>> -                                             "samsung,syscon-phandle");
> >>>>> -             if (IS_ERR(wdt->pmureg))
> >>>>> -                     return dev_err_probe(dev, PTR_ERR(wdt->pmureg),
> >>>>> -                                          "syscon regmap lookup failed.\n");
> >>>>
> >>>>
> >>>> Continuing topic from the binding: I don't see how you handle probe
> >>>> deferral, suspend ordering.
> >>>
> >>> The current implementation is simply relying on exynos-pmu being
> >>> postcore_initcall level.
> >>>
> >>> I was just looking around for any existing Linux APIs that could be a
> >>> more robust solution. It looks like
> >>>
> >>> of_parse_phandle()
> >>> and
> >>> of_find_device_by_node();
> >>>
> >>> Are often used to solve this type of probe deferral issue between
> >>> devices. Is that what you would recommend using? Or is there something
> >>> even better?
> >>
> >> I think you should keep the phandle and then set device link based on
> >> of_find_device_by_node(). This would actually improve the code, because
> >> syscon_regmap_lookup_by_phandle() does not create device links.
> >
> > I kinda agree with this. Just because we no longer use a syscon API to
> > find the PMU register address doesn't mean the WDT doesn't depend on
> > the PMU.
> >
> > However, I think we should move to a generic "syscon" property. Then I
> > can add support for "syscon" property to fw_devlink and then things
> > will just work in terms of probe ordering, suspend/resume and also
> > showing the dependency in DT even if you don't use the syscon APIs.
> >
> > Side note 1:
> >
> > I think we really should officially document a generic syscon DT
> > property similar to how we have a generic "clocks" or "dmas" property.
> > Then we can have a syscon_get_regmap() that's like so:
> >
> > struct regmap *syscon_get_regmap(struct device *dev)
> > {
> >         return syscon_regmap_lookup_by_phandle(dev->of_node, "syscon");
> > }
> >
> > Instead of every device defining its own bespoke DT property to do the
> > exact same thing. I did a quick "back of the envelope" grep on this
> > and I get about 143 unique properties just to get the syscon regmap.
> > $ git grep -A1 syscon_regmap_lookup_by_phandle | grep '"' | sed -e
> > 's/^[^"]*//' -e 's/"[^"]*$/"/' | sort | uniq | wc -l
> > 143
>
> Sorry, generic "syscon" property won't fly with DT maintainers, because
> there is no such thing as syscon in any of hardware.

Then why do we allow a "syscon" compatible string and nodes? If the
"syscon" property isn't clear enough, we can make it something like
gpios and have it be <whatever>-syscon or have syscon-names property
if you want to give it a name.
143 bespoke properties all to say "here are some registers I need to
twiddle that's outside my regmap" doesn't seem great.

> >
> > Side note 2:
> >
> > How are we making sure that it's the exynos-pmu driver that ends up
> > probing the PMU and not the generic syscon driver? Both of these are
> > platform drivers. And the exynos PMU device lists both the exynos
> > compatible string and the syscon property. Is it purely a link order
> > coincidence?
>
> initcall ordering

Both these drivers usr postcore_initcall(). So it's purely because
soc/ is listed earlier in drivers/Makefile than mfd/. And as soon as
drivers are made into modules this is going to break. This is
terrible. If you want to have a modular system, this is going to throw
in a wrench.

-Saravana