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

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Date: Wed, 24 Jan 2024 07:27:02 +0100
From: Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>
To: Saravana Kannan <saravanak@...gle.com>
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
Subject: Re: [PATCH 3/9] watchdog: s3c2410_wdt: update to use new
 exynos_pmu_*() apis

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.

> 
> 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

Best regards,
Krzysztof