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

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Message-ID: <04411aaf-6f2c-4f43-83b4-aa0741ccd25f@linaro.org>
Date: Thu, 25 Jan 2024 08:37:35 +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,
 Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: Re: [PATCH 3/9] watchdog: s3c2410_wdt: update to use new
 exynos_pmu_*() apis

On 24/01/2024 22:27, Saravana Kannan wrote:
> 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

To bind Linux drivers.

> "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.

This could work.

> 143 bespoke properties all to say "here are some registers I need to
> twiddle that's outside my regmap" doesn't seem great.

Why? 143 of these registers are not the same.

> 
>>>
>>> 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

Oh... great :/.

> 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.

Best regards,
Krzysztof