Sender: Guenter Roeck <groeck7@gmail.com>
Message-ID: <68131735-005b-890f-4052-12f598e2bce6@roeck-us.net>
Date:   Mon, 13 Feb 2023 12:09:45 -0800
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Subject: Re: [PATCH v4 1/5] dt-bindings: watchdog: mt7621-wdt: add phandle to
 access system controller registers
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To:     Sergio Paracuellos <sergio.paracuellos@gmail.com>
Cc:     Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>,
        =?UTF-8?B?QXLEsW7DpyDDnE5BTA==?= <arinc.unal@arinc9.com>,
        linux-watchdog@vger.kernel.org, wim@linux-watchdog.org,
        robh+dt@kernel.org, krzysztof.kozlowski+dt@linaro.org,
        matthias.bgg@gmail.com, tsbogend@alpha.franken.de,
        p.zabel@pengutronix.de, linux-kernel@vger.kernel.org,
        devicetree@vger.kernel.org, linux-mips@vger.kernel.org
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 <76353597-0170-e0d9-9f5d-f208a03e44e8@linaro.org>
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From:   Guenter Roeck <linux@roeck-us.net>
In-Reply-To: <CAMhs-H9UW=zkLs+cPkzXUj=TEytH75Y0dX_7NP1E2s-JeoANzA@mail.gmail.com>
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On 2/13/23 11:57, Sergio Paracuellos wrote:
> On Mon, Feb 13, 2023 at 8:36 PM Guenter Roeck <linux@roeck-us.net> wrote:
>>
>> On Mon, Feb 13, 2023 at 09:59:35AM +0100, Sergio Paracuellos wrote:
>>> On Sun, Feb 12, 2023 at 4:27 PM Guenter Roeck <linux@roeck-us.net> wrote:
>>>>
>>>> On 2/12/23 00:13, Sergio Paracuellos wrote:
>>>>> On Sat, Feb 11, 2023 at 12:42 PM Krzysztof Kozlowski
>>>>> <krzysztof.kozlowski@linaro.org> wrote:
>>>>>>
>>>>>> On 11/02/2023 12:01, Sergio Paracuellos wrote:
>>>>>>> On Sat, Feb 11, 2023 at 11:47 AM Arınç ÜNAL <arinc.unal@arinc9.com> wrote:
>>>>>>>>
>>>>>>>> On 11.02.2023 13:41, Sergio Paracuellos wrote:
>>>>>>>>> On Sat, Feb 11, 2023 at 10:10 AM Arınç ÜNAL <arinc.unal@arinc9.com> wrote:
>>>>>>>>>>
>>>>>>>>>> Is this mediatek,sysctl property required after your changes on the
>>>>>>>>>> watchdog code?
>>>>>>>>>
>>>>>>>>> I don't really understand the question :-) Yes, it is. Since we have
>>>>>>>>> introduced a new phandle in the watchdog node to be able to access the
>>>>>>>>> reset status register through the 'sysc' syscon node.
>>>>>>>>> We need the bindings to be aligned with the mt7621.dtsi file and we
>>>>>>>>> are getting the syscon regmap handler via
>>>>>>>>> 'syscon_regmap_lookup_by_phandle()'. See PATCH 5 of the series, Arınç.
>>>>>>>>
>>>>>>>> I believe you need to put mediatek,sysctl under "required:".
>>>>>>>
>>>>>>> Ah, I understood your question now :-). You meant 'required' property.
>>>>>>> I need more coffee, I guess :-). I am not sure if you can add
>>>>>>> properties as required after bindings are already mainlined for
>>>>>>> compatibility issues. The problem with this SoC is that drivers become
>>>>>>> mainlined before the device tree was so if things are properly fixed
>>>>>>> now this kind of issues appear.  Let's see Krzysztof and Rob comments
>>>>>>> for this.
>>>>>>
>>>>>> If your driver fails to probe without mediatek,sysctl, you already made
>>>>>> it required (thus broke the ABI) regardless what dt-binding is saying.
>>>>>> In such case you should update dt-binding to reflect reality.
>>>>>>
>>>>>> Now ABI break is different case. Usually you should not break it without
>>>>>> valid reasons (e.g. it was never working before). Your commit msg
>>>>>> suggests that you only improve the code, thus ABI break is not really
>>>>>> justified. In such case - binding is correct, driver should be reworked
>>>>>> to accept DTS without the new property.
>>>>>
>>>>> Thanks for clarification, Krzysztof. Ok, so if this is the case I need
>>>>> to add this property required (as Arinc was properly pointing out in
>>>>> previous mail) since without it the driver is going to fail on probe
>>>>> (PATCH 5 of the series). I understand the "it was never working
>>>>> before" argument reason for ABI breaks. What happens if the old driver
>>>>> code was not ideal and totally dependent on architecture specific
>>>>> operations when this could be totally avoided and properly make arch
>>>>> independent agnostic drivers? This driver was added in 2016 [0]. There
>>>>> was not a device tree file in the kernel for this SoC mainlined until
>>>>> 2022 [1]. I also personally migrated this watchdog binding in 2022
>>>>> from text to YAML and maintained it without changes [2]. When this was
>>>>> mainlined not all drivers were properly reviewed and the current code
>>>>> was just maintained as it is. Most users of this SoC are in the
>>>>> openWRT community where the dtsi of the mainline is not used yet and
>>>>> they maintain their own mt7621.dtsi files. Also, when a new version of
>>>>> the openWRT selected kernel is added they also modify and align with
>>>>> its mt7621.dtsi file without maintaining previous dtb's. If "make the
>>>>> driver arch independent to be able to be compile tested" and this kind
>>>>> of arguments are not valid at all I need to know because I have
>>>>> started to review driver code for this SoC and other drivers also have
>>>>> the same arch dependency that ideally should be avoided in the same
>>>>> way. This at the end means to break the ABI again in the future for
>>>>> those drivers / bindings. So I can just let them be as it is and not
>>>>> provide any change at all and continue without being compile tested
>>>>> and other beneficial features to detect future driver breakage.
>>>>>
>>>>
>>>> Problem is that there are (presumably) shipped systems out there with
>>>> the old devicetree file. The watchdog driver would no longer instantiate
>>>> on those systems.
>>>
>>> Ok, I will maintain only the PATCH that changes the driver to not use
>>> globals and send v5.
>>>
>>
>> Other options might be to search for the "syscon" node name or to search
>> for the "mediatek,mt7621-sysc" compatible.
> 
> Thanks for the hint. I didn't know about
> 'syscon_regmap_lookup_by_compatible()'. I will use this to avoid DTB
> ABI breakage and allow the driver to be selected for COMPILE_TEST..
> 

I didn't know about that one either. I thought about of_find_compatible_node()
or of_find_node_by_name(). syscon_regmap_lookup_by_compatible() is widely used,
though, so it seems to be a much better option.

Thanks,
Guenter