linux-kernel - Re: [PATCH v6 3/3] watchdog: mt7621-wdt: avoid ralink architecture dependent code

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Date:   Tue, 14 Feb 2023 10:50:32 +0100
From:   Sergio Paracuellos <sergio.paracuellos@...il.com>
To:     Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>
Cc:     linux-watchdog@...r.kernel.org, wim@...ux-watchdog.org,
        linux@...ck-us.net, robh+dt@...nel.org,
        krzysztof.kozlowski+dt@...aro.org, matthias.bgg@...il.com,
        arinc.unal@...nc9.com, tsbogend@...ha.franken.de,
        p.zabel@...gutronix.de, linux-kernel@...r.kernel.org,
        devicetree@...r.kernel.org, linux-mips@...r.kernel.org
Subject: Re: [PATCH v6 3/3] watchdog: mt7621-wdt: avoid ralink architecture
 dependent code

On Tue, Feb 14, 2023 at 10:33 AM Krzysztof Kozlowski
<krzysztof.kozlowski@...aro.org> wrote:
>
> On 14/02/2023 09:54, Sergio Paracuellos wrote:
> >>>       return 0;
> >>> @@ -143,6 +147,10 @@ static int mt7621_wdt_probe(struct platform_device *pdev)
> >>>       if (!drvdata)
> >>>               return -ENOMEM;
> >>>
> >>> +     drvdata->sysc = syscon_regmap_lookup_by_compatible("mediatek,mt7621-sysc");
> >>> +     if (IS_ERR(drvdata->sysc))
> >>> +             return PTR_ERR(drvdata->sysc);
> >>
> >> This should be the backup/error path for original code using syscon
> >> property. Looking up by compatible is really not portable/re-usable.
> >
> > I can change the code in the way you are pointing out here but...
> > Why is it not re-usable? Compatible is not supposed to be changed
> > since in other cases the DTB ABI will be broken. I am totally lost
> > about what is an ABI breakage, then.
>
> How do you use it on other platform?

I see, thanks.

So if I am understanding you correctly I have to maintain the
'mediatek,sysctl' phandle and use it in the first instance and if it
fails I have to use regmap_loopup_by_compatible() stuff, right?
Something like:

...

drvdata->sysc = syscon_regmap_lookup_by_phandle(np, "mediatek,sysctl");
if (IS_ERR(drvdata->sysc)) {
         drvdata->sysc =
syscon_regmap_lookup_by_compatible("mediatek,mt7621-sysc");
         if (IS_ERR(drvdata->sysc))
              return PTR_ERR(drvdata->sysc);
}

....

So in that case, we can add the new phandle to the bindings without
any kind of ABI breakage and we can use old dtbs using new watchdog
driver code.

I really hope I finally understood this properly. Thanks for your
patience, Krzysztof.

Best regards,
    Sergio Paracuellos

>
> Best regards,
> Krzysztof
>