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Date:   Tue, 2 Aug 2022 13:04:53 +0200
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To:     Chen-Yu Tsai <wenst@chromium.org>,
        Allen-KH Cheng <allen-kh.cheng@mediatek.com>,
        Wim Van Sebroeck <wim@linux-watchdog.org>,
        Guenter Roeck <linux@roeck-us.net>,
        Rob Herring <robh+dt@kernel.org>
Cc:     Krzysztof Kozlowski <krzk+dt@kernel.org>, nfraprado@collabora.com,
        angelogioacchino.delregno@collabora.com,
        Project_Global_Chrome_Upstream_Group@mediatek.com,
        devicetree@vger.kernel.org, linux-arm-kernel@lists.infradead.org,
        linux-kernel@vger.kernel.org, linux-mediatek@lists.infradead.org
References: <20220721014845.19044-1-allen-kh.cheng@mediatek.com>
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From:   Matthias Brugger <matthias.bgg@gmail.com>
Subject: Re: [PATCH v4 1/4] dt-bindings: mediatek: watchdog: Fix compatible
 fallbacks and example
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On 01/08/2022 11:29, Chen-Yu Tsai wrote:
> On Thu, Jul 21, 2022 at 10:50 AM Allen-KH Cheng
> <allen-kh.cheng@mediatek.com> wrote:
>>
>> The watchdog timer of mt8186. mt8195 and mt7986 have their DT data.
>> We should not use 'mediatek,mt6589-wdt' as fallback.
>>
>> For mediatek,wdt example of mt8183, We remove mediatek,mt6589-wdt fallback.
> 
> I think this needs some more information.
> 
> Right now on the kernel side, mt6589-wdt provides just watchdog support.
> The SoC-specific compatibles that are touched by this patch provide reset
> controls in addition to the standard watchdog, which remains the same.
> 
> If that is the case, then the fallback compatibles are correct. A fallback
> says that the new hardware is compatible with some older hardware, and
> can be run with the driver supporting that older hardware, likely with
> reduced functionality.
> 

My understanding is, that we add a fallback because although at the time we 
entered the compatible, the functionality of the device is the same as the 
fallback. Nonetheless we add a compatible specific for the device in case in the 
future we realize that the device has some functionality that is not and can not 
be covered by the fallback.

This is the case here. Actually adding the fallback in the first place was 
wrong, because the driver since ever supports the extra function for the device, 
the reset.

So this is a mere cleanup of the binding to reflect what was always present in 
the driver.

Regards,
Matthias

> As an example, if mt8195-wdt is backward compatible with mt6589-wdt,
> then it should run as mt6589-wdt, and would just be missing new
> functionality, in this case the reset controls.
> 
> So either mt6589-wdt also contains a reset control that is not the same
> as the other newer chips, or has some other functionality that the other
> chips contain, and justifies the removal of the fallback, or this patch
> is incorrect. Note that mt2701-wdt and mt762*-wdt are still listed as
> compatible with mt6589-wdt. So I think a better explanation is required.
> 
> 
> Regards
> ChenYu
> 
> 
>> Fixes:a45b408a020b("dt-bindings: watchdog: Add compatible for MediaTek MT8186")
>> Fixes:b326f2c85f3d("dt-bindings: watchdog: Add compatible for Mediatek MT8195")
>> Fixes:41e73feb1024("dt-bindings: watchdog: Add compatible for Mediatek MT7986")
>> Fixes:f43f97a0fc0e("dt-bindings: mediatek: mt8183: Add #reset-cells")
>> Signed-off-by: Allen-KH Cheng <allen-kh.cheng@mediatek.com>
>> Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
>> ---
>>   Documentation/devicetree/bindings/watchdog/mtk-wdt.txt | 9 ++++-----
>>   1 file changed, 4 insertions(+), 5 deletions(-)
>>
>> diff --git a/Documentation/devicetree/bindings/watchdog/mtk-wdt.txt b/Documentation/devicetree/bindings/watchdog/mtk-wdt.txt
>> index 762c62e428ef..67ef991ec4cf 100644
>> --- a/Documentation/devicetree/bindings/watchdog/mtk-wdt.txt
>> +++ b/Documentation/devicetree/bindings/watchdog/mtk-wdt.txt
>> @@ -14,12 +14,12 @@ Required properties:
>>          "mediatek,mt7622-wdt", "mediatek,mt6589-wdt": for MT7622
>>          "mediatek,mt7623-wdt", "mediatek,mt6589-wdt": for MT7623
>>          "mediatek,mt7629-wdt", "mediatek,mt6589-wdt": for MT7629
>> -       "mediatek,mt7986-wdt", "mediatek,mt6589-wdt": for MT7986
>> +       "mediatek,mt7986-wdt": for MT7986
>>          "mediatek,mt8183-wdt": for MT8183
>> -       "mediatek,mt8186-wdt", "mediatek,mt6589-wdt": for MT8186
>> +       "mediatek,mt8186-wdt": for MT8186
>>          "mediatek,mt8516-wdt", "mediatek,mt6589-wdt": for MT8516
>>          "mediatek,mt8192-wdt": for MT8192
>> -       "mediatek,mt8195-wdt", "mediatek,mt6589-wdt": for MT8195
>> +       "mediatek,mt8195-wdt": for MT8195
>>
>>   - reg : Specifies base physical address and size of the registers.
>>
>> @@ -32,8 +32,7 @@ Optional properties:
>>   Example:
>>
>>   watchdog: watchdog@10007000 {
>> -       compatible = "mediatek,mt8183-wdt",
>> -                    "mediatek,mt6589-wdt";
>> +       compatible = "mediatek,mt8183-wdt";
>>          mediatek,disable-extrst;
>>          reg = <0 0x10007000 0 0x100>;
>>          interrupts = <GIC_SPI 139 IRQ_TYPE_NONE>;
>> --
>> 2.18.0
>>
>>