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Date:   Tue, 26 Jul 2022 06:25:35 -0700
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Subject: Re: [PATCH] watchdog: mtk_wdt: Move mt6589-wdt fallback compatible to
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To:     AngeloGioacchino Del Regno 
        <angelogioacchino.delregno@collabora.com>,
        Chen-Yu Tsai <wenst@chromium.org>,
        Wim Van Sebroeck <wim@linux-watchdog.org>,
        Matthias Brugger <matthias.bgg@gmail.com>,
        Philipp Zabel <p.zabel@pengutronix.de>
Cc:     linux-watchdog@vger.kernel.org,
        linux-arm-kernel@lists.infradead.org,
        linux-mediatek@lists.infradead.org, linux-kernel@vger.kernel.org
References: <20220726062057.320342-1-wenst@chromium.org>
 <91613700-3335-7499-1b48-3d88edd9da36@collabora.com>
From:   Guenter Roeck <linux@roeck-us.net>
In-Reply-To: <91613700-3335-7499-1b48-3d88edd9da36@collabora.com>
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On 7/26/22 01:37, AngeloGioacchino Del Regno wrote:
> Il 26/07/22 08:20, Chen-Yu Tsai ha scritto:
>> The watchdog facility in the MediaTek SoCs are all the same, but the
>> hardware block also contains a reset controller, which differs in the
>> number of resets used between different SoCs. This difference is
>> supported with of_device_get_match_data() supplying the number of reset
>> lines for the newer compatible strings. The original mt6589-wdt only
>> covers the watchdog facility.
>>
>> of_device_is_compatible(), and by extension of_device_get_match_data(),
>> works by going through the given list of compatible strings sequentially,
>> and checks if any of the device node's compatible strings match.
>>
>> To avoid early matching on the "mediatek,mt6589-wdt" fallback compatible,
>> which only provides watchdog functionality and no reset controller, move
>> the fallback entry to the end of the list, so that other, more specific
>> compatible strings have a chance at getting matched.
>>
>> Fixes: c254e103082b ("watchdog: mtk_wdt: mt8183: Add reset controller")
>> Fixes: adc318a34066 ("watchdog: mt8192: add wdt support")
>> Fixes: 8c6b5ea6ac68 ("watchdog: mediatek: mt8195: add wdt support")
>> Fixes: 4dbabc4d9e8c ("watchdog: mediatek: mt8186: add wdt support")
>> Fixes: 711a5b25bac9 ("watchdog: mtk_wdt: mt7986: Add toprgu reset controller support")
>> Signed-off-by: Chen-Yu Tsai <wenst@chromium.org>
> 
> Uhm, I don't think that this is an issue?
> 
> Ordering precedence is given to the list that you specify in devicetree, that's why
> "the second one" is a fallback, meaning: ("impossible" example below)
> 
> compatible = "mediatek,mt8195-wdt", "mediatek,mt8183-wdt", "mediatek,mt6589-wdt";
> 
> This gets walked as per the order in which you wrote the compatibles, so:
> * Check match for mt8195-wdt, does not exist?
>   * Check match for mt8183-wdt, exists!
>   * Put everything into dev->of_node (having mediatek,mt8183-wdt only!)
> 
> __of_device_is_compatible() gets dev->of_node and compares that to all of the
> possible matches.
> 
> struct device_node for this device hence does *not* contain any of the other
> compatibles that we specified in devicetree, so it does *not* contain any of
> "mediatek,mt8195-wdt", or "mediatek,mt6589-wdt", because we have previously
> successfully matches 8183.
> 
> I don't think that I've misinterpreted this flow, but if I have, let's pull
> in devicetree people and check with them?
> 

I don't see the problem either. The fallback needs to be listed last in the
compatible property. If it isn't, having it last in struct of_device_id
won't help either.

> Cheers,
> Angelo
> 
>> ---
>>
>> This change complements the removal of the fallback compatible from the
>> bindings and DTSI files [1].
>>

If the fallback isn't listed in the dtsi files, it won't match at all,
no matter where it is located in struct of_device_id.

Guenter

>> [1] https://lore.kernel.org/linux-mediatek/20220721014845.19044-1-allen-kh.cheng@mediatek.com/
>>
>>   drivers/watchdog/mtk_wdt.c | 3 ++-
>>   1 file changed, 2 insertions(+), 1 deletion(-)
>>
>> diff --git a/drivers/watchdog/mtk_wdt.c b/drivers/watchdog/mtk_wdt.c
>> index e97787536792..3d5a239b93ba 100644
>> --- a/drivers/watchdog/mtk_wdt.c
>> +++ b/drivers/watchdog/mtk_wdt.c
>> @@ -425,12 +425,13 @@ static int mtk_wdt_resume(struct device *dev)
>>   static const struct of_device_id mtk_wdt_dt_ids[] = {
>>       { .compatible = "mediatek,mt2712-wdt", .data = &mt2712_data },
>> -    { .compatible = "mediatek,mt6589-wdt" },
>>       { .compatible = "mediatek,mt7986-wdt", .data = &mt7986_data },
>>       { .compatible = "mediatek,mt8183-wdt", .data = &mt8183_data },
>>       { .compatible = "mediatek,mt8186-wdt", .data = &mt8186_data },
>>       { .compatible = "mediatek,mt8192-wdt", .data = &mt8192_data },
>>       { .compatible = "mediatek,mt8195-wdt", .data = &mt8195_data },
>> +    /* Fallback compatible string must be last entry */
>> +    { .compatible = "mediatek,mt6589-wdt" },
>>       { /* sentinel */ }
>>   };
>>   MODULE_DEVICE_TABLE(of, mtk_wdt_dt_ids);
> 
>