linux-kernel - Re: [PATCH v3 2/2] net: dsa: mt7530: Use GPIO polarity to generate correct reset sequence

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Message-ID: <0d85e1e6-ea75-4f20-aef1-90d446b4bfa1@kernel.org>
Date: Mon, 1 Dec 2025 08:48:51 +0100
From: Krzysztof Kozlowski <krzk@...nel.org>
To: Andrew Lunn <andrew@...n.ch>, Vladimir Oltean <olteanv@...il.com>
Cc: Chen Minqiang <ptpt52@...il.com>, Rob Herring <robh@...nel.org>,
 Krzysztof Kozlowski <krzk+dt@...nel.org>, Conor Dooley
 <conor+dt@...nel.org>, Matthias Brugger <matthias.bgg@...il.com>,
 AngeloGioacchino Del Regno <angelogioacchino.delregno@...labora.com>,
 "Chester A. Unal" <chester.a.unal@...nc9.com>,
 Daniel Golle <daniel@...rotopia.org>, DENG Qingfang <dqfext@...il.com>,
 Sean Wang <sean.wang@...iatek.com>, linux-kernel@...r.kernel.org,
 linux-arm-kernel@...ts.infradead.org, linux-mediatek@...ts.infradead.org,
 netdev@...r.kernel.org
Subject: Re: [PATCH v3 2/2] net: dsa: mt7530: Use GPIO polarity to generate
 correct reset sequence

On 30/11/2025 21:17, Andrew Lunn wrote:
> On Sun, Nov 30, 2025 at 10:07:31AM +0200, Vladimir Oltean wrote:
>> On Sun, Nov 30, 2025 at 02:11:05AM +0100, Andrew Lunn wrote:
>>>> -		gpiod_set_value_cansleep(priv->reset, 0);
>>>> +		int is_active_low = !!gpiod_is_active_low(priv->reset);
>>>> +		gpiod_set_value_cansleep(priv->reset, is_active_low);
>>>
>>> I think you did not correctly understand what Russell said. You pass
>>> the logical value to gpiod_set_value(). If the GPIO has been marked as
>>> active LOW, the GPIO core will invert the logical values to the raw
>>> value. You should not be using gpiod_is_active_low().
>>>
>>> But as i said to the previous patch, i would just leave everything as
>>> it is, except document the issue.
>>>
>>> 	Andrew
>>>
>>
>> It was my suggestion to do it like this (but I don't understand why I'm
>> again not in CC).
>>
>> We _know_ that the reset pin of the switch should be active low. So by
>> using gpiod_is_active_low(), we can determine whether the device tree is
>> wrong or not, and we can work with a wrong device tree too (just invert
>> the logical values).
> 
> Assuming there is not a NOT gate placed between the GPIO and the reset
> pin, because the board designer decided to do that for some reason?

Yeah, I cannot imagine how this could possibly support old and new DTS
without breaking some users, unless people over-simplified and discarded
some cases. But then this should clearly mark these broken cases instead
of falsely claim that impossible task of rewriting the flag is done
correctly.

BTW, the code clearly does not handle such cases, so "we can determine
whether the device tree is wrong or not" statement is obviously not true.

Best regards,
Krzysztof