linux-kernel - Re: [PATCH v2 2/2] dt-bindings: hwmon: pwm-fan: add retain-state-shutdown property

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Message-ID:
 <OSQPR06MB72525057883A59578441E0988B512@OSQPR06MB7252.apcprd06.prod.outlook.com>
Date: Mon, 4 Nov 2024 07:45:56 +0000
From: Billy Tsai <billy_tsai@...eedtech.com>
To: Akinobu Mita <akinobu.mita@...il.com>, Krzysztof Kozlowski
	<krzk@...nel.org>
CC: "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-hwmon@...r.kernel.org" <linux-hwmon@...r.kernel.org>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>, Jean Delvare
	<jdelvare@...e.com>, Guenter Roeck <linux@...ck-us.net>, Rob Herring
	<robh@...nel.org>, Krzysztof Kozlowski <krzk+dt@...nel.org>, Conor Dooley
	<conor+dt@...nel.org>
Subject: Re: [PATCH v2 2/2] dt-bindings: hwmon: pwm-fan: add
 retain-state-shutdown property

> >
> > On 28/10/2024 15:57, Akinobu Mita wrote:
> > >>>>
> > >>>> You described the desired Linux feature or behavior, not the actual
> > >>>> hardware. The bindings are about the latter, so instead you need to
> > >>>> rephrase the property and its description to match actual hardware
> > >>>> capabilities/features/configuration etc.
> > >>>
> > >>> Is this description okay?
> > >>> (Reused the description of retain-state-shutdown in leds-gpio.yaml)
> > >>>
> > >>> description:
> > >>>   Retain the state of the PWM on shutdown. Useful in BMC systems, for
> > >>>   example, when the BMC is rebooted while the host remains up, the fan
> > >>>   will not stop.
> > >>
> > >> Nothing improved in the property. You still say what the system should
> > >> do. This is user-space choice, not DT.
> > >
> > > It seems better to implement it as a device attribute.
> >
> > I don't know about that. To repeat: if you say what system is supposed
> > to be doing, it is a policy. Describe the hardware and its configuration
> > and maybe this would be suitable for DT.

> Billy, could you please write a proper description for this property?
> I'm not the right person for this.

In our hardware, if the system reboots and power remains on the PWM controller
will retain its original settings. However, the pwm-fan.c driver currently disables
the PWM controller during a system reboot. I need this property to prevent pwm-fan.c
from disabling the PWM when the system reboots.
In my point of view, the description can be:
Retain the state of the PWM on shutdown. Some platforms (e.g., BMC) will maintain
the PWM status after the system reboot. Add this property to prevent the PWM from being
disabled during the system reboot.

Thanks

Billy Tsai