linux-kernel - Re: [PATCH] hwmon: pwm-fan: set usage

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Message-Id: <LGXNRR.0Q7OXCG5DVDQ2@brun.one>
Date:   Fri, 17 Mar 2023 12:33:57 +0100
From:   Lorenz Brun <lorenz@...n.one>
To:     Guenter Roeck <linux@...ck-us.net>
Cc:     Jean Delvare <jdelvare@...e.com>, linux-hwmon@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] hwmon: pwm-fan: set usage_power on PWM state

Am Mi, 15. Mär 2023 um 19:26:26 -07:00:00 schrieb Guenter Roeck 
<linux@...ck-us.net>:
> This doesn't seem to be used anywhere else. I do not understand the
> rationale, and I do not understand the practical impact of this 
> change.
> This needs to be confirmed by someone who understands what the flag is
> supposed to be used for, its impact, and if it is indeed appropriate
> in this context.
> 
> Also, since in practice there is no such "less flexible hardware"
> in the upstream kernel, please describe the use case in more detail.
> The only pwm driver that supports usage_power is pwm-pca9685,
> and that driver uses the flag for EMI reasons, not because it is
> "less flexible hardware". I am not inclined to accept such a change
> without specific use case or need.

The reason for this is that I have a PWM-driven fan on a Mediatek 
MT7986a whose PWM signal is inverted by a MOSFET.
The PWM peripheral of the MT7986a however doesn't support inverted 
output, at least not in a strict sense (i.e. if inverted mathematically 
it is out of phase). The maintainer of the PWM subsystem recommended 
that I only mathematically invert if usage_power is set as this means 
that phase doesn't matter.

See 
https://lore.kernel.org/linux-pwm/20230309010410.2106525-1-lorenz@brun.one/ 
for the PWM-side patch.

Regards,
Lorenz