linux-kernel - Re: [PATCH RESEND v3 2/4] pwm: Add Apple PWM controller

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Message-ID: <29c800dd-efdf-ddab-3ad2-466e1a8eb7d2@marcan.st>
Date:   Wed, 23 Nov 2022 11:37:08 +0900
From:   Hector Martin <marcan@...can.st>
To:     Rob Herring <robh@...nel.org>,
        Sasha Finkelstein <fnkl.kernel@...il.com>
Cc:     thierry.reding@...il.com, krzysztof.kozlowski+dt@...aro.org,
        sven@...npeter.dev, alyssa@...enzweig.io, asahi@...ts.linux.dev,
        linux-arm-kernel@...ts.infradead.org, linux-pwm@...r.kernel.org,
        devicetree@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH RESEND v3 2/4] pwm: Add Apple PWM controller

On 23/11/2022 11.24, Rob Herring wrote:
> On Mon, Nov 21, 2022 at 08:42:26PM +0300, Sasha Finkelstein wrote:
>> diff --git a/drivers/pwm/pwm-apple.c b/drivers/pwm/pwm-apple.c
>> new file mode 100644
>> index 000000000000..b0c3f86fd578
>> --- /dev/null
>> +++ b/drivers/pwm/pwm-apple.c
>> @@ -0,0 +1,127 @@
>> +// SPDX-License-Identifier: GPL-2.0 OR MIT
> 
> Kernel code is generally GPL-2.0 only. No other PWM driver is MIT 
> licensed. So why this one.
> 
> Mixing licenses is a problem because few people look at the licenses 
> when copying code around.

*Sigh*. We encourage the use of MIT dual-licensing as a project to allow
other OSes to port the drivers over without having to rewrite them, for
any driver written from scratch. We've had this conversation quite a few
times already...

>> +
>> +	ret = devm_pwmchip_add(&pdev->dev, &pwm->chip);
> 
> This symbol is EXPORT_SYMBOL_GPL. So how can this module be MIT 
> licensed?

Because they are compatible licenses. The combination of this driver and
the kernel is GPL, but this driver itself is MIT. People are free to
port it to other OSes and reimplement devm_pwmchip_add or replace the
call with something else.

The EXPORT_SYMBOL_GPL stuff is about blocking *proprietary*
GPL-incompatible modules from using those symbols. This is a
GPL-compatible, explicitly dual-licensed module.

In this case the driver is trivial enough there isn't much to gain from
dual-licensing since the parts that matter (the reverse engineering) are
not copyrightable, but I still find it silly that we keep getting told
more permissive licensing is a problem. People are free to dual-license
their work as they see fit, it's a fundamental freedom in free software,
and plenty of kernel code is dual-licensed like this (including much of
DRM and entire GPU drivers).

- Hector