linux-kernel - Re: [RFC PATCH v4 7/8] pwm: Add dummy pwmchip for orphan pwms

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Date:   Tue, 17 Oct 2017 10:04:29 -0700
From:   Brian Norris <briannorris@...omium.org>
To:     Thierry Reding <thierry.reding@...il.com>
Cc:     Jeffy Chen <jeffy.chen@...k-chips.com>,
        linux-kernel@...r.kernel.org, dmitry.torokhov@...il.com,
        heiko@...ech.de, rjw@...ysocki.net, dianders@...omium.org,
        tfiga@...omium.org, broonie@...nel.org, seanpaul@...omium.org,
        linux-pwm@...r.kernel.org
Subject: Re: [RFC PATCH v4 7/8] pwm: Add dummy pwmchip for orphan pwms

Hi,

On Tue, Oct 17, 2017 at 02:40:31PM +0200, Thierry Reding wrote:
> On Tue, Oct 17, 2017 at 06:16:23PM +0800, Jeffy Chen wrote:
> > When the pwm driver is unbound while the pwm is still requested, the
> > pwm core would not actually remove the pwmchip(return -EBUSY instead).
> > 
> > So it would hold some references to the invalid resources(e.g. pwmchip).
> > 
> > And the customer who requested the pwm would have those references too,
> > and may crash the kernel when trying to access them later.
> > 
> > Add a dummy pwmchip, and assign orphan pwms to it to avoid that.
> > 
> > Signed-off-by: Jeffy Chen <jeffy.chen@...k-chips.com>
> > ---
> > 
> > Changes in v4:
> > Fix compile warning.
> > 
> > Changes in v3:
> > Assign orphan pwms to dummy pwmchip instead of adding device link in the
> > customer driver.
> 
> What happened to this? Device links were specifically designed to avoid
> situations like these.

I think Jeffy came up with this as an odd response to my suggestion on
v2 that we could just handle the device links in the PWM core. I don't
fully understand why the complete change in direction...

BTW, since you seem to have an opinion about device links: is it
expected that all consumer drivers will make explicit calls to
device_link_add()? I thought this should be avoided, if possible (e.g.,
this can be handled in pwm_get()).

> A dummy PWM chip doesn't seem like the right solution to this.

Agreed.

Brian