linux-kernel - Re: [PATCH RFC 3/4] backlight: add kernel-internal backlight API

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Date:	Thu, 11 Sep 2014 13:21:34 +0200
From:	Thierry Reding <thierry.reding@...il.com>
To:	David Herrmann <dh.herrmann@...il.com>
Cc:	"dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>,
	Matthew Garrett <matthew.garrett@...ula.com>,
	Daniel Vetter <daniel.vetter@...ll.ch>,
	Bryan Wu <cooloney@...il.com>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Lee Jones <lee.jones@...aro.org>
Subject: Re: [PATCH RFC 3/4] backlight: add kernel-internal backlight API

On Thu, Sep 11, 2014 at 01:14:31PM +0200, David Herrmann wrote:
> Hi
> 
> On Thu, Sep 11, 2014 at 1:10 PM, Thierry Reding
> <thierry.reding@...il.com> wrote:
> > On Wed, Sep 10, 2014 at 05:54:22PM +0200, David Herrmann wrote:
> > [...]
> >> +void backlight_set_brightness(struct backlight_device *bd, unsigned int value,
> >> +                           enum backlight_update_reason reason)
> >> +{
> >> +     mutex_lock(&bd->ops_lock);
> >> +     if (bd->ops) {
> >> +             value = clamp(value, 0U, (unsigned)bd->props.max_brightness);
> >
> > max_brightness should really be unsigned to begin with...
> >
> >> +             pr_debug("set brightness to %u\n", value);
> >
> > dev_dbg(&bd->dev, ...)?
> 
> I agree with both comments, but I tried to be consistent with what
> brightness_store() does.

Fair enough, this can be cleaned up in separate patches.

> >> diff --git a/include/linux/backlight.h b/include/linux/backlight.h
> >> index adb14a8..bcc0dec 100644
> >> --- a/include/linux/backlight.h
> >> +++ b/include/linux/backlight.h
> >> @@ -141,6 +141,22 @@ extern bool backlight_device_registered(enum backlight_type type);
> >>  extern int backlight_register_notifier(struct notifier_block *nb);
> >>  extern int backlight_unregister_notifier(struct notifier_block *nb);
> >>
> >> +struct backlight_device *backlight_device_lookup(const char *name);
> >> +void backlight_set_brightness(struct backlight_device *bd, unsigned int value,
> >> +                           enum backlight_update_reason reason);
> >> +
> >> +static inline void backlight_device_ref(struct backlight_device *bd)
> >> +{
> >> +     if (bd)
> >> +             get_device(&bd->dev);
> >> +}
> >
> > Perhaps for consistency with get_device() this should return bd? That
> > way you can chain things like so:
> >
> >         priv->backlight = backlight_device_ref(bd);
> 
> Makes sense, will change it. Same is actually true for _unref(), which
> should return NULL unconditionally. This way, you can use:
>   priv->backlight = backlight_device_unref(priv->backlight);
> to release a reference and reset the pointer at the same time.

That looks somewhat odd to me. Wouldn't priv->backlight typically go
away after the unref anyway (presumably because priv is going to get
freed soon after)?

But I have no strong objections to returning NULL from _unref(), if code
doesn't need it it can always choose not to use the return value.

Thierry

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