linux-kernel - Re: [PATCH v3 1/5] ASoC: hdmi-codec: Add an op to set callback function for plug event

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Date:   Tue, 16 Jul 2019 15:59:36 +0800
From:   Cheng-yi Chiang <cychiang@...omium.org>
To:     Tzung-Bi Shih <tzungbi@...gle.com>
Cc:     Russell King - ARM Linux admin <linux@...linux.org.uk>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        ALSA development <alsa-devel@...a-project.org>,
        Douglas Anderson <dianders@...omium.org>,
        Heiko Stuebner <heiko@...ech.de>,
        linux-rockchip@...ts.infradead.org,
        David Airlie <airlied@...ux.ie>,
        dri-devel@...ts.freedesktop.org, Takashi Iwai <tiwai@...e.com>,
        Liam Girdwood <lgirdwood@...il.com>, tzungbi@...omium.org,
        Jaroslav Kysela <perex@...ex.cz>,
        Hans Verkuil <hverkuil@...all.nl>,
        Andrzej Hajda <a.hajda@...sung.com>,
        Mark Brown <broonie@...nel.org>,
        Laurent Pinchart <Laurent.pinchart@...asonboard.com>,
        Daniel Vetter <daniel@...ll.ch>,
        Dylan Reid <dgreid@...omium.org>,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v3 1/5] ASoC: hdmi-codec: Add an op to set callback
 function for plug event

On Mon, Jul 15, 2019 at 11:56 PM Tzung-Bi Shih <tzungbi@...gle.com> wrote:
>
> On Fri, Jul 12, 2019 at 6:58 PM Russell King - ARM Linux admin
> <linux@...linux.org.uk> wrote:
> >
> > On Fri, Jul 12, 2019 at 06:04:39PM +0800, Cheng-Yi Chiang wrote:
> > > Add an op in hdmi_codec_ops so codec driver can register callback
> > > function to handle plug event.
> > >
> > > Driver in DRM can use this callback function to report connector status.
> > >
> > > Signed-off-by: Cheng-Yi Chiang <cychiang@...omium.org>
> > > ---
> > >  include/sound/hdmi-codec.h    | 16 +++++++++++++
> > >  sound/soc/codecs/hdmi-codec.c | 45 +++++++++++++++++++++++++++++++++++
> > >  2 files changed, 61 insertions(+)
> > >
> > > diff --git a/include/sound/hdmi-codec.h b/include/sound/hdmi-codec.h
> > > index 7fea496f1f34..9a8661680256 100644
> > > --- a/include/sound/hdmi-codec.h
> > > +++ b/include/sound/hdmi-codec.h
> > > @@ -47,6 +47,9 @@ struct hdmi_codec_params {
> > >       int channels;
> > >  };
> > >
> > > +typedef void (*hdmi_codec_plugged_cb)(struct device *dev,
> > > +                                   bool plugged);
> > > +
> >
> > I'd like to pose a question for people to think about.
> >
> > Firstly, typedefs are generally shunned in the kernel.  However, for
> > these cases it seems to make sense.
> >
> > However, should the "pointer"-ness be part of the typedef or not?  To
> > see what I mean, consider:
> >
> >         typedef void (*hdmi_foo)(void);
> >
> >         int register_foo(hdmi_foo foo);
> >
> > vs
> >
> >         typedef void hdmi_foo(void);
> >
> >         int register_foo(hdmi_foo *foo);
> >
> > which is more in keeping with how we code non-typedef'd code - it's
> > obvious that foo is a pointer while reading the code.
> I have a different opinion.  Its suffix "_cb" self-described it is a
> callback function.  Since function and function pointer are equivalent
> in the language, I think we don't need to emphasize that it is a
> function "pointer".
>
>

Hi Russell and Tzungbi, thank you for the review.
Regarding this typedef of callback function, I found a thread
discussing this very long time ago:

https://yarchive.net/comp/linux/typedefs.html

>From that thread, Linus gave an example of using typedef for function
pointer that is following to this pattern.
I also looked around how other driver use it:
$ git grep typedef | grep _cb | less | wc -l
138
$ git grep typedef | grep _cb | grep "(\*" | wc -l
115
Most of the typedef of callback function use this pattern.
So I think this should be fine.
Thanks!


> > It seems to me that the latter better matches what is in the kernel's
> > coding style, which states:
> >
> >   In general, a pointer, or a struct that has elements that can
> >   reasonably be directly accessed should **never** be a typedef.
> >
> > or maybe Documentation/process/coding-style.rst needs updating?