linux-kernel - Re: [RFC PATCH 2/7] ALSA: ac97: add an ac97 bus

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Date:	Mon, 16 May 2016 07:40:56 +0200
From:	Takashi Iwai <tiwai@...e.de>
To:	Robert Jarzmik <robert.jarzmik@...e.fr>
Cc:	"Haojian Zhuang" <haojian.zhuang@...il.com>,
	"Liam Girdwood" <lgirdwood@...il.com>,
	"Mark Brown" <broonie@...nel.org>,
	"Jaroslav Kysela" <perex@...ex.cz>,
	"Daniel Mack" <daniel@...que.org>, <alsa-devel@...a-project.org>,
	<linux-arm-kernel@...ts.infradead.org>,
	<patches@...nsource.wolfsonmicro.com>,
	<linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH 2/7] ALSA: ac97: add an ac97 bus

On Sun, 15 May 2016 23:29:27 +0200,
Robert Jarzmik wrote:
> 
> Takashi Iwai <tiwai@...e.de> writes:
> 
> > On Sat, 14 May 2016 11:50:50 +0200,
> > Robert Jarzmik wrote:
> >> >> +unsigned int ac97_bus_scan_one(struct ac97_controller *ac97,
> >> >> +				      int codec_num)
> >> >> +{
> >> >> +	struct ac97_codec_device codec;
> >> >> +	unsigned short vid1, vid2;
> >> >> +	int ret;
> >> >> +
> >> >> +	codec.dev = *ac97->dev;
> >> >> +	codec.num = codec_num;
> >> >> +	ret = ac97->ops->read(&codec, AC97_VENDOR_ID1);
> >> >> +	vid1 = (ret & 0xffff);
> >> >> +	if (ret < 0)
> >> >> +		return 0;
> >> >
> >> > Hmm.  This looks pretty hackish and dangerous.
> >> You mean returning 0 even if the read failed, right ?
> >
> > No, my concern is that it's creating a dummy codec object temporarily
> > on the stack just by copying some fields and calling the ops with it.
> > (And actually the current code may work wrongly because lack of
> >  zero-clear of the object.)
> Ah yes, I remember now, the on-stack generated device, indeed ugly.
> 
> > IMO, a cleaner way would be to define the ops passed with both
> > controller and codec objects as arguments, and pass NULL codec here.
> It's rather unusual to need both the device and its controller in bus
> operations. I must admit I have no better idea so far, so I'll try that just to
> see how it looks like, and let's see next ...

Thinking of this again, I wonder now why we need to pass the codec
object at all.  It's the read/write ops via ac97, so we just need the
ac97_controller object and the address slot of the accessed codec?


Takashi