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

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Date:	Mon, 16 May 2016 10:53:56 +0200
From:	Robert Jarzmik <robert.jarzmik@...e.fr>
To:	Takashi Iwai <tiwai@...e.de>
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

Takashi Iwai <tiwai@...e.de> writes:

> 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,
>> >
>> > 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?
So far it would work. The only objection I would see is if in the future the bus
operation needs a specialization which is ac97 codec dependent, such as a flag
or a mask in ac97_codec_device structure.

Even if I'd like to not have these in bus operations, the struct snd_ac97 had a
need for a 'caps', 'ext_id', ... fields for example. Yet these could be
contained in the ac97_codec_device structure and not exposed to bus operations.

Another worry is the pattern (as an example) in atmel_ac97c_write() in
sound/atmel/ac97.c, where the codec structure is used to get the controller
through a container_of() type call. Yet passing the controller to bus operations
takes care of this one.

>From a "purely API" point of view the couple (ac97_controller, ac97_slot_id) is
what will route an ac97 bus operation, be that a read/write/reset/..., the
remaining question is will it cover the cases we've not thought of ?

Cheers.

-- 
Robert