Re: [PATCH RFC v4 2/8] ALSA: pcm: add IEC958 channel status helper for hw_params

From: Clemens Ladisch <clemens@ladisch.de>
To: Jyri Sarha <jsarha@ti.com>,
	Russell King - ARM Linux <linux@arm.linux.org.uk>
Cc: devicetree@vger.kernel.org, alsa-devel@alsa-project.org,
	liam.r.girdwood@linux.intel.com, moinejf@free.fr,
	airlied@linux.ie, broonie@kernel.org,
	dri-devel@lists.freedesktop.org, peter.ujfalusi@ti.com,
	Takashi Iwai <tiwai@suse.de>,
	tony@atomide.com, tomi.valkeinen@ti.com, bcousson@baylibre.com,
	linux-omap@vger.kernel.org
Subject: Re: [PATCH RFC v4 2/8] ALSA: pcm: add IEC958 channel	status helper for hw_params
Date: Mon, 21 Sep 2015 17:08:15 +0200	[thread overview]
Message-ID: <56001D5F.1010903@ladisch.de> (raw)
In-Reply-To: <560016BE.6010801@ti.com>

Jyri Sarha wrote:
> On 09/21/15 12:37, Russell King - ARM Linux wrote:
>> ALSA documentation (which may be out of date) says that the hw_params
>> callback can be called multiple times during stream setup.  Do we want
>> to be repeatedly programming the HDMI infoframe with different settings,
>> potentially confusing the attached device, or would it be better to do
>> it slightly later (in the prepare callback) after the parameters have
>> been fully negotiated?
>
> If it is possible that hw_params() can be called multiple times, at
> least it does not happen very often. I wonder if it would ever happen
> again after a successful hw_params() call.

It is certainly allowed, and it happens regularly when using OSS
emulation.  AFAIK PulseAudio does something similar when enumerating
devices.

> But there is no downside in using prepare callback.

The prepare callback also can be called multiple times.  This certainly
happens when the stream is stopped/restarted multiple times.

If something does not need to be done again when a stream is restarted,
then it should be done in the hw_params callback.

Regards,
Clemens