Re: [PATCH - JACK 1/1] jack: Use internal snd_pcm_state to reduce amount of additional variables

From: "Wischer, Timo (ADITG/ESB)" <twischer@de.adit-jv.com>
To: Takashi Iwai <tiwai@suse.de>
Cc: "alsa-devel@alsa-project.org" <alsa-devel@alsa-project.org>
Subject: Re: [PATCH - JACK 1/1] jack: Use internal snd_pcm_state to reduce amount of additional variables
Date: Thu, 15 Mar 2018 15:07:31 +0000	[thread overview]
Message-ID: <B0FB33DC1499054591F62C0EF1E013D7684FDCEB@HI2EXCH01.adit-jv.com> (raw)
In-Reply-To: <s5hk1ud7744.wl-tiwai@suse.de>

> We need to check it, of course.  But the behavior of the prepare
> function is certainly different from others, so it's more likely a
> bug.

I could provide a fix for the IO plug API for prepare and draining.

> This should be notified to ioplug via snd_pcm_ioplug_set_state().

Without an additional variable snd_pcm_ioplug_set_state() has to be called from the JACK thread.
But snd_pcm_ioplug_set_state() is not thread safe.

> That'd be great if everything is ready, but this can't be a reason to
> add a shadow state and modify the code as in the patch for now.

At the end I would like to use functions similar to READ_ONCE() and WRITE_ONCE() to access the state (known from the Linux kernel [1]).
But I think this should not be part of the ALSA library because it is for most IO plugins not required.
Therefore I need to have an internal copy of the state which will only be accessed with READ_ONCE() and WRITE_ONCE().

You are right, it is not yet required but at the end I have to implement it.

Do you think we should implement such a synchronization into the ALSA lib?

[1] https://www.kernel.org/doc/Documentation/memory-barriers.txt

Best regards

Timo Wischer

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