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Date: Thu, 01 Oct 2015 13:46:33 +0200
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From: Takashi Iwai <tiwai@suse.de>
To: Felipe Tonello <eu@felipetonello.com>
Cc: Clemens Ladisch <clemens@ladisch.de>,
	linux-bluetooth@vger.kernel.org, alsa-devel@alsa-project.org
Subject: Re: [alsa-devel] [RFC] MIDI over Bluetooth Low Energy
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On Thu, 01 Oct 2015 13:35:48 +0200,
Felipe Tonello wrote:
> 
> Hi Clemens,
> 
> On Thu, Oct 1, 2015 at 12:27 PM, Clemens Ladisch <clemens@ladisch.de> wrote:
> > Felipe Tonello wrote:
> >> I am planning to start the support of MIDI BLE profile[1].
> >>
> >> I suggest two main goals:
> >>  * To be transparent to applications, i.e., use rawmidi and sequencer
> >> ALSA interfaces to interact.
> >>  * To support peripheral and central BLE roles.
> >>
> >> My question is: what is the best way possible of doing it?
> >
> > Just write a (user-space) sequencer client.
> 
> But this will limit to seq interface only. It will not be available
> via rawmidi interface, right? Unless I load snd-virtmidi, correct?

Right.

> That was what I was suggesting at first, but I don't really like the
> idea of loading snd-virtmidi for that.

Well, many audio/music apps may talk directly sequencer API, too, so
it's not too bad.  Whether loading snd-virmidi is a system setup
question, and it'd be more or less static configuration like snd-seq
core module.

> >> [...]
> >> They all have the problem of context switching between bluez plugin
> >> and alsa midi driver.
> >
> > Why would context switches be a problem?
> 
> It is just too much travelling around. GATT messages are parsed in
> userspace by BlueZ, so it sounds unnecessary to copy buffers to the
> kernel and copy back to user space to the application (ALSA).

Of course it'd be optimal if we can avoid context switching, but
usually its latency isn't too critical for MIDI, even on a much slower
machine in 20 years ago.

So, IMO, we should go for a simpler implementation at first.


thanks,

Takashi