linux-kernel - Re: [PATCH 1/5] firewire: fix use of multiple AV/C devices, allow multiple FCP listeners

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Message-ID: <4B361A23.8090009@s5r6.in-berlin.de>
Date:	Sat, 26 Dec 2009 15:13:55 +0100
From:	Stefan Richter <stefanr@...6.in-berlin.de>
To:	Pieter Palmers <pieterp@...w.be>
CC:	linux-kernel@...r.kernel.org,
	Clemens Ladisch <cladisch@...tmail.net>,
	linux1394-devel@...ts.sourceforge.net
Subject: Re: [PATCH 1/5] firewire: fix use of multiple AV/C devices, allow
 multiple FCP listeners

Pieter Palmers wrote:
[lack of system-wide FCP transaction serialization]
> This is not a 'practical' problem because most people (learn to) leave
> enough time between starting jackd and starting the mixer. This makes
> things usable, but I don't really consider this to be a satisfactory
> solution.

Ah, so this is yet another of those software flaws which are not talked
about because people take the workaround as something inevitable.

>>> I think this can only be implemented by tracking which AVC commands have
>>> been sent to what device, regardless of the userspace application (or
>>> kernel driver) that sent them, and disallowing any transactions that
>>> result in scenarios such as the above.

BTW, what about AV/C RESERVE?  According to the spec, it means that a
controller gets exclusive FCP access to a target.  At this point, a
node-wide FCP serializer will have to decide on a policy:  The simpler
policy would be to remain oblivious of RESERVE.  The more difficult to
implement policy would be to keep track of reservations and only allow a
single controller process (or user, or...) to perform subsequent
transactions for the duration of a reservation.  (I for one would keep
it simple.)
-- 
Stefan Richter
-=====-==--= ==-- ==-=-
http://arcgraph.de/sr/
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