Re: [PATCH 03/25] ALSA: firewire-lib: add helper functions for asynchronous MIDI port

[Stefan Richter <stefanr@s5r6.in-berlin.de>]

On Aug 16 Takashi Iwai wrote:
> On Sat, 15 Aug 2015 12:15:54 +0200,
> Stefan Richter wrote:
> > 
> > On Aug 13 Takashi Sakamoto wrote:
> > > On Aug 13 2015 15:31, Takashi Iwai wrote:
> > > > On Thu, 13 Aug 2015 02:19:59 +0200,
> > > > Takashi Sakamoto wrote:
> > > >> +	generation = port->parent->generation;
> > > >> +	smp_rmb(); /* node_id vs. generation */
> > > >
> > > > Why this barrier is needed?
> > > 
> > > Oops. I forgot to refine it after copying from the other place. Exactly, 
> > > it's not need here.
> > > 
> > > >> +	fw_send_request(port->parent->card, &port->transaction, type,
> > > >> +			port->parent->node_id, generation,
> > > >> +			port->parent->max_speed, port->addr,
> > > >> +			port->buf, port->len, async_midi_port_callback,
> > > >> +			port);
> > > >> +}
> > 
> > The barrier is normally needed because:
[there is a dependency between node_ID and generation and]
> >   - The node_ID:generation tuple is accessed in the Linux firewire stack in
> >     a lockless manner:
[...]
> Thanks, that clarifies.  But wouldn't it be more helpful to have some
> macro than open-coding at each place?

I agree that a cooked-up accessor via <linux/firewire.h> would be good,
but I have no immediate idea how that accessor should look like.  Looking
at the various uses of fw_send_request
[http://lxr.free-electrons.com/ident?i=fw_send_request],
it's clear that this API between protocol drivers and firewire-core is
quite low-level and flexible by nature.  In particular, the different users
of the API take node_id:generation from different sources.¹  

As mentioned, another way would be to fuse node_id:generation into a single
atomic datum throughout all firewire drivers.  I.e. instead of ensuring the
proper access order everytime, just access both data simultaneously
everytime.  Not having tried this yet, I am not sure which way would be
more readable.
------------
 ¹) Those differences between fw_send_request users are typically due to
    bus reset handling in highlevel drivers on top of firewire-core's
    bus reset handling.
    [firewire-core::bus reset handler]
      - analyze bus topology
      - write fw_device instances' node_id:generation tuple
      - notify protocol drivers
    [protocol_driver::bus reset handler]
      - perform protocol-specific reconnection procedure
      - write protocol_driver_device instance's node_id:generation tuple
    [protocol_driver::outbound IO]
      - read protocol_driver_device instance's node_id:generation tuple
      - send request
    Here the request is not only guaranteed to hit the topologically
    correct target, it is also guaranteed to hit only while in a
    protocol-specific connected state.  (Requests with a stale generation
    won't go out to the bus.)
-- 
Stefan Richter
-=====-===== =--- =----
http://arcgraph.de/sr/
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