16 Aug
2015
16 Aug
'15
9:47 a.m.
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:
--- a/sound/firewire/lib.c +++ b/sound/firewire/lib.c
[...]
+static void midi_port_tasklet(unsigned long data) +{
- struct snd_fw_async_midi_port *port =
(struct snd_fw_async_midi_port *)data;- struct snd_rawmidi_substream *substream = ACCESS_ONCE(port->substream);
- int generation;
- int type;
[...]
- /* Start this transaction. */
- 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:
- "generation" is the IEEE 1394 bus generation, which is a counter that distinguishes periods between bus resets.
- At a bus reset, nodes may assume different node IDs from before the reset. Hence whenever a node is addressed by means of node ID, the generation value must be given too.
- The node_ID:generation tuple is accessed in the Linux firewire stack in a lockless manner¹:
This way highlevel always only uses current IDs in current generations or current IDs in outdated generations (the latter case being extremely rare and doing no harm because the link layer hardware will not emit this sort of requests to the bus), while on the other hand highlevel never uses outdated IDs in current generations (which would cause requests to be misdirected to a wrong node).
- firewire-core's bus reset handler writes them by set node_id; smp_wmb(); set generation
- higher-level code reads them by get generation; smp_rmb(); get node_id
Thanks, that clarifies. But wouldn't it be more helpful to have some macro than open-coding at each place?
Takashi