[alsa-devel] [PATCH 2/4] ALSA: dice: postpone card registration

[Stefan Richter]

On Dec 24 Takashi Sakamoto wrote:
> --- a/sound/firewire/dice/dice.c
> +++ b/sound/firewire/dice/dice.c
[...]
>  static void dice_bus_reset(struct fw_unit *unit)
>  {
>  	struct snd_dice *dice = dev_get_drvdata(&unit->device);
>  
> +	/* Postpone a workqueue for deferred registration. */
> +	if (!dice->registered) {
> +		schedule_registration(dice);
> +		return;
> +	}
> +
>  	/* The handler address register becomes initialized. */
>  	snd_dice_transaction_reinit(dice);
>  

In previous versions of the patch, you used
	schedule_delayed_work(&dice->dwork, delay);
in dice_probe() and
	mod_delayed_work(dice->dwork.wq, &dice->dwork, delay);
in dice_bus_reset().

Now you are using schedule_delayed_work() in both.  This means that
dice_bus_reset() is now unable to further defer the work.  Is this
intentional?

By the way, in drivers/firewire/core-cdev.c, I used a somewhat different
workqueue scheduling scheme.  Problem:
  - The first 1000 ms after a bus reset are to be used for re-allocations
    of previously allocated isochronous resources.  Attempts for new iso
    resource allocations shall be deferred until after 1000 ms after the
    latest bus reset.
My solution:
  - The work which is to perform allocations/ reallocations/ deallocations
    is scheduled immediately.  But the worker function (iso_resource_work)
    reschedules itself if it notices that (1.) its job is to allocate and
    (2.) the last bus reset was less than 1 s ago.

I used the same principle in drivers/firewire/core-card.c.  Problem:
  - If system software wants to reset the bus, it shall wait at least
    2000 ms until after the last bus reset.  The reason is to allow for
    various bus reset handling protocols to be performed first (e.g.
    isochronous resource allocations, re-logins and the likes).
My solution:
  - br_work() reschedules itself if it detects that the last bus reset was
    less than 2 s ago.

Admittedly there is a small remaining window after the worker looked at
card->reset_jiffies and before it performs its real work, and another bus
rest could happen in this window.  I suppose if I wanted to close even
this window, I would have to couple card->reset_jiffies with a
bus generation counter and then make the remaining work depended on this
bus generation.

Back to your patch:  I am not sure how strictly you want to guarantee the
delay between last reset and do_registration()'s execution.  Maybe it
would be beneficial to put the check for card->reset_jiffies and self-
rescheduling into do_registration(), similar to the two examples from
firewire-core, or maybe that's not really necessary for your purposes.
-- 
Stefan Richter
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http://arcgraph.de/sr/