dmaengine: Can we schedule new transfer from dma callback routine??

[linux@arm.linux.org.uk (Russell King - ARM Linux)]

On Mon, Apr 11, 2011 at 01:25:04PM +0530, viresh kumar wrote:
> 
> Hello,
> 
> In dw_dmac.c driver, dwc_descriptor_complete() routine, following is
> mentioned before calling callback:
> 
> 	/*
> 	 * The API requires that no submissions are done from a
> 	 * callback, so we don't need to drop the lock here
> 	 */
> 	if (callback)
> 		callback(param);
> 
> Does this hold true for dmaengine??

Not for slave devices - see Dan's reply:

http://lists.arm.linux.org.uk/lurker/message/20101223.005313.a38d7bf0.en.html

As the slave API hasn't been well documented, there's a lot of
inconsistency of behaviour between DMA engine slave implementations.
I'd suggest at least fixing slave DMA engine drivers to ensure that:

(a) the callback is always called in tasklet context
(b) the callback can submit new slave transactions (iow, the spinlock
    which prep_slave_sg takes must not be held during the callback.)

The way that others solve this is to move the completed txd structures
to a local 'completed' list, and then walk this list after the spinlocks
have been dropped.

IOW, something like this:

my_tasklet()
{
	INIT_LIST_HEAD(completed);

	spin_lock_irqsave(my_chan->lock);
	for_each_txd(my_txd, my_chan) {
		if (has_completed(my_txd))
			list_add_tail(my_txd->node, &completed);
	}
	spin_unlock_irqrestore(my_chan->lock);

	list_for_each_entry_safe(my_txd, next, &completed, node) {
		void *callback_param = my_txd->txd.callback_param;
		void (*fn)(void *) = my_txd->txd.callback;

		my_txd_free(my_chan, my_txd);

		fn(callback_param);
	}	
}