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Darwin 0.2 Driver Kit

descriptorList_t	*head, *tail;	// software's list of descriptors

/*
 * Returns CHAN_RUNNING/ CHAN_NOT_RUNNING.  CHAN_NOT_RUNNING implies that
 * a 'load descriptor' operation is required to start the channel.
 */
emptyIndication_t enqueueDescriptors(descriptorList_t *descFrame)
{
	if (channel's DNLE == 0) {
		/*
		 * hw out of descriptors, channel MUST be stopped.
		 * Set-up hardware to start with this descriptor if
		 * current list is empty, else enqueue on end of 
		 * (non-running) list. 
		 */
		if (tail == NULL)
			head = descFrame;
		else
			tail->next = descFrame;
		tail = tailOf(descFrame);
		hw_current_desc = descFrame;
		hw_count = -1;
		return CHAN_NOT_RUNNING;
	} else {
		/*
		 * Channel has descriptors to process.
		 * It may, or may not be currently running.
		 *
		 * First, just do what's necessary to enqueue descriptors
		 * in memory.
		 *
		 * Software queue CAN'T be empty....
		 */
		tail->next = descFrame;
		tail = tailOf(descFrame);
		
		/*
		 * Now, need to check to see if the memory enqueue "made it".
		 *
		 * The first test is to check the hw_next_desc.
		 *
		 * If the channel is running and hw_next_desc is non-null,
		 * then the update must have been in time (channel is running
		 * with more descriptors ready).
		 *
		 * Of course, if the channel is not running, there's no race
		 * (we wouldn't be here...??)
		 */
		if (hw_next_desc != NULL)
			return CHAN_RUNNING;
		/*
		 * At this point, the channel MUST have been running since
		 * the last time this routine was called because that's the 
		 * only way that hw_next_desc can become null.
		 *
		 * Race to update hw_next_desc before the transfer engine
		 * needs to advance.
		 */
		hw_next_desc = descFrame;
		
		/*
		 * Have to do an implementation-dependent test for desc list
		 * not empty - updates to TE registers are atomic, but the
		 * update of the channel status register is not. Current
		 * desc_list_not_empty() test is to read channel's DNLE bit
		 * 'n' times here; any read of 0 is a failure (i.e., it's 
		 * empty).
		 */
		if (desc_list_not_empty() == 1)
			return CHAN_RUNNING;
			
		/*
		 * Lost the race.  The channel MUST be stopped.
		 *
		 * Set-up the channel so that a driver ENABLE will restart
		 * it with descFrame.
		 */
		hw_current_desc = descFrame;
		hw_count = -1;
		return CHAN_NOT_RUNNING;
	}
}

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