iop_adma: document how to calculate the minimum descriptor pool size

static void iop_chan_start_null_memcpy(struct iop_adma_chan *iop_chan);

static void iop_chan_start_null_xor(struct iop_adma_chan *iop_chan);

/* returns the number of allocated descriptors */

/**

 * iop_adma_alloc_chan_resources -  returns the number of allocated descriptors

 * @chan - allocate descriptor resources for this channel

 * @client - current client requesting the channel be ready for requests

 *

 * Note: We keep the slots for 1 operation on iop_chan->chain at all times.  To

 * avoid deadlock, via async_xor, num_descs_in_pool must at a minimum be

 * greater than 2x the number slots needed to satisfy a device->max_xor

 * request.

 * */

static int iop_adma_alloc_chan_resources(struct dma_chan *chan,

					 struct dma_client *client)

{