dmaengine: kill tx_list

The tx_list attribute of struct dma_async_tx_descriptor is common to
most, but not all dma driver implementations.  None of the upper level
code (dmaengine/async_tx) uses it, so allow drivers to implement it
locally if they need it.  This saves sizeof(struct list_head) bytes for
drivers that do not manage descriptors with a linked list (e.g.: ioatdma
v2,3).
Signed-off-by: Dan Williams <dan.j.williams@intel.com>

drivers/dma/dmaengine.c

@@ -933,7 +933,6 @@ void dma_async_tx_descriptor_init(struct dma_async_tx_descriptor *tx,
 {
 	tx->chan = chan;
 	spin_lock_init(&tx->lock);
-	INIT_LIST_HEAD(&tx->tx_list);
 }
 EXPORT_SYMBOL(dma_async_tx_descriptor_init);
 

include/linux/dmaengine.h

@@ -180,8 +180,6 @@ typedef void (*dma_async_tx_callback)(void *dma_async_param);
  * @flags: flags to augment operation preparation, control completion, and
  * 	communicate status
  * @phys: physical address of the descriptor
- * @tx_list: driver common field for operations that require multiple
- *	descriptors
  * @chan: target channel for this operation
  * @tx_submit: set the prepared descriptor(s) to be executed by the engine
  * @callback: routine to call after this operation is complete
@@ -195,7 +193,6 @@ struct dma_async_tx_descriptor {
 	dma_cookie_t cookie;
 	enum dma_ctrl_flags flags; /* not a 'long' to pack with cookie */
 	dma_addr_t phys;
-	struct list_head tx_list;
 	struct dma_chan *chan;
 	dma_cookie_t (*tx_submit)(struct dma_async_tx_descriptor *tx);
 	dma_async_tx_callback callback;