dmaengine, async_tx: add a "no channel switch" allocator

Channel switching is problematic for some dmaengine drivers as the
architecture precludes separating the ->prep from ->submit.  In these
cases the driver can select ASYNC_TX_DISABLE_CHANNEL_SWITCH to modify
the async_tx allocator to only return channels that support all of the
required asynchronous operations.

For example MD_RAID456=y selects support for asynchronous xor, xor
validate, pq, pq validate, and memcpy.  When
ASYNC_TX_DISABLE_CHANNEL_SWITCH=y any channel with all these
capabilities is marked DMA_ASYNC_TX allowing async_tx_find_channel() to
quickly locate compatible channels with the guarantee that dependency
chains will remain on one channel.  When
ASYNC_TX_DISABLE_CHANNEL_SWITCH=n async_tx_find_channel() may select
channels that lead to operation chains that need to cross channel
boundaries using the async_tx channel switch capability.

Signed-off-by: Dan Williams <dan.j.williams@intel.com>

1 files changed, 9 insertions, 1 deletions

diff --git a/include/linux/dmaengine.h b/include/linux/dmaengine.h
index 4d6c1c9..86853ed 100644
--- a/include/linux/dmaengine.h
+++ b/include/linux/dmaengine.h
@@ -48,6 +48,9 @@ enum dma_status {
 
 /**
  * enum dma_transaction_type - DMA transaction types/indexes
+ *
+ * Note: The DMA_ASYNC_TX capability is not to be set by drivers.  It is
+ * automatically set as dma devices are registered.
  */
 enum dma_transaction_type {
 	DMA_MEMCPY,
@@ -61,6 +64,7 @@ enum dma_transaction_type {
 	DMA_MEMCPY_CRC32C,
 	DMA_INTERRUPT,
 	DMA_PRIVATE,
+	DMA_ASYNC_TX,
 	DMA_SLAVE,
 };
 
@@ -396,7 +400,11 @@ static inline void net_dmaengine_put(void)
 #ifdef CONFIG_ASYNC_TX_DMA
 #define async_dmaengine_get()	dmaengine_get()
 #define async_dmaengine_put()	dmaengine_put()
+#ifdef CONFIG_ASYNC_TX_DISABLE_CHANNEL_SWITCH
+#define async_dma_find_channel(type) dma_find_channel(DMA_ASYNC_TX)
+#else
 #define async_dma_find_channel(type) dma_find_channel(type)
+#endif /* CONFIG_ASYNC_TX_DISABLE_CHANNEL_SWITCH */
 #else
 static inline void async_dmaengine_get(void)
 {
@@ -409,7 +417,7 @@ async_dma_find_channel(enum dma_transaction_type type)
 {
 	return NULL;
 }
-#endif
+#endif /* CONFIG_ASYNC_TX_DMA */
 
 dma_cookie_t dma_async_memcpy_buf_to_buf(struct dma_chan *chan,
 	void *dest, void *src, size_t len);