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>
diff --git a/crypto/async_tx/async_tx.c b/crypto/async_tx/async_tx.c
index 60615fe..f9cdf04 100644
--- a/crypto/async_tx/async_tx.c
+++ b/crypto/async_tx/async_tx.c
@@ -81,6 +81,10 @@
struct dma_device *device = chan->device;
struct dma_async_tx_descriptor *intr_tx = (void *) ~0;
+ #ifdef CONFIG_ASYNC_TX_DISABLE_CHANNEL_SWITCH
+ BUG();
+ #endif
+
/* first check to see if we can still append to depend_tx */
spin_lock_bh(&depend_tx->lock);
if (depend_tx->parent && depend_tx->chan == tx->chan) {