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, 4 insertions, 0 deletions

diff --git a/drivers/dma/Kconfig b/drivers/dma/Kconfig
index 912a51b5cbd3..ddcd9793b25c 100644
--- a/drivers/dma/Kconfig
+++ b/drivers/dma/Kconfig
@@ -17,11 +17,15 @@ if DMADEVICES
 
 comment "DMA Devices"
 
+config ASYNC_TX_DISABLE_CHANNEL_SWITCH
+        bool
+
 config INTEL_IOATDMA
         tristate "Intel I/OAT DMA support"
         depends on PCI && X86
         select DMA_ENGINE
         select DCA
+        select ASYNC_TX_DISABLE_CHANNEL_SWITCH
         help
           Enable support for the Intel(R) I/OAT DMA engine present
           in recent Intel Xeon chipsets.