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/drivers/dma/Kconfig b/drivers/dma/Kconfig
index 912a51b..ddcd979 100644
--- a/drivers/dma/Kconfig
+++ b/drivers/dma/Kconfig
@@ -17,11 +17,15 @@
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.