async_tx: add support for asynchronous RAID6 recovery operations

 async_raid6_2data_recov() recovers two data disk failures

 async_raid6_datap_recov() recovers a data disk and the P disk

These routines are a port of the synchronous versions found in
drivers/md/raid6recov.c.  The primary difference is breaking out the xor
operations into separate calls to async_xor.  Two helper routines are
introduced to perform scalar multiplication where needed.
async_sum_product() multiplies two sources by scalar coefficients and
then sums (xor) the result.  async_mult() simply multiplies a single
source by a scalar.

This implemention also includes, in contrast to the original
synchronous-only code, special case handling for the 4-disk and 5-disk
array cases.  In these situations the default N-disk algorithm will
present 0-source or 1-source operations to dma devices.  To cover for
dma devices where the minimum source count is 2 we implement 4-disk and
5-disk handling in the recovery code.

[ Impact: asynchronous raid6 recovery routines for 2data and datap cases ]

Cc: Yuri Tikhonov <yur@emcraft.com>
Cc: Ilya Yanok <yanok@emcraft.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: David Woodhouse <David.Woodhouse@intel.com>
Reviewed-by: Andre Noll <maan@systemlinux.org>
Acked-by: Maciej Sosnowski <maciej.sosnowski@intel.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>

Author: djbw

Documentation/crypto/async-tx-api.txt

@@ -67,6 +67,10 @@ xor_val - xor a series of source buffers and set a flag if the
 pq	- generate the p+q (raid6 syndrome) from a series of source buffers
 pq_val  - validate that a p and or q buffer are in sync with a given series of
 	  sources
+datap	- (raid6_datap_recov) recover a raid6 data block and the p block
+	  from the given sources
+2data	- (raid6_2data_recov) recover 2 raid6 data blocks from the given
+	  sources
 
 3.3 Descriptor management:
 The return value is non-NULL and points to a 'descriptor' when the operation

crypto/async_tx/Kconfig

@@ -18,3 +18,8 @@ config ASYNC_PQ
 	tristate
 	select ASYNC_CORE
 
+config ASYNC_RAID6_RECOV
+	tristate
+	select ASYNC_CORE
+	select ASYNC_PQ
+

crypto/async_tx/Makefile

@@ -3,3 +3,4 @@ obj-$(CONFIG_ASYNC_MEMCPY) += async_memcpy.o
 obj-$(CONFIG_ASYNC_MEMSET) += async_memset.o
 obj-$(CONFIG_ASYNC_XOR) += async_xor.o
 obj-$(CONFIG_ASYNC_PQ) += async_pq.o
+obj-$(CONFIG_ASYNC_RAID6_RECOV) += async_raid6_recov.o