async_tx: add support for asynchronous GF multiplication

[ Based on an original patch by Yuri Tikhonov ]

This adds support for doing asynchronous GF multiplication by adding
two additional functions to the async_tx API:

async_gen_syndrome() does simultaneous XOR and Galois field
multiplication of sources.

async_syndrome_val() validates the given source buffers against known P
and Q values.

When a request is made to run async_pq against more than the hardware
maximum number of supported sources we need to reuse the previous
generated P and Q values as sources into the next operation. Care must
be taken to remove Q from P' and P from Q'. For example to perform a 5
source pq op with hardware that only supports 4 sources at a time the
following approach is taken:

p, q = PQ(src0, src1, src2, src3, COEF({01}, {02}, {04}, {08}))
p', q' = PQ(p, q, q, src4, COEF({00}, {01}, {00}, {10}))

p' = p + q + q + src4 = p + src4
q' = {00}*p + {01}*q + {00}*q + {10}*src4 = q + {10}*src4

Note: 4 is the minimum acceptable maxpq otherwise we punt to
synchronous-software path.

The DMA_PREP_CONTINUE flag indicates to the driver to reuse p and q as
sources (in the above manner) and fill the remaining slots up to maxpq
with the new sources/coefficients.

Note1: Some devices have native support for P+Q continuation and can skip
this extra work. Devices with this capability can advertise it with
dma_set_maxpq. It is up to each driver how to handle the
DMA_PREP_CONTINUE flag.

Note2: The api supports disabling the generation of P when generating Q,
this is ignored by the synchronous path but is implemented by some dma
devices to save unnecessary writes. In this case the continuation
algorithm is simplified to only reuse Q as a source.

Cc: H. Peter Anvin <[e-mail]>
Cc: David Woodhouse <[e-mail]>
Signed-off-by: Yuri Tikhonov <[e-mail]>
Signed-off-by: Ilya Yanok <[e-mail]>
Reviewed-by: Andre Noll <[e-mail]>
Acked-by: Maciej Sosnowski <[e-mail]>
Signed-off-by: Dan Williams <[e-mail]>

Documentation/crypto/async-tx-api.txt

···
 xor_val - xor a series of source buffers and set a flag if the
 	  result is zero.  The implementation attempts to prevent
 	  writes to memory
+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
 
 3.3 Descriptor management:
 The return value is non-NULL and points to a 'descriptor' when the operation

arch/arm/mach-iop13xx/setup.c

···
 			dma_cap_set(DMA_MEMSET, plat_data->cap_mask);
 			dma_cap_set(DMA_MEMCPY_CRC32C, plat_data->cap_mask);
 			dma_cap_set(DMA_INTERRUPT, plat_data->cap_mask);
-			dma_cap_set(DMA_PQ_XOR, plat_data->cap_mask);
+			dma_cap_set(DMA_PQ, plat_data->cap_mask);
 			dma_cap_set(DMA_PQ_UPDATE, plat_data->cap_mask);
 			dma_cap_set(DMA_PQ_VAL, plat_data->cap_mask);
 			break;

crypto/async_tx/Kconfig

···
 	tristate
 	select ASYNC_CORE
 
+config ASYNC_PQ
+	tristate
+	select ASYNC_CORE
+

crypto/async_tx/Makefile

···
 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

crypto/async_tx/async_pq.c

···
+/*
+ * Copyright(c) 2007 Yuri Tikhonov <[e-mail]>
+ * Copyright(c) 2009 Intel Corporation
+ *
+ * This program is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License as published by the Free
+ * Software Foundation; either version 2 of the License, or (at your option)
+ * any later version.
+ *
+ * This program is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License for
+ * more details.
+ *
+ * You should have received a copy of the GNU General Public License along with
+ * this program; if not, write to the Free Software Foundation, Inc., 59
+ * Temple Place - Suite 330, Boston, MA  02111-1307, USA.
+ *
+ * The full GNU General Public License is included in this distribution in the
+ * file called COPYING.
+ */
+#include <linux/kernel.h>
+#include <linux/interrupt.h>
+#include <linux/dma-mapping.h>
+#include <linux/raid/pq.h>
+#include <linux/async_tx.h>
+
+/**
+ * scribble - space to hold throwaway P buffer for synchronous gen_syndrome
+ */
+static struct page *scribble;
+
+static bool is_raid6_zero_block(struct page *p)
+{
+	return p == (void *) raid6_empty_zero_page;
+}
+
+/* the struct page *blocks[] parameter passed to async_gen_syndrome()
+ * and async_syndrome_val() contains the 'P' destination address at
+ * blocks[disks-2] and the 'Q' destination address at blocks[disks-1]
+ *
+ * note: these are macros as they are used as lvalues
+ */
+#define P(b, d) (b[d-2])
+#define Q(b, d) (b[d-1])
+
+/**
+ * do_async_gen_syndrome - asynchronously calculate P and/or Q
+ */
+static __async_inline struct dma_async_tx_descriptor *
+do_async_gen_syndrome(struct dma_chan *chan, struct page **blocks,
+		      const unsigned char *scfs, unsigned int offset, int disks,
+		      size_t len, dma_addr_t *dma_src,
+		      struct async_submit_ctl *submit)
+{
+	struct dma_async_tx_descriptor *tx = NULL;
+	struct dma_device *dma = chan->device;
+	enum dma_ctrl_flags dma_flags = 0;
+	enum async_tx_flags flags_orig = submit->flags;
+	dma_async_tx_callback cb_fn_orig = submit->cb_fn;
+	dma_async_tx_callback cb_param_orig = submit->cb_param;
+	int src_cnt = disks - 2;
+	unsigned char coefs[src_cnt];
+	unsigned short pq_src_cnt;
+	dma_addr_t dma_dest[2];
+	int src_off = 0;
+	int idx;
+	int i;
+
+	/* DMAs use destinations as sources, so use BIDIRECTIONAL mapping */
+	if (P(blocks, disks))
+		dma_dest[0] = dma_map_page(dma->dev, P(blocks, disks), offset,
+					   len, DMA_BIDIRECTIONAL);
+	else
+		dma_flags |= DMA_PREP_PQ_DISABLE_P;
+	if (Q(blocks, disks))
+		dma_dest[1] = dma_map_page(dma->dev, Q(blocks, disks), offset,
+					   len, DMA_BIDIRECTIONAL);
+	else
+		dma_flags |= DMA_PREP_PQ_DISABLE_Q;
+
+	/* convert source addresses being careful to collapse 'empty'
+	 * sources and update the coefficients accordingly
+	 */
+	for (i = 0, idx = 0; i < src_cnt; i++) {
+		if (is_raid6_zero_block(blocks[i]))
+			continue;
+		dma_src[idx] = dma_map_page(dma->dev, blocks[i], offset, len,
+					    DMA_TO_DEVICE);
+		coefs[idx] = scfs[i];
+		idx++;
+	}
+	src_cnt = idx;
+
+	while (src_cnt > 0) {
+		submit->flags = flags_orig;
+		pq_src_cnt = min(src_cnt, dma_maxpq(dma, dma_flags));
+		/* if we are submitting additional pqs, leave the chain open,
+		 * clear the callback parameters, and leave the destination
+		 * buffers mapped
+		 */
+		if (src_cnt > pq_src_cnt) {
+			submit->flags &= ~ASYNC_TX_ACK;
+			dma_flags |= DMA_COMPL_SKIP_DEST_UNMAP;
+			submit->cb_fn = NULL;
+			submit->cb_param = NULL;
+		} else {
+			dma_flags &= ~DMA_COMPL_SKIP_DEST_UNMAP;
+			submit->cb_fn = cb_fn_orig;
+			submit->cb_param = cb_param_orig;
+			if (cb_fn_orig)
+				dma_flags |= DMA_PREP_INTERRUPT;
+		}
+
+		/* Since we have clobbered the src_list we are committed
+		 * to doing this asynchronously.  Drivers force forward
+		 * progress in case they can not provide a descriptor
+		 */
+		for (;;) {
+			tx = dma->device_prep_dma_pq(chan, dma_dest,
+						     &dma_src[src_off],
+						     pq_src_cnt,
+						     &coefs[src_off], len,
+						     dma_flags);
+			if (likely(tx))
+				break;
+			async_tx_quiesce(&submit->depend_tx);
+			dma_async_issue_pending(chan);
+		}
+
+		async_tx_submit(chan, tx, submit);
+		submit->depend_tx = tx;
+
+		/* drop completed sources */
+		src_cnt -= pq_src_cnt;
+		src_off += pq_src_cnt;
+
+		dma_flags |= DMA_PREP_CONTINUE;
+	}
+
+	return tx;
+}
+
+/**
+ * do_sync_gen_syndrome - synchronously calculate a raid6 syndrome
+ */
+static void
+do_sync_gen_syndrome(struct page **blocks, unsigned int offset, int disks,
+		     size_t len, struct async_submit_ctl *submit)
+{
+	void **srcs;
+	int i;
+
+	if (submit->scribble)
+		srcs = submit->scribble;
+	else
+		srcs = (void **) blocks;
+
+	for (i = 0; i < disks; i++) {
+		if (is_raid6_zero_block(blocks[i])) {
+			BUG_ON(i > disks - 3); /* P or Q can't be zero */
+			srcs[i] = blocks[i];
+		} else
+			srcs[i] = page_address(blocks[i]) + offset;
+	}
+	raid6_call.gen_syndrome(disks, len, srcs);
+	async_tx_sync_epilog(submit);
+}
+
+/**
+ * async_gen_syndrome - asynchronously calculate a raid6 syndrome
+ * @blocks: source blocks from idx 0..disks-3, P @ disks-2 and Q @ disks-1
+ * @offset: common offset into each block (src and dest) to start transaction
+ * @disks: number of blocks (including missing P or Q, see below)
+ * @len: length of operation in bytes
+ * @submit: submission/completion modifiers
+ *
+ * General note: This routine assumes a field of GF(2^8) with a
+ * primitive polynomial of 0x11d and a generator of {02}.
+ *
+ * 'disks' note: callers can optionally omit either P or Q (but not
+ * both) from the calculation by setting blocks[disks-2] or
+ * blocks[disks-1] to NULL.  When P or Q is omitted 'len' must be <=
+ * PAGE_SIZE as a temporary buffer of this size is used in the
+ * synchronous path.  'disks' always accounts for both destination
+ * buffers.
+ *
+ * 'blocks' note: if submit->scribble is NULL then the contents of
+ * 'blocks' may be overridden
+ */
+struct dma_async_tx_descriptor *
+async_gen_syndrome(struct page **blocks, unsigned int offset, int disks,
+		   size_t len, struct async_submit_ctl *submit)
+{
+	int src_cnt = disks - 2;
+	struct dma_chan *chan = async_tx_find_channel(submit, DMA_PQ,
+						      &P(blocks, disks), 2,
+						      blocks, src_cnt, len);
+	struct dma_device *device = chan ? chan->device : NULL;
+	dma_addr_t *dma_src = NULL;
+
+	BUG_ON(disks > 255 || !(P(blocks, disks) || Q(blocks, disks)));
+
+	if (submit->scribble)
+		dma_src = submit->scribble;
+	else if (sizeof(dma_addr_t) <= sizeof(struct page *))
+		dma_src = (dma_addr_t *) blocks;
+
+	if (dma_src && device &&
+	    (src_cnt <= dma_maxpq(device, 0) ||
+	     dma_maxpq(device, DMA_PREP_CONTINUE) > 0)) {
+		/* run the p+q asynchronously */
+		pr_debug("%s: (async) disks: %d len: %zu\n",
+			 __func__, disks, len);
+		return do_async_gen_syndrome(chan, blocks, raid6_gfexp, offset,
+					     disks, len, dma_src, submit);
+	}
+
+	/* run the pq synchronously */
+	pr_debug("%s: (sync) disks: %d len: %zu\n", __func__, disks, len);
+
+	/* wait for any prerequisite operations */
+	async_tx_quiesce(&submit->depend_tx);
+
+	if (!P(blocks, disks)) {
+		P(blocks, disks) = scribble;
+		BUG_ON(len + offset > PAGE_SIZE);
+	}
+	if (!Q(blocks, disks)) {
+		Q(blocks, disks) = scribble;
+		BUG_ON(len + offset > PAGE_SIZE);
+	}
+	do_sync_gen_syndrome(blocks, offset, disks, len, submit);
+
+	return NULL;
+}
+EXPORT_SYMBOL_GPL(async_gen_syndrome);
+
+/**
+ * async_syndrome_val - asynchronously validate a raid6 syndrome
+ * @blocks: source blocks from idx 0..disks-3, P @ disks-2 and Q @ disks-1
+ * @offset: common offset into each block (src and dest) to start transaction
+ * @disks: number of blocks (including missing P or Q, see below)
+ * @len: length of operation in bytes
+ * @pqres: on val failure SUM_CHECK_P_RESULT and/or SUM_CHECK_Q_RESULT are set
+ * @spare: temporary result buffer for the synchronous case
+ * @submit: submission / completion modifiers
+ *
+ * The same notes from async_gen_syndrome apply to the 'blocks',
+ * and 'disks' parameters of this routine.  The synchronous path
+ * requires a temporary result buffer and submit->scribble to be
+ * specified.
+ */
+struct dma_async_tx_descriptor *
+async_syndrome_val(struct page **blocks, unsigned int offset, int disks,
+		   size_t len, enum sum_check_flags *pqres, struct page *spare,
+		   struct async_submit_ctl *submit)
+{
+	struct dma_chan *chan = async_tx_find_channel(submit, DMA_PQ_VAL,
+						      NULL, 0,  blocks, disks,
+						      len);
+	struct dma_device *device = chan ? chan->device : NULL;
+	struct dma_async_tx_descriptor *tx;
+	enum dma_ctrl_flags dma_flags = submit->cb_fn ? DMA_PREP_INTERRUPT : 0;
+	dma_addr_t *dma_src = NULL;
+
+	BUG_ON(disks < 4);
+
+	if (submit->scribble)
+		dma_src = submit->scribble;
+	else if (sizeof(dma_addr_t) <= sizeof(struct page *))
+		dma_src = (dma_addr_t *) blocks;
+
+	if (dma_src && device && disks <= dma_maxpq(device, 0)) {
+		struct device *dev = device->dev;
+		dma_addr_t *pq = &dma_src[disks-2];
+		int i;
+
+		pr_debug("%s: (async) disks: %d len: %zu\n",
+			 __func__, disks, len);
+		if (!P(blocks, disks))
+			dma_flags |= DMA_PREP_PQ_DISABLE_P;
+		if (!Q(blocks, disks))
+			dma_flags |= DMA_PREP_PQ_DISABLE_Q;
+		for (i = 0; i < disks; i++)
+			if (likely(blocks[i])) {
+				BUG_ON(is_raid6_zero_block(blocks[i]));
+				dma_src[i] = dma_map_page(dev, blocks[i],
+							  offset, len,
+							  DMA_TO_DEVICE);
+			}
+
+		for (;;) {
+			tx = device->device_prep_dma_pq_val(chan, pq, dma_src,
+							    disks - 2,
+							    raid6_gfexp,
+							    len, pqres,
+							    dma_flags);
+			if (likely(tx))
+				break;
+			async_tx_quiesce(&submit->depend_tx);
+			dma_async_issue_pending(chan);
+		}
+		async_tx_submit(chan, tx, submit);
+
+		return tx;
+	} else {
+		struct page *p_src = P(blocks, disks);
+		struct page *q_src = Q(blocks, disks);
+		enum async_tx_flags flags_orig = submit->flags;
+		dma_async_tx_callback cb_fn_orig = submit->cb_fn;
+		void *scribble = submit->scribble;
+		void *cb_param_orig = submit->cb_param;
+		void *p, *q, *s;
+
+		pr_debug("%s: (sync) disks: %d len: %zu\n",
+			 __func__, disks, len);
+
+		/* caller must provide a temporary result buffer and
+		 * allow the input parameters to be preserved
+		 */
+		BUG_ON(!spare || !scribble);
+
+		/* wait for any prerequisite operations */
+		async_tx_quiesce(&submit->depend_tx);
+
+		/* recompute p and/or q into the temporary buffer and then
+		 * check to see the result matches the current value
+		 */
+		tx = NULL;
+		*pqres = 0;
+		if (p_src) {
+			init_async_submit(submit, ASYNC_TX_XOR_ZERO_DST, NULL,
+					  NULL, NULL, scribble);
+			tx = async_xor(spare, blocks, offset, disks-2, len, submit);
+			async_tx_quiesce(&tx);
+			p = page_address(p_src) + offset;
+			s = page_address(spare) + offset;
+			*pqres |= !!memcmp(p, s, len) << SUM_CHECK_P;
+		}
+
+		if (q_src) {
+			P(blocks, disks) = NULL;
+			Q(blocks, disks) = spare;
+			init_async_submit(submit, 0, NULL, NULL, NULL, scribble);
+			tx = async_gen_syndrome(blocks, offset, disks, len, submit);
+			async_tx_quiesce(&tx);
+			q = page_address(q_src) + offset;
+			s = page_address(spare) + offset;
+			*pqres |= !!memcmp(q, s, len) << SUM_CHECK_Q;
+		}
+
+		/* restore P, Q and submit */
+		P(blocks, disks) = p_src;
+		Q(blocks, disks) = q_src;
+
+		submit->cb_fn = cb_fn_orig;
+		submit->cb_param = cb_param_orig;
+		submit->flags = flags_orig;
+		async_tx_sync_epilog(submit);
+
+		return NULL;
+	}
+}
+EXPORT_SYMBOL_GPL(async_syndrome_val);
+
+static int __init async_pq_init(void)
+{
+	scribble = alloc_page(GFP_KERNEL);
+
+	if (scribble)
+		return 0;
+
+	pr_err("%s: failed to allocate required spare page\n", __func__);
+
+	return -ENOMEM;
+}
+
+static void __exit async_pq_exit(void)
+{
+	put_page(scribble);
+}
+
+module_init(async_pq_init);
+module_exit(async_pq_exit);
+
+MODULE_DESCRIPTION("asynchronous raid6 syndrome generation/validation");
+MODULE_LICENSE("GPL");

crypto/async_tx/async_xor.c

···
 	while (src_cnt) {
 		submit->flags = flags_orig;
 		dma_flags = 0;
-		xor_src_cnt = min(src_cnt, dma->max_xor);
+		xor_src_cnt = min(src_cnt, (int)dma->max_xor);
 		/* if we are submitting additional xors, leave the chain open,
 		 * clear the callback parameters, and leave the destination
 		 * buffer mapped

drivers/dma/dmaengine.c

···
 		!device->device_prep_dma_xor);
 	BUG_ON(dma_has_cap(DMA_XOR_VAL, device->cap_mask) &&
 		!device->device_prep_dma_xor_val);
+	BUG_ON(dma_has_cap(DMA_PQ, device->cap_mask) &&
+		!device->device_prep_dma_pq);
+	BUG_ON(dma_has_cap(DMA_PQ_VAL, device->cap_mask) &&
+		!device->device_prep_dma_pq_val);
 	BUG_ON(dma_has_cap(DMA_MEMSET, device->cap_mask) &&
 		!device->device_prep_dma_memset);
 	BUG_ON(dma_has_cap(DMA_INTERRUPT, device->cap_mask) &&

drivers/dma/iop-adma.c

···
 
 	dev_printk(KERN_INFO, &pdev->dev, "Intel(R) IOP: "
 	  "( %s%s%s%s%s%s%s%s%s%s)\n",
-	  dma_has_cap(DMA_PQ_XOR, dma_dev->cap_mask) ? "pq_xor " : "",
+	  dma_has_cap(DMA_PQ, dma_dev->cap_mask) ? "pq " : "",
 	  dma_has_cap(DMA_PQ_UPDATE, dma_dev->cap_mask) ? "pq_update " : "",
 	  dma_has_cap(DMA_PQ_VAL, dma_dev->cap_mask) ? "pq_val " : "",
 	  dma_has_cap(DMA_XOR, dma_dev->cap_mask) ? "xor " : "",

include/linux/async_tx.h

···
 
 struct dma_async_tx_descriptor *async_trigger_callback(struct async_submit_ctl *submit);
 
+struct dma_async_tx_descriptor *
+async_gen_syndrome(struct page **blocks, unsigned int offset, int src_cnt,
+		   size_t len, struct async_submit_ctl *submit);
+
+struct dma_async_tx_descriptor *
+async_syndrome_val(struct page **blocks, unsigned int offset, int src_cnt,
+		   size_t len, enum sum_check_flags *pqres, struct page *spare,
+		   struct async_submit_ctl *submit);
+
 void async_tx_quiesce(struct dma_async_tx_descriptor **tx);
 #endif /* _ASYNC_TX_H_ */

include/linux/dmaengine.h

···
 enum dma_transaction_type {
 	DMA_MEMCPY,
 	DMA_XOR,
-	DMA_PQ_XOR,
+	DMA_PQ,
 	DMA_DUAL_XOR,
 	DMA_PQ_UPDATE,
 	DMA_XOR_VAL,
···
 
 /**
  * enum dma_ctrl_flags - DMA flags to augment operation preparation,
- * 	control completion, and communicate status.
+ *  control completion, and communicate status.
  * @DMA_PREP_INTERRUPT - trigger an interrupt (callback) upon completion of
- * 	this transaction
+ *  this transaction
  * @DMA_CTRL_ACK - the descriptor cannot be reused until the client
- * 	acknowledges receipt, i.e. has has a chance to establish any
- * 	dependency chains
+ *  acknowledges receipt, i.e. has has a chance to establish any dependency
+ *  chains
  * @DMA_COMPL_SKIP_SRC_UNMAP - set to disable dma-unmapping the source buffer(s)
  * @DMA_COMPL_SKIP_DEST_UNMAP - set to disable dma-unmapping the destination(s)
+ * @DMA_PREP_PQ_DISABLE_P - prevent generation of P while generating Q
+ * @DMA_PREP_PQ_DISABLE_Q - prevent generation of Q while generating P
+ * @DMA_PREP_CONTINUE - indicate to a driver that it is reusing buffers as
+ *  sources that were the result of a previous operation, in the case of a PQ
+ *  operation it continues the calculation with new sources
  */
 enum dma_ctrl_flags {
 	DMA_PREP_INTERRUPT = (1 << 0),
 	DMA_CTRL_ACK = (1 << 1),
 	DMA_COMPL_SKIP_SRC_UNMAP = (1 << 2),
 	DMA_COMPL_SKIP_DEST_UNMAP = (1 << 3),
+	DMA_PREP_PQ_DISABLE_P = (1 << 4),
+	DMA_PREP_PQ_DISABLE_Q = (1 << 5),
+	DMA_PREP_CONTINUE = (1 << 6),
 };
 
 /**
···
  * @global_node: list_head for global dma_device_list
  * @cap_mask: one or more dma_capability flags
  * @max_xor: maximum number of xor sources, 0 if no capability
+ * @max_pq: maximum number of PQ sources and PQ-continue capability
  * @dev_id: unique device ID
  * @dev: struct device reference for dma mapping api
  * @device_alloc_chan_resources: allocate resources and return the
···
  * @device_prep_dma_memcpy: prepares a memcpy operation
  * @device_prep_dma_xor: prepares a xor operation
  * @device_prep_dma_xor_val: prepares a xor validation operation
+ * @device_prep_dma_pq: prepares a pq operation
+ * @device_prep_dma_pq_val: prepares a pqzero_sum operation
  * @device_prep_dma_memset: prepares a memset operation
  * @device_prep_dma_interrupt: prepares an end of chain interrupt operation
  * @device_prep_slave_sg: prepares a slave dma operation
···
 	struct list_head channels;
 	struct list_head global_node;
 	dma_cap_mask_t  cap_mask;
-	int max_xor;
+	unsigned short max_xor;
+	unsigned short max_pq;
+	#define DMA_HAS_PQ_CONTINUE (1 << 15)
 
 	int dev_id;
 	struct device *dev;
···
 	struct dma_async_tx_descriptor *(*device_prep_dma_xor_val)(
 		struct dma_chan *chan, dma_addr_t *src,	unsigned int src_cnt,
 		size_t len, enum sum_check_flags *result, unsigned long flags);
+	struct dma_async_tx_descriptor *(*device_prep_dma_pq)(
+		struct dma_chan *chan, dma_addr_t *dst, dma_addr_t *src,
+		unsigned int src_cnt, const unsigned char *scf,
+		size_t len, unsigned long flags);
+	struct dma_async_tx_descriptor *(*device_prep_dma_pq_val)(
+		struct dma_chan *chan, dma_addr_t *pq, dma_addr_t *src,
+		unsigned int src_cnt, const unsigned char *scf, size_t len,
+		enum sum_check_flags *pqres, unsigned long flags);
 	struct dma_async_tx_descriptor *(*device_prep_dma_memset)(
 		struct dma_chan *chan, dma_addr_t dest, int value, size_t len,
 		unsigned long flags);
···
 			dma_cookie_t *used);
 	void (*device_issue_pending)(struct dma_chan *chan);
 };
+
+static inline void
+dma_set_maxpq(struct dma_device *dma, int maxpq, int has_pq_continue)
+{
+	dma->max_pq = maxpq;
+	if (has_pq_continue)
+		dma->max_pq |= DMA_HAS_PQ_CONTINUE;
+}
+
+static inline bool dmaf_continue(enum dma_ctrl_flags flags)
+{
+	return (flags & DMA_PREP_CONTINUE) == DMA_PREP_CONTINUE;
+}
+
+static inline bool dmaf_p_disabled_continue(enum dma_ctrl_flags flags)
+{
+	enum dma_ctrl_flags mask = DMA_PREP_CONTINUE | DMA_PREP_PQ_DISABLE_P;
+
+	return (flags & mask) == mask;
+}
+
+static inline bool dma_dev_has_pq_continue(struct dma_device *dma)
+{
+	return (dma->max_pq & DMA_HAS_PQ_CONTINUE) == DMA_HAS_PQ_CONTINUE;
+}
+
+static unsigned short dma_dev_to_maxpq(struct dma_device *dma)
+{
+	return dma->max_pq & ~DMA_HAS_PQ_CONTINUE;
+}
+
+/* dma_maxpq - reduce maxpq in the face of continued operations
+ * @dma - dma device with PQ capability
+ * @flags - to check if DMA_PREP_CONTINUE and DMA_PREP_PQ_DISABLE_P are set
+ *
+ * When an engine does not support native continuation we need 3 extra
+ * source slots to reuse P and Q with the following coefficients:
+ * 1/ {00} * P : remove P from Q', but use it as a source for P'
+ * 2/ {01} * Q : use Q to continue Q' calculation
+ * 3/ {00} * Q : subtract Q from P' to cancel (2)
+ *
+ * In the case where P is disabled we only need 1 extra source:
+ * 1/ {01} * Q : use Q to continue Q' calculation
+ */
+static inline int dma_maxpq(struct dma_device *dma, enum dma_ctrl_flags flags)
+{
+	if (dma_dev_has_pq_continue(dma) || !dmaf_continue(flags))
+		return dma_dev_to_maxpq(dma);
+	else if (dmaf_p_disabled_continue(flags))
+		return dma_dev_to_maxpq(dma) - 1;
+	else if (dmaf_continue(flags))
+		return dma_dev_to_maxpq(dma) - 3;
+	BUG();
+}
 
 /* --- public DMA engine API --- */