arch/tile: common DMA code for the GXIO IORPC subsystem

The dma_queue support is used by both the mPipe (networking)
and Trio (PCI) hardware shims on tilegx.  This common code is
selected when either of those drivers is built.

Signed-off-by: Chris Metcalf <cmetcalf@tilera.com>

arch/tile/include/gxio/dma_queue.h

@@ -0,0 +1,161 @@
+/*
+ * Copyright 2012 Tilera Corporation. All Rights Reserved.
+ *
+ *   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, version 2.
+ *
+ *   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, GOOD TITLE or
+ *   NON INFRINGEMENT.  See the GNU General Public License for
+ *   more details.
+ */
+
+#ifndef _GXIO_DMA_QUEUE_H_
+#define _GXIO_DMA_QUEUE_H_
+
+/*
+ * DMA queue management APIs shared between TRIO and mPIPE.
+ */
+
+#include "common.h"
+
+/* The credit counter lives in the high 32 bits. */
+#define DMA_QUEUE_CREDIT_SHIFT 32
+
+/*
+ * State object that tracks a DMA queue's head and tail indices, as
+ * well as the number of commands posted and completed.  The
+ * structure is accessed via a thread-safe, lock-free algorithm.
+ */
+typedef struct {
+	/*
+	 * Address of a MPIPE_EDMA_POST_REGION_VAL_t,
+	 * TRIO_PUSH_DMA_REGION_VAL_t, or TRIO_PULL_DMA_REGION_VAL_t
+	 * register.  These register have identical encodings and provide
+	 * information about how many commands have been processed.
+	 */
+	void *post_region_addr;
+
+	/*
+	 * A lazily-updated count of how many edescs the hardware has
+	 * completed.
+	 */
+	uint64_t hw_complete_count __attribute__ ((aligned(64)));
+
+	/*
+	 * High 32 bits are a count of available egress command credits,
+	 * low 24 bits are the next egress "slot".
+	 */
+	int64_t credits_and_next_index;
+
+} __gxio_dma_queue_t;
+
+/* Initialize a dma queue. */
+extern void __gxio_dma_queue_init(__gxio_dma_queue_t *dma_queue,
+				  void *post_region_addr,
+				  unsigned int num_entries);
+
+/*
+ * Update the "credits_and_next_index" and "hw_complete_count" fields
+ * based on pending hardware completions.  Note that some other thread
+ * may have already done this and, importantly, may still be in the
+ * process of updating "credits_and_next_index".
+ */
+extern void __gxio_dma_queue_update_credits(__gxio_dma_queue_t *dma_queue);
+
+/* Wait for credits to become available. */
+extern int64_t __gxio_dma_queue_wait_for_credits(__gxio_dma_queue_t *dma_queue,
+						 int64_t modifier);
+
+/* Reserve slots in the queue, optionally waiting for slots to become
+ * available, and optionally returning a "completion_slot" suitable for
+ * direct comparison to "hw_complete_count".
+ */
+static inline int64_t __gxio_dma_queue_reserve(__gxio_dma_queue_t *dma_queue,
+					       unsigned int num, bool wait,
+					       bool completion)
+{
+	uint64_t slot;
+
+	/*
+	 * Try to reserve 'num' egress command slots.  We do this by
+	 * constructing a constant that subtracts N credits and adds N to
+	 * the index, and using fetchaddgez to only apply it if the credits
+	 * count doesn't go negative.
+	 */
+	int64_t modifier = (((int64_t)(-num)) << DMA_QUEUE_CREDIT_SHIFT) | num;
+	int64_t old =
+		__insn_fetchaddgez(&dma_queue->credits_and_next_index,
+				   modifier);
+
+	if (unlikely(old + modifier < 0)) {
+		/*
+		 * We're out of credits.  Try once to get more by checking for
+		 * completed egress commands.  If that fails, wait or fail.
+		 */
+		__gxio_dma_queue_update_credits(dma_queue);
+		old = __insn_fetchaddgez(&dma_queue->credits_and_next_index,
+					 modifier);
+		if (old + modifier < 0) {
+			if (wait)
+				old = __gxio_dma_queue_wait_for_credits
+					(dma_queue, modifier);
+			else
+				return GXIO_ERR_DMA_CREDITS;
+		}
+	}
+
+	/* The bottom 24 bits of old encode the "slot". */
+	slot = (old & 0xffffff);
+
+	if (completion) {
+		/*
+		 * A "completion_slot" is a "slot" which can be compared to
+		 * "hw_complete_count" at any time in the future.  To convert
+		 * "slot" into a "completion_slot", we access "hw_complete_count"
+		 * once (knowing that we have reserved a slot, and thus, it will
+		 * be "basically" accurate), and combine its high 40 bits with
+		 * the 24 bit "slot", and handle "wrapping" by adding "1 << 24"
+		 * if the result is LESS than "hw_complete_count".
+		 */
+		uint64_t complete;
+		complete = ACCESS_ONCE(dma_queue->hw_complete_count);
+		slot |= (complete & 0xffffffffff000000);
+		if (slot < complete)
+			slot += 0x1000000;
+	}
+
+	/*
+	 * If any of our slots mod 256 were equivalent to 0, go ahead and
+	 * collect some egress credits, and update "hw_complete_count", and
+	 * make sure the index doesn't overflow into the credits.
+	 */
+	if (unlikely(((old + num) & 0xff) < num)) {
+		__gxio_dma_queue_update_credits(dma_queue);
+
+		/* Make sure the index doesn't overflow into the credits. */
+#ifdef __BIG_ENDIAN__
+		*(((uint8_t *)&dma_queue->credits_and_next_index) + 4) = 0;
+#else
+		*(((uint8_t *)&dma_queue->credits_and_next_index) + 3) = 0;
+#endif
+	}
+
+	return slot;
+}
+
+/* Non-inlinable "__gxio_dma_queue_reserve(..., true)". */
+extern int64_t __gxio_dma_queue_reserve_aux(__gxio_dma_queue_t *dma_queue,
+					    unsigned int num, int wait);
+
+/* Check whether a particular "completion slot" has completed.
+ *
+ * Note that this function requires a "completion slot", and thus
+ * cannot be used with the result of any "reserve_fast" function.
+ */
+extern int __gxio_dma_queue_is_complete(__gxio_dma_queue_t *dma_queue,
+					int64_t completion_slot, int update);
+
+#endif /* !_GXIO_DMA_QUEUE_H_ */