[PATCH RFC 07/13] dmaengine: sdxi: Import descriptor enqueue code from spec

Nathan Lynch via B4 Relay
2025-09-05T18:48:51Z

From: Nathan Lynch <nathan.lynch@amd.com>

Import the example code from the "SDXI Descriptor Ring Operation"
chapter of the SDXI 1.0 spec[1], which demonstrates lockless
descriptor submission to the ring. Lightly alter the code
to (somewhat) comply with Linux coding style, and use byte order-aware
types as well as kernel atomic and barrier APIs.

Ultimately we may not really need a lockless submission path, and it
would be better for it to more closely integrate with the rest of the
driver.

[1] https://www.snia.org/sites/default/files/technical-work/sdxi/release/SNIA-SDXI-Specification-v1.0a.pdf

Signed-off-by: Nathan Lynch <nathan.lynch@amd.com>
---
drivers/dma/sdxi/enqueue.c | 136 +++++++++++++++++++++++++++++++++++++++++++++
drivers/dma/sdxi/enqueue.h | 16 ++++++
2 files changed, 152 insertions(+)

diff --git a/drivers/dma/sdxi/enqueue.c b/drivers/dma/sdxi/enqueue.c
new file mode 100644
index 0000000000000000000000000000000000000000..822d9b890fa3538dcc09e99ef562a6d8419290f0
--- /dev/null
+++ b/drivers/dma/sdxi/enqueue.c
@@ -0,0 +1,136 @@
+// SPDX-License-Identifier: BSD-3-Clause
+/*
+ *
+ * Copyright (c) 2024, The Storage Networking Industry Association.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ *
+ * * Redistributions of source code must retain the above copyright
+ * notice, this list of conditions and the following disclaimer.
+ *
+ * * Redistributions in binary form must reproduce the above copyright
+ * notice, this list of conditions and the following disclaimer in the
+ * documentation and/or other materials provided with the
+ * distribution.
+ *
+ * * Neither the name of The Storage Networking Industry Association
+ * (SNIA) nor the names of its contributors may be used to endorse or
+ * promote products derived from this software without specific prior
+ * written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+ * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+ * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
+ * FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE
+ * COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
+ * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES
+ * (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
+ * SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
+ * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT,
+ * STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
+ * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED
+ * OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+#include <asm/barrier.h>
+#include <asm/byteorder.h>
+#include <asm/rwonce.h>
+#include <linux/atomic.h>
+#include <linux/errno.h>
+#include <linux/io-64-nonatomic-lo-hi.h>
+#include <linux/processor.h>
+#include <linux/types.h>
+
+#include "enqueue.h"
+
+/*
+ * Code adapted from the "SDXI Descriptor Ring Operation" chapter of
+ * the SDXI spec, specifically the example code in "Enqueuing one or
+ * more Descriptors."
+ */
+
+#define SDXI_DESCR_SIZE 64
+#define SDXI_DS_NUM_QW (SDXI_DESCR_SIZE / sizeof(__le64))
+#define SDXI_MULTI_PRODUCER 1 /* Define to 0 if single-producer. */
+
+static int update_ring(const __le64 *enq_entries, /* Ptr to entries to enqueue */
+ u64 enq_num, /* Number of entries to enqueue */
+ __le64 *ring_base, /* Ptr to ring location */
+ u64 ring_size, /* (Ring Size in bytes)/64 */
+ u64 index) /* Starting ring index to update */
+{
+ for (u64 i = 0; i < enq_num; i++) {
+ __le64 *ringp = ring_base + ((index + i) % ring_size) * SDXI_DS_NUM_QW;
+ const __le64 *entryp = enq_entries + (i * SDXI_DS_NUM_QW);
+
+ for (u64 j = 1; j < SDXI_DS_NUM_QW; j++)
+ *(ringp + j) = *(entryp + j);
+ }
+
+ /* Now write the first QW of the new entries to the ring. */
+ dma_wmb();
+ for (u64 i = 0; i < enq_num; i++) {
+ __le64 *ringp = ring_base + ((index + i) % ring_size) * SDXI_DS_NUM_QW;
+ const __le64 *entryp = enq_entries + (i * SDXI_DS_NUM_QW);
+
+ *ringp = *entryp;
+ }
+
+ return 0;
+}
+
+int sdxi_enqueue(const __le64 *enq_entries, /* Ptr to entries to enqueue */
+ u64 enq_num, /* Number of entries to enqueue */
+ __le64 *ring_base, /* Ptr to ring location */
+ u64 ring_size, /* (Ring Size in bytes)/64 */
+ __le64 const volatile * const Read_Index, /* Ptr to Read_Index location */
+ __le64 volatile * const Write_Index, /* Ptr to Write_Index location */
+ __le64 __iomem *Door_Bell) /* Ptr to Ring Doorbell location */
+{
+ u64 old_write_idx;
+ u64 new_idx;
+
+ while (true) {
+ u64 read_idx;
+
+ read_idx = le64_to_cpu(READ_ONCE(*Read_Index));
+ dma_rmb(); /* Get Read_Index before Write_Index to always get consistent values */
+ old_write_idx = le64_to_cpu(READ_ONCE(*Write_Index));
+
+ if (read_idx > old_write_idx) {
+ /* Only happens if Write_Index wraps or ring has bad setup */
+ return -EIO;
+ }
+
+ new_idx = old_write_idx + enq_num;
+ if (new_idx - read_idx > ring_size) {
+ cpu_relax();
+ continue; /* Not enough free entries, try again */
+ }
+
+ if (SDXI_MULTI_PRODUCER) {
+ /* Try to atomically update Write_Index. */
+ bool success = cmpxchg(Write_Index,
+ cpu_to_le64(old_write_idx),
+ cpu_to_le64(new_idx)) == cpu_to_le64(old_write_idx);
+ if (success)
+ break; /* Updated Write_Index, no need to try again. */
+ } else {
+ /* Single-Producer case */
+ WRITE_ONCE(*Write_Index, cpu_to_le64(new_idx));
+ dma_wmb(); /* Make the Write_Index update visible before the Door_Bell update. */
+ break; /* Always successful for single-producer */
+ }
+ /* Couldn"t update Write_Index, try again. */
+ }
+
+ /* Write_Index is now advanced. Let's write out entries to the ring. */
+ update_ring(enq_entries, enq_num, ring_base, ring_size, old_write_idx);
+
+ /* Door_Bell write required; only needs ordering wrt update of Write_Index. */
+ iowrite64(new_idx, Door_Bell);
+
+ return 0;
+}
diff --git a/drivers/dma/sdxi/enqueue.h b/drivers/dma/sdxi/enqueue.h
new file mode 100644
index 0000000000000000000000000000000000000000..28c1493779db1119ff0d682fa6623b016998042a
--- /dev/null
+++ b/drivers/dma/sdxi/enqueue.h
@@ -0,0 +1,16 @@
+/* SPDX-License-Identifier: BSD-3-Clause */
+/* Copyright (c) 2024, The Storage Networking Industry Association. */
+#ifndef DMA_SDXI_ENQUEUE_H
+#define DMA_SDXI_ENQUEUE_H
+
+#include <linux/types.h>
+
+int sdxi_enqueue(const __le64 *enq_entries, /* Ptr to entries to enqueue */
+ u64 enq_num, /* Number of entries to enqueue */
+ __le64 *ring_base, /* Ptr to ring location */
+ u64 ring_size, /* (Ring Size in bytes)/64 */
+ __le64 const volatile * const Read_Index, /* Ptr to Read_Index location */
+ __le64 volatile * const Write_Index, /* Ptr to Write_Index location */
+ __le64 __iomem *Door_Bell); /* Ptr to Ring Doorbell location */
+
+#endif /* DMA_SDXI_ENQUEUE_H */

--
2.39.5