Add a shim header to support locking without ldrex/strex.

Required to allow the USB queue code to be built for the M0.

firmware/common/locking.h

@@ -0,0 +1,47 @@
+/*
+ * Copyright 2024 Great Scott Gadgets <[email protected]>
+ *
+ * This file is part of HackRF.
+ *
+ * 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, 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; see the file COPYING.  If not, write to
+ * the Free Software Foundation, Inc., 51 Franklin Street,
+ * Boston, MA 02110-1301, USA.
+ */
+
+#ifndef __LOCKING_H__
+#define __LOCKING_H__
+
+#include <stdint.h>
+
+#include <libopencm3/cm3/sync.h>
+
+// Use ldrex and strex directly if available.
+// Otherwise, disable interrupts to ensure exclusivity.
+#if defined(__ARM_ARCH_7M__) || defined(__ARM_ARCH_7EM__)
+#define load_exclusive __ldrex
+#define store_exclusive __strex
+#else
+static inline uint32_t load_exclusive(volatile uint32_t* addr) {
+    __disable_irq();
+    return *addr;
+}
+
+static inline uint32_t store_exclusive(uint32_t val, volatile uint32_t* addr) {
+    *addr = val;
+    __enable_irq();
+    return 0;
+}
+#endif
+
+#endif /* __LOCKING_H__ */

firmware/common/usb_queue.c

@@ -27,8 +27,8 @@
 #include <assert.h>
 
 #include <libopencm3/cm3/cortex.h>
-#include <libopencm3/cm3/sync.h>
 
+#include "locking.h"
 #include "usb.h"
 #include "usb_queue.h"
 
@@ -73,10 +73,10 @@ static usb_transfer_t* allocate_transfer(usb_queue_t* const queue)
 	}
 
 	do {
-		transfer = (void*) __ldrex((uint32_t*) &queue->free_transfers);
-		aborted =
-			__strex((uint32_t) transfer->next,
-				(uint32_t*) &queue->free_transfers);
+		transfer = (void*) load_exclusive((uint32_t*) &queue->free_transfers);
+		aborted = store_exclusive(
+			(uint32_t) transfer->next,
+			(uint32_t*) &queue->free_transfers);
 	} while (aborted);
 	transfer->next = NULL;
 	return transfer;
@@ -88,9 +88,11 @@ static void free_transfer(usb_transfer_t* const transfer)
 	usb_queue_t* const queue = transfer->queue;
 	bool aborted;
 	do {
-		transfer->next = (void*) __ldrex((uint32_t*) &queue->free_transfers);
-		aborted =
-			__strex((uint32_t) transfer, (uint32_t*) &queue->free_transfers);
+		transfer->next =
+			(void*) load_exclusive((uint32_t*) &queue->free_transfers);
+		aborted = store_exclusive(
+			(uint32_t) transfer,
+			(uint32_t*) &queue->free_transfers);
 	} while (aborted);
 }