diff --git a/arch/m68k/include/asm/mac_pdma.h b/arch/m68k/include/asm/mac_pdma.h
new file mode 100644
index 000000000000..44e2fb6d4b1c
--- /dev/null
+++ b/arch/m68k/include/asm/mac_pdma.h
@@ -0,0 +1,179 @@
+/* SPDX-License-Identifier: GPL-2.0 OR MIT */
+/* Copyright (C) 2019 Finn Thain */
+
+#ifndef _ASM_MAC_PDMA_H_
+#define _ASM_MAC_PDMA_H_
+
+#include <linux/delay.h>
+
+/*
+ * According to "Inside Macintosh: Devices", Mac OS requires disk drivers to
+ * specify the number of bytes between the delays expected from a SCSI target.
+ * This allows the operating system to "prevent bus errors when a target fails
+ * to deliver the next byte within the processor bus error timeout period."
+ * Linux SCSI drivers lack knowledge of the timing behaviour of SCSI targets
+ * so bus errors are unavoidable.
+ *
+ * If a MOVE.B instruction faults, we assume that zero bytes were transferred
+ * and simply retry. That assumption probably depends on target behaviour but
+ * seems to hold up okay. The NOP provides synchronization: without it the
+ * fault can sometimes occur after the program counter has moved past the
+ * offending instruction. Post-increment addressing can't be used.
+ */
+
+#define MOVE_BYTE(operands) \
+	asm volatile ( \
+		"1:     moveb " operands "     \n" \
+		"11:    nop                    \n" \
+		"       addq #1,%0             \n" \
+		"       subq #1,%1             \n" \
+		"40:                           \n" \
+		"                              \n" \
+		".section .fixup,\"ax\"        \n" \
+		".even                         \n" \
+		"90:    movel #1, %2           \n" \
+		"       jra 40b                \n" \
+		".previous                     \n" \
+		"                              \n" \
+		".section __ex_table,\"a\"     \n" \
+		".align  4                     \n" \
+		".long   1b,90b                \n" \
+		".long  11b,90b                \n" \
+		".previous                     \n" \
+		: "+a" (addr), "+r" (n), "+r" (result) : "a" (io))
+
+/*
+ * If a MOVE.W (or MOVE.L) instruction faults, it cannot be retried because
+ * the residual byte count would be uncertain. In that situation the MOVE_WORD
+ * macro clears n in the fixup section to abort the transfer.
+ */
+
+#define MOVE_WORD(operands) \
+	asm volatile ( \
+		"1:     movew " operands "     \n" \
+		"11:    nop                    \n" \
+		"       subq #2,%1             \n" \
+		"40:                           \n" \
+		"                              \n" \
+		".section .fixup,\"ax\"        \n" \
+		".even                         \n" \
+		"90:    movel #0, %1           \n" \
+		"       movel #2, %2           \n" \
+		"       jra 40b                \n" \
+		".previous                     \n" \
+		"                              \n" \
+		".section __ex_table,\"a\"     \n" \
+		".align  4                     \n" \
+		".long   1b,90b                \n" \
+		".long  11b,90b                \n" \
+		".previous                     \n" \
+		: "+a" (addr), "+r" (n), "+r" (result) : "a" (io))
+
+#define MOVE_16_WORDS(operands) \
+	asm volatile ( \
+		"1:     movew " operands "     \n" \
+		"2:     movew " operands "     \n" \
+		"3:     movew " operands "     \n" \
+		"4:     movew " operands "     \n" \
+		"5:     movew " operands "     \n" \
+		"6:     movew " operands "     \n" \
+		"7:     movew " operands "     \n" \
+		"8:     movew " operands "     \n" \
+		"9:     movew " operands "     \n" \
+		"10:    movew " operands "     \n" \
+		"11:    movew " operands "     \n" \
+		"12:    movew " operands "     \n" \
+		"13:    movew " operands "     \n" \
+		"14:    movew " operands "     \n" \
+		"15:    movew " operands "     \n" \
+		"16:    movew " operands "     \n" \
+		"17:    nop                    \n" \
+		"       subl  #32,%1           \n" \
+		"40:                           \n" \
+		"                              \n" \
+		".section .fixup,\"ax\"        \n" \
+		".even                         \n" \
+		"90:    movel #0, %1           \n" \
+		"       movel #2, %2           \n" \
+		"       jra 40b                \n" \
+		".previous                     \n" \
+		"                              \n" \
+		".section __ex_table,\"a\"     \n" \
+		".align  4                     \n" \
+		".long   1b,90b                \n" \
+		".long   2b,90b                \n" \
+		".long   3b,90b                \n" \
+		".long   4b,90b                \n" \
+		".long   5b,90b                \n" \
+		".long   6b,90b                \n" \
+		".long   7b,90b                \n" \
+		".long   8b,90b                \n" \
+		".long   9b,90b                \n" \
+		".long  10b,90b                \n" \
+		".long  11b,90b                \n" \
+		".long  12b,90b                \n" \
+		".long  13b,90b                \n" \
+		".long  14b,90b                \n" \
+		".long  15b,90b                \n" \
+		".long  16b,90b                \n" \
+		".long  17b,90b                \n" \
+		".previous                     \n" \
+		: "+a" (addr), "+r" (n), "+r" (result) : "a" (io))
+
+#define MAC_PDMA_DELAY		32
+
+static inline int mac_pdma_recv(void __iomem *io, unsigned char *start, int n)
+{
+	unsigned char *addr = start;
+	int result = 0;
+
+	if (n >= 1) {
+		MOVE_BYTE("%3@,%0@");
+		if (result)
+			goto out;
+	}
+	if (n >= 1 && ((unsigned long)addr & 1)) {
+		MOVE_BYTE("%3@,%0@");
+		if (result)
+			goto out;
+	}
+	while (n >= 32)
+		MOVE_16_WORDS("%3@,%0@+");
+	while (n >= 2)
+		MOVE_WORD("%3@,%0@+");
+	if (result)
+		return start - addr; /* Negated to indicate uncertain length */
+	if (n == 1)
+		MOVE_BYTE("%3@,%0@");
+out:
+	return addr - start;
+}
+
+static inline int mac_pdma_send(unsigned char *start, void __iomem *io, int n)
+{
+	unsigned char *addr = start;
+	int result = 0;
+
+	if (n >= 1) {
+		MOVE_BYTE("%0@,%3@");
+		if (result)
+			goto out;
+	}
+	if (n >= 1 && ((unsigned long)addr & 1)) {
+		MOVE_BYTE("%0@,%3@");
+		if (result)
+			goto out;
+	}
+	while (n >= 32)
+		MOVE_16_WORDS("%0@+,%3@");
+	while (n >= 2)
+		MOVE_WORD("%0@+,%3@");
+	if (result)
+		return start - addr; /* Negated to indicate uncertain length */
+	if (n == 1)
+		MOVE_BYTE("%0@,%3@");
+out:
+	return addr - start;
+}
+
+#endif /* _ASM_MAC_PDMA_H_ */
diff --git a/drivers/scsi/mac_scsi.c b/drivers/scsi/mac_scsi.c
index ba1afcaadae8..e83b47a7e4b5 100644
--- a/drivers/scsi/mac_scsi.c
+++ b/drivers/scsi/mac_scsi.c
@@ -21,6 +21,7 @@
 
 #include <asm/hwtest.h>
 #include <asm/io.h>
+#include <asm/mac_pdma.h>
 #include <asm/macints.h>
 #include <asm/setup.h>
 
@@ -89,101 +90,47 @@ static int __init mac_scsi_setup(char *str)
 __setup("mac5380=", mac_scsi_setup);
 #endif /* !MODULE */
 
-/* Pseudo DMA asm originally by Ove Edlund */
-
-#define CP_IO_TO_MEM(s,d,n)				\
-__asm__ __volatile__					\
-    ("    cmp.w  #4,%2\n"				\
-     "    bls    8f\n"					\
-     "    move.w %1,%%d0\n"				\
-     "    neg.b  %%d0\n"				\
-     "    and.w  #3,%%d0\n"				\
-     "    sub.w  %%d0,%2\n"				\
-     "    bra    2f\n"					\
-     " 1: move.b (%0),(%1)+\n"				\
-     " 2: dbf    %%d0,1b\n"				\
-     "    move.w %2,%%d0\n"				\
-     "    lsr.w  #5,%%d0\n"				\
-     "    bra    4f\n"					\
-     " 3: move.l (%0),(%1)+\n"				\
-     "31: move.l (%0),(%1)+\n"				\
-     "32: move.l (%0),(%1)+\n"				\
-     "33: move.l (%0),(%1)+\n"				\
-     "34: move.l (%0),(%1)+\n"				\
-     "35: move.l (%0),(%1)+\n"				\
-     "36: move.l (%0),(%1)+\n"				\
-     "37: move.l (%0),(%1)+\n"				\
-     " 4: dbf    %%d0,3b\n"				\
-     "    move.w %2,%%d0\n"				\
-     "    lsr.w  #2,%%d0\n"				\
-     "    and.w  #7,%%d0\n"				\
-     "    bra    6f\n"					\
-     " 5: move.l (%0),(%1)+\n"				\
-     " 6: dbf    %%d0,5b\n"				\
-     "    and.w  #3,%2\n"				\
-     "    bra    8f\n"					\
-     " 7: move.b (%0),(%1)+\n"				\
-     " 8: dbf    %2,7b\n"				\
-     "    moveq.l #0, %2\n"				\
-     " 9: \n"						\
-     ".section .fixup,\"ax\"\n"				\
-     "    .even\n"					\
-     "91: moveq.l #1, %2\n"				\
-     "    jra 9b\n"					\
-     "94: moveq.l #4, %2\n"				\
-     "    jra 9b\n"					\
-     ".previous\n"					\
-     ".section __ex_table,\"a\"\n"			\
-     "   .align 4\n"					\
-     "   .long  1b,91b\n"				\
-     "   .long  3b,94b\n"				\
-     "   .long 31b,94b\n"				\
-     "   .long 32b,94b\n"				\
-     "   .long 33b,94b\n"				\
-     "   .long 34b,94b\n"				\
-     "   .long 35b,94b\n"				\
-     "   .long 36b,94b\n"				\
-     "   .long 37b,94b\n"				\
-     "   .long  5b,94b\n"				\
-     "   .long  7b,91b\n"				\
-     ".previous"					\
-     : "=a"(s), "=a"(d), "=d"(n)			\
-     : "0"(s), "1"(d), "2"(n)				\
-     : "d0")
-
 static inline int macscsi_pread(struct NCR5380_hostdata *hostdata,
                                 unsigned char *dst, int len)
 {
 	u8 __iomem *s = hostdata->pdma_io + (INPUT_DATA_REG << 4);
 	unsigned char *d = dst;
-	int n = len;
-	int transferred;
+
+	hostdata->pdma_residual = len;
 
 	while (!NCR5380_poll_politely(hostdata, BUS_AND_STATUS_REG,
 	                              BASR_DRQ | BASR_PHASE_MATCH,
 	                              BASR_DRQ | BASR_PHASE_MATCH, HZ / 64)) {
-		CP_IO_TO_MEM(s, d, n);
+		int bytes;
+
+		bytes = mac_pdma_recv(s, d, min(hostdata->pdma_residual, 512));
 
-		transferred = d - dst - n;
-		hostdata->pdma_residual = len - transferred;
+		if (bytes > 0) {
+			d += bytes;
+			hostdata->pdma_residual -= bytes;
+		}
 
-		/* No bus error. */
-		if (n == 0)
+		if (hostdata->pdma_residual == 0)
 			return 0;
 
-		/* Target changed phase early? */
 		if (NCR5380_poll_politely2(hostdata, STATUS_REG, SR_REQ, SR_REQ,
-		                           BUS_AND_STATUS_REG, BASR_ACK, BASR_ACK, HZ / 64) < 0)
-			scmd_printk(KERN_ERR, hostdata->connected,
+		                           BUS_AND_STATUS_REG, BASR_ACK,
+		                           BASR_ACK, HZ / 64) < 0)
+			scmd_printk(KERN_DEBUG, hostdata->connected,
 			            "%s: !REQ and !ACK\n", __func__);
 		if (!(NCR5380_read(BUS_AND_STATUS_REG) & BASR_PHASE_MATCH))
 			return 0;
 
+		if (bytes == 0)
+			udelay(MAC_PDMA_DELAY);
+
+		if (bytes >= 0)
+			continue;
+
 		dsprintk(NDEBUG_PSEUDO_DMA, hostdata->host,
-		         "%s: bus error (%d/%d)\n", __func__, transferred, len);
+		         "%s: bus error (%d/%d)\n", __func__, d - dst, len);
 		NCR5380_dprint(NDEBUG_PSEUDO_DMA, hostdata->host);
-		d = dst + transferred;
-		n = len - transferred;
+		return -1;
 	}
 
 	scmd_printk(KERN_ERR, hostdata->connected,
@@ -192,93 +139,27 @@ static inline int macscsi_pread(struct NCR5380_hostdata *hostdata,
 	return -1;
 }
 
-
-#define CP_MEM_TO_IO(s,d,n)				\
-__asm__ __volatile__					\
-    ("    cmp.w  #4,%2\n"				\
-     "    bls    8f\n"					\
-     "    move.w %0,%%d0\n"				\
-     "    neg.b  %%d0\n"				\
-     "    and.w  #3,%%d0\n"				\
-     "    sub.w  %%d0,%2\n"				\
-     "    bra    2f\n"					\
-     " 1: move.b (%0)+,(%1)\n"				\
-     " 2: dbf    %%d0,1b\n"				\
-     "    move.w %2,%%d0\n"				\
-     "    lsr.w  #5,%%d0\n"				\
-     "    bra    4f\n"					\
-     " 3: move.l (%0)+,(%1)\n"				\
-     "31: move.l (%0)+,(%1)\n"				\
-     "32: move.l (%0)+,(%1)\n"				\
-     "33: move.l (%0)+,(%1)\n"				\
-     "34: move.l (%0)+,(%1)\n"				\
-     "35: move.l (%0)+,(%1)\n"				\
-     "36: move.l (%0)+,(%1)\n"				\
-     "37: move.l (%0)+,(%1)\n"				\
-     " 4: dbf    %%d0,3b\n"				\
-     "    move.w %2,%%d0\n"				\
-     "    lsr.w  #2,%%d0\n"				\
-     "    and.w  #7,%%d0\n"				\
-     "    bra    6f\n"					\
-     " 5: move.l (%0)+,(%1)\n"				\
-     " 6: dbf    %%d0,5b\n"				\
-     "    and.w  #3,%2\n"				\
-     "    bra    8f\n"					\
-     " 7: move.b (%0)+,(%1)\n"				\
-     " 8: dbf    %2,7b\n"				\
-     "    moveq.l #0, %2\n"				\
-     " 9: \n"						\
-     ".section .fixup,\"ax\"\n"				\
-     "    .even\n"					\
-     "91: moveq.l #1, %2\n"				\
-     "    jra 9b\n"					\
-     "94: moveq.l #4, %2\n"				\
-     "    jra 9b\n"					\
-     ".previous\n"					\
-     ".section __ex_table,\"a\"\n"			\
-     "   .align 4\n"					\
-     "   .long  1b,91b\n"				\
-     "   .long  3b,94b\n"				\
-     "   .long 31b,94b\n"				\
-     "   .long 32b,94b\n"				\
-     "   .long 33b,94b\n"				\
-     "   .long 34b,94b\n"				\
-     "   .long 35b,94b\n"				\
-     "   .long 36b,94b\n"				\
-     "   .long 37b,94b\n"				\
-     "   .long  5b,94b\n"				\
-     "   .long  7b,91b\n"				\
-     ".previous"					\
-     : "=a"(s), "=a"(d), "=d"(n)			\
-     : "0"(s), "1"(d), "2"(n)				\
-     : "d0")
-
 static inline int macscsi_pwrite(struct NCR5380_hostdata *hostdata,
                                  unsigned char *src, int len)
 {
 	unsigned char *s = src;
 	u8 __iomem *d = hostdata->pdma_io + (OUTPUT_DATA_REG << 4);
-	int n = len;
-	int transferred;
+
+	hostdata->pdma_residual = len;
 
 	while (!NCR5380_poll_politely(hostdata, BUS_AND_STATUS_REG,
 	                              BASR_DRQ | BASR_PHASE_MATCH,
 	                              BASR_DRQ | BASR_PHASE_MATCH, HZ / 64)) {
-		CP_MEM_TO_IO(s, d, n);
+		int bytes;
 
-		transferred = s - src - n;
-		hostdata->pdma_residual = len - transferred;
+		bytes = mac_pdma_send(s, d, min(hostdata->pdma_residual, 512));
 
-		/* Target changed phase early? */
-		if (NCR5380_poll_politely2(hostdata, STATUS_REG, SR_REQ, SR_REQ,
-		                           BUS_AND_STATUS_REG, BASR_ACK, BASR_ACK, HZ / 64) < 0)
-			scmd_printk(KERN_ERR, hostdata->connected,
-			            "%s: !REQ and !ACK\n", __func__);
-		if (!(NCR5380_read(BUS_AND_STATUS_REG) & BASR_PHASE_MATCH))
-			return 0;
+		if (bytes > 0) {
+			s += bytes;
+			hostdata->pdma_residual -= bytes;
+		}
 
-		/* No bus error. */
-		if (n == 0) {
+		if (hostdata->pdma_residual == 0) {
 			if (NCR5380_poll_politely(hostdata, TARGET_COMMAND_REG,
 			                          TCR_LAST_BYTE_SENT,
 			                          TCR_LAST_BYTE_SENT, HZ / 64) < 0)
@@ -287,17 +168,29 @@ static inline int macscsi_pwrite(struct NCR5380_hostdata *hostdata,
 			return 0;
 		}
 
+		if (NCR5380_poll_politely2(hostdata, STATUS_REG, SR_REQ, SR_REQ,
+		                           BUS_AND_STATUS_REG, BASR_ACK,
+		                           BASR_ACK, HZ / 64) < 0)
+			scmd_printk(KERN_DEBUG, hostdata->connected,
+			            "%s: !REQ and !ACK\n", __func__);
+		if (!(NCR5380_read(BUS_AND_STATUS_REG) & BASR_PHASE_MATCH))
+			return 0;
+
+		if (bytes == 0)
+			udelay(MAC_PDMA_DELAY);
+
+		if (bytes >= 0)
+			continue;
+
 		dsprintk(NDEBUG_PSEUDO_DMA, hostdata->host,
-		         "%s: bus error (%d/%d)\n", __func__, transferred, len);
+		         "%s: bus error (%d/%d)\n", __func__, s - src, len);
 		NCR5380_dprint(NDEBUG_PSEUDO_DMA, hostdata->host);
-		s = src + transferred;
-		n = len - transferred;
+		return -1;
 	}
 
 	scmd_printk(KERN_ERR, hostdata->connected,
 	            "%s: phase mismatch or !DRQ\n", __func__);
 	NCR5380_dprint(NDEBUG_PSEUDO_DMA, hostdata->host);
-
 	return -1;
 }
 
