scsi: mac_scsi: Disallow bus errors during PDMA send

SD cards can produce write latency spikes on the order of a hundred
milliseconds. If the target firmware does not hide that latency during DATA
IN and OUT phases it can cause the PDMA circuitry to raise a processor bus
fault which in turn leads to an unreliable byte count and a DMA overrun.

The Last Byte Sent flag is used to detect the overrun but this mechanism is
unreliable on some systems. Instead, set a DID_ERROR result whenever there
is a bus fault during a PDMA send, unless the cause was a phase mismatch.

Cc: [email protected] # 5.15+
Reported-and-tested-by: Stan Johnson <[email protected]>
Fixes: 7c1f3e3 ("scsi: mac_scsi: Treat Last Byte Sent time-out as failure")
Signed-off-by: Finn Thain <[email protected]>
Link: https://lore.kernel.org/r/cc38df687ace2c4ffc375a683b2502fc476b600d.1723001788.git.fthain@linux-m68k.org
Signed-off-by: Martin K. Petersen <[email protected]>

Author: fthain

drivers/scsi/mac_scsi.c

@@ -102,11 +102,15 @@ __setup("mac5380=", mac_scsi_setup);
  * 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.
+ * If a MOVE.B instruction faults during a receive operation, we assume the
+ * target sent nothing and try again. That assumption probably depends on
+ * target firmware but it seems to hold up okay. If a fault happens during a
+ * send operation, the target may or may not have seen /ACK and got the byte.
+ * It's uncertain so the whole SCSI command gets retried.
+ *
+ * The NOP is needed for synchronization because the fault address in the
+ * exception stack frame may or may not be the instruction that actually
+ * caused the bus error. Post-increment addressing can't be used.
  */
 
 #define MOVE_BYTE(operands) \
@@ -243,22 +247,21 @@ static inline int mac_pdma_send(unsigned char *start, void __iomem *io, int n)
 	if (n >= 1) {
 		MOVE_BYTE("%0@,%3@");
 		if (result)
-			goto out;
+			return -1;
 	}
 	if (n >= 1 && ((unsigned long)addr & 1)) {
 		MOVE_BYTE("%0@,%3@");
 		if (result)
-			goto out;
+			return -2;
 	}
 	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 */
+		return start - addr - 1; /* Negated to indicate uncertain length */
 	if (n == 1)
 		MOVE_BYTE("%0@,%3@");
-out:
 	return addr - start;
 }
 
@@ -307,7 +310,6 @@ static inline int macscsi_pread(struct NCR5380_hostdata *hostdata,
 {
 	u8 __iomem *s = hostdata->pdma_io + (INPUT_DATA_REG << 4);
 	unsigned char *d = dst;
-	int result = 0;
 
 	hostdata->pdma_residual = len;
 
@@ -343,19 +345,19 @@ static inline int macscsi_pread(struct NCR5380_hostdata *hostdata,
 		if (bytes == 0)
 			continue;
 
-		result = -1;
+		if (macscsi_wait_for_drq(hostdata) <= 0)
+			set_host_byte(hostdata->connected, DID_ERROR);
 		break;
 	}
 
-	return result;
+	return 0;
 }
 
 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 result = 0;
 
 	hostdata->pdma_residual = len;
 
@@ -377,17 +379,8 @@ static inline int macscsi_pwrite(struct NCR5380_hostdata *hostdata,
 			hostdata->pdma_residual -= bytes;
 		}
 
-		if (hostdata->pdma_residual == 0) {
-			if (NCR5380_poll_politely(hostdata, TARGET_COMMAND_REG,
-			                          TCR_LAST_BYTE_SENT,
-			                          TCR_LAST_BYTE_SENT,
-			                          0) < 0) {
-				scmd_printk(KERN_ERR, hostdata->connected,
-				            "%s: Last Byte Sent timeout\n", __func__);
-				result = -1;
-			}
+		if (hostdata->pdma_residual == 0)
 			break;
-		}
 
 		if (bytes > 0)
 			continue;
@@ -400,11 +393,12 @@ static inline int macscsi_pwrite(struct NCR5380_hostdata *hostdata,
 		if (bytes == 0)
 			continue;
 
-		result = -1;
+		if (macscsi_wait_for_drq(hostdata) <= 0)
+			set_host_byte(hostdata->connected, DID_ERROR);
 		break;
 	}
 
-	return result;
+	return 0;
 }
 
 static int macscsi_dma_xfer_len(struct NCR5380_hostdata *hostdata,