scsi: g_NCR5380: Two DTC436 PDMA workarounds

Limit PDMA send to 512 B to avoid data corruption on DTC3181E. The
corruption is always the same: one byte missing at the beginning of a
128 B block. It happens only with slow Quantum LPS 240 drive, not with
faster IBM DORS-32160. It's not clear what causes this. Documentation
for the DTC436 chip has not been made available.

Signed-off-by: Finn Thain <fthain@telegraphics.com.au>
Tested-by: Ondrej Zary <linux@rainbow-software.org>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

drivers/scsi/g_NCR5380.c

@@ -45,7 +45,8 @@
 	int c400_blk_cnt; \
 	int c400_host_buf; \
 	int io_width; \
-	int pdma_residual
+	int pdma_residual; \
+	int board
 
 #define NCR5380_dma_xfer_len            generic_NCR5380_dma_xfer_len
 #define NCR5380_dma_recv_setup          generic_NCR5380_precv
@@ -316,6 +317,7 @@ static int generic_NCR5380_init_one(struct scsi_host_template *tpnt,
 	}
 	hostdata = shost_priv(instance);
 
+	hostdata->board = board;
 	hostdata->io = iomem;
 	hostdata->region_size = region_size;
 
@@ -492,6 +494,8 @@ static void wait_for_53c80_access(struct NCR5380_hostdata *hostdata)
 	int count = 10000;
 
 	do {
+		if (hostdata->board == BOARD_DTC3181E)
+			udelay(4); /* DTC436 chip hangs without this */
 		if (NCR5380_read(hostdata->c400_ctl_status) & CSR_53C80_REG)
 			return;
 	} while (--count > 0);
@@ -665,7 +669,12 @@ static int generic_NCR5380_dma_xfer_len(struct NCR5380_hostdata *hostdata,
 
 	/* 53C400 datasheet: non-modulo-128-byte transfers should use PIO */
 	if (transfersize % 128)
-		transfersize = 0;
+		return 0;
+
+	/* Limit PDMA send to 512 B to avoid random corruption on DTC3181E */
+	if (hostdata->board == BOARD_DTC3181E &&
+	    cmd->sc_data_direction == DMA_TO_DEVICE)
+		transfersize = min(cmd->SCp.this_residual, 512);
 
 	return min(transfersize, DMA_MAX_SIZE);
 }