am53c974: BLAST residual handling

The am53c974 has an design issue where a single byte might be
left in the SCSI FIFO after a DMA transfer.
As the handling code is currently untested add a WARN_ON()
statement here.

Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Acked-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: Christoph Hellwig <hch@lst.de>

drivers/scsi/am53c974.c

@@ -200,6 +200,12 @@ static void pci_esp_dma_drain(struct esp *esp)
 	}
 	pci_esp_write8(esp, ESP_DMA_CMD_DIR | ESP_DMA_CMD_IDLE, ESP_DMA_CMD);
 	esp_dma_log("DMA blast done (%d tries, %d bytes left)\n", lim, resid);
+	/* BLAST residual handling is currently untested */
+	if (WARN_ON_ONCE(resid == 1)) {
+		struct esp_cmd_entry *ent = esp->active_cmd;
+
+		ent->flags |= ESP_CMD_FLAG_RESIDUAL;
+	}
 }
 
 static void pci_esp_dma_invalidate(struct esp *esp)