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>

1 files changed, 29 insertions, 0 deletions

diff --git a/drivers/scsi/esp_scsi.c b/drivers/scsi/esp_scsi.c
index 25cc6fa0535c..4366011cd400 100644
--- a/drivers/scsi/esp_scsi.c
+++ b/drivers/scsi/esp_scsi.c
@@ -1334,6 +1334,35 @@ static int esp_data_bytes_sent(struct esp *esp, struct esp_cmd_entry *ent,
         bytes_sent = esp->data_dma_len;
         bytes_sent -= ecount;
 
+        /*
+         * The am53c974 has a DMA 'pecularity'. The doc states:
+         * In some odd byte conditions, one residual byte will
+         * be left in the SCSI FIFO, and the FIFO Flags will
+         * never count to '0 '. When this happens, the residual
+         * byte should be retrieved via PIO following completion
+         * of the BLAST operation.
+         */
+        if (fifo_cnt == 1 && ent->flags & ESP_CMD_FLAG_RESIDUAL) {
+                size_t count = 1;
+                size_t offset = bytes_sent;
+                u8 bval = esp_read8(ESP_FDATA);
+
+                if (ent->flags & ESP_CMD_FLAG_AUTOSENSE)
+                        ent->sense_ptr[bytes_sent] = bval;
+                else {
+                        struct esp_cmd_priv *p = ESP_CMD_PRIV(cmd);
+                        u8 *ptr;
+
+                        ptr = scsi_kmap_atomic_sg(p->cur_sg, p->u.num_sg,
+                                                  &offset, &count);
+                        if (likely(ptr)) {
+                                *(ptr + offset) = bval;
+                                scsi_kunmap_atomic_sg(ptr);
+                        }
+                }
+                bytes_sent += fifo_cnt;
+                ent->flags &= ~ESP_CMD_FLAG_RESIDUAL;
+        }
         if (!(ent->flags & ESP_CMD_FLAG_WRITE))
                 bytes_sent -= fifo_cnt;