3 files changed, 36 insertions, 0 deletions

diff --git a/drivers/scsi/am53c974.c b/drivers/scsi/am53c974.c
index a16c9cf0f0a2..a42e8db057a4 100644
--- a/drivers/scsi/am53c974.c
+++ b/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)
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;
 
diff --git a/drivers/scsi/esp_scsi.h b/drivers/scsi/esp_scsi.h
index 27dcaf84e717..5fa456cc5e8b 100644
--- a/drivers/scsi/esp_scsi.h
+++ b/drivers/scsi/esp_scsi.h
@@ -269,6 +269,7 @@ struct esp_cmd_entry {
 #define ESP_CMD_FLAG_WRITE      0x01 /* DMA is a write */
 #define ESP_CMD_FLAG_ABORT      0x02 /* being aborted */
 #define ESP_CMD_FLAG_AUTOSENSE  0x04 /* Doing automatic REQUEST_SENSE */
+#define ESP_CMD_FLAG_RESIDUAL   0x08 /* AM53c974 BLAST residual */
 
         u8                      tag[2];
         u8                      orig_tag[2];