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 <[e-mail]>
Acked-by: David S. Miller <[e-mail]>
Signed-off-by: Hannes Reinecke <[e-mail]>
Signed-off-by: Christoph Hellwig <[e-mail]>

drivers/scsi/am53c974.c

···
 	}
 	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)

drivers/scsi/esp_scsi.c

···
 	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;
 

drivers/scsi/esp_scsi.h

···
 #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];