Detect the case where the guest underflows TC by requesting a DMA transfer which
is larger than the available data. If this case is detected, immediately
complete the SCSI request and handle any remaining FIFO accesses in the STATUS
phase by raising INTR_BS once the FIFO is below the threshold.

Note that handling the premature SCSI bus phase change in the case of TC
underflow fixes booting EMILE on m68k once again.

Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
---
 hw/scsi/esp.c | 25 ++++++++++++++++++++++++-
 1 file changed, 24 insertions(+), 1 deletion(-)

diff --git a/hw/scsi/esp.c b/hw/scsi/esp.c
index 8ea100ee9c..a3e18bb3d7 100644
--- a/hw/scsi/esp.c
+++ b/hw/scsi/esp.c
@@ -579,6 +579,12 @@ static void esp_do_dma(ESPState *s)
             s->async_len -= len;
             s->ti_size -= len;
 
+            if (s->async_len == 0 && s->ti_size == 0 && esp_get_tc(s)) {
+                /* If the guest underflows TC then terminate SCSI request */
+                scsi_req_continue(s->current_req);
+                return;
+            }
+
             if (s->async_len == 0 && fifo8_num_used(&s->fifo) < 2) {
                 /* Defer until the scsi layer has completed */
                 scsi_req_continue(s->current_req);
@@ -596,6 +602,12 @@ static void esp_do_dma(ESPState *s)
             esp_set_tc(s, esp_get_tc(s) - len);
             esp_raise_drq(s);
 
+            if (s->async_len == 0 && s->ti_size == 0 && esp_get_tc(s)) {
+                /* If the guest underflows TC then terminate SCSI request */
+                scsi_req_continue(s->current_req);
+                return;
+            }
+
             if (s->async_len == 0 && fifo8_num_used(&s->fifo) < 2) {
                 /* Defer until the scsi layer has completed */
                 scsi_req_continue(s->current_req);
@@ -630,6 +642,15 @@ static void esp_do_dma(ESPState *s)
                 }
             }
             break;
+
+        default:
+            /* Consume remaining data if the guest underflows TC */
+            if (fifo8_num_used(&s->fifo) < 2) {
+                s->rregs[ESP_RINTR] |= INTR_BS;
+                esp_raise_irq(s);
+                esp_lower_drq(s);
+            }
+            break;
         }
         break;
 
@@ -884,7 +905,9 @@ void esp_command_complete(SCSIRequest *req, size_t resid)
     esp_set_phase(s, STAT_ST);
     s->rregs[ESP_RINTR] |= INTR_BS;
     esp_raise_irq(s);
-    esp_lower_drq(s);
+
+    /* Ensure DRQ is set correctly for TC underflow or normal completion */
+    esp_dma_ti_check(s);
 
     if (s->current_req) {
         scsi_req_unref(s->current_req);
-- 
2.39.2