esp.c: implement DMA Transfer Pad command for DATA phases

The Transfer Pad command is used to either drop incoming FIFO data during the
DATA IN phase or generate a series of zero bytes in the FIFO during the DATA
OUT phase.

Implement the DMA Transfer Pad command for the DATA phases which is used by
the NeXTCube firmware in the DATA IN phase to ignore part of the incoming SCSI
data as it is copied into memory.

Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Tested-by: Helge Deller <deller@gmx.de>
Tested-by: Thomas Huth <thuth@redhat.com>
Message-Id: <20240112125420.514425-85-mark.cave-ayland@ilande.co.uk>
Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>

hw/scsi/esp.c

@@ -387,6 +387,15 @@ static void handle_satn_stop(ESPState *s)
     }
 }
 
+static void handle_pad(ESPState *s)
+{
+    if (s->dma) {
+        esp_do_dma(s);
+    } else {
+        esp_do_nodma(s);
+    }
+}
+
 static void write_response(ESPState *s)
 {
     trace_esp_write_response(s->status);
@@ -518,20 +527,38 @@ static void esp_do_dma(ESPState *s)
             len = s->async_len;
         }
 
-        if (s->dma_memory_read) {
-            s->dma_memory_read(s->dma_opaque, s->async_buf, len);
-            esp_set_tc(s, esp_get_tc(s) - len);
-        } else {
-            /* Copy FIFO data to device */
-            len = MIN(s->async_len, ESP_FIFO_SZ);
-            len = MIN(len, fifo8_num_used(&s->fifo));
-            len = esp_fifo_pop_buf(&s->fifo, s->async_buf, len);
-            esp_raise_drq(s);
-        }
+        switch (s->rregs[ESP_CMD]) {
+        case CMD_TI | CMD_DMA:
+            if (s->dma_memory_read) {
+                s->dma_memory_read(s->dma_opaque, s->async_buf, len);
+                esp_set_tc(s, esp_get_tc(s) - len);
+            } else {
+                /* Copy FIFO data to device */
+                len = MIN(s->async_len, ESP_FIFO_SZ);
+                len = MIN(len, fifo8_num_used(&s->fifo));
+                len = esp_fifo_pop_buf(&s->fifo, s->async_buf, len);
+                esp_raise_drq(s);
+            }
 
-        s->async_buf += len;
-        s->async_len -= len;
-        s->ti_size += len;
+            s->async_buf += len;
+            s->async_len -= len;
+            s->ti_size += len;
+            break;
+
+        case CMD_PAD | CMD_DMA:
+            /* Copy TC zero bytes into the incoming stream */
+            if (!s->dma_memory_read) {
+                len = MIN(s->async_len, ESP_FIFO_SZ);
+                len = MIN(len, fifo8_num_free(&s->fifo));
+            }
+
+            memset(s->async_buf, 0, len);
+
+            s->async_buf += len;
+            s->async_len -= len;
+            s->ti_size += len;
+            break;
+        }
 
         if (s->async_len == 0 && fifo8_num_used(&s->fifo) < 2) {
             /* Defer until the scsi layer has completed */
@@ -554,19 +581,35 @@ static void esp_do_dma(ESPState *s)
             len = s->async_len;
         }
 
-        if (s->dma_memory_write) {
-            s->dma_memory_write(s->dma_opaque, s->async_buf, len);
-        } else {
-            /* Copy device data to FIFO */
-            len = MIN(len, fifo8_num_free(&s->fifo));
-            fifo8_push_all(&s->fifo, s->async_buf, len);
-            esp_raise_drq(s);
-        }
+        switch (s->rregs[ESP_CMD]) {
+        case CMD_TI | CMD_DMA:
+            if (s->dma_memory_write) {
+                s->dma_memory_write(s->dma_opaque, s->async_buf, len);
+            } else {
+                /* Copy device data to FIFO */
+                len = MIN(len, fifo8_num_free(&s->fifo));
+                fifo8_push_all(&s->fifo, s->async_buf, len);
+                esp_raise_drq(s);
+            }
+
+            s->async_buf += len;
+            s->async_len -= len;
+            s->ti_size -= len;
+            esp_set_tc(s, esp_get_tc(s) - len);
+            break;
+
+        case CMD_PAD | CMD_DMA:
+            /* Drop TC bytes from the incoming stream */
+            if (!s->dma_memory_write) {
+                len = MIN(len, fifo8_num_free(&s->fifo));
+            }
 
-        s->async_buf += len;
-        s->async_len -= len;
-        s->ti_size -= len;
-        esp_set_tc(s, esp_get_tc(s) - len);
+            s->async_buf += len;
+            s->async_len -= len;
+            s->ti_size -= len;
+            esp_set_tc(s, esp_get_tc(s) - len);
+            break;
+        }
 
         if (s->async_len == 0 && s->ti_size == 0 && esp_get_tc(s)) {
             /* If the guest underflows TC then terminate SCSI request */
@@ -1087,9 +1130,7 @@ static void esp_run_cmd(ESPState *s)
         break;
     case CMD_PAD:
         trace_esp_mem_writeb_cmd_pad(cmd);
-        s->rregs[ESP_RSTAT] = STAT_TC;
-        s->rregs[ESP_RINTR] |= INTR_FC;
-        s->rregs[ESP_RSEQ] = 0;
+        handle_pad(s);
         break;
     case CMD_SATN:
         trace_esp_mem_writeb_cmd_satn(cmd);