[PATCH v2 4/7] esp.c: only call dma_memory_read function if transfer length is non-zero

[Mark Cave-Ayland]

In the cases where mixed DMA/non-DMA transfers are used or no data is
available, it is possible to for the calculated transfer length to be
zero. Only call the dma_memory_read function where the transfer length
is non-zero to avoid invoking the DMA engine for a zero length transfer
which can have side-effects (along with generating additional tracing
noise).

Signed-off-by: Mark Cave-Ayland <mark.cave-ayl...@ilande.co.uk>
---
 hw/scsi/esp.c | 20 +++++++++++++-------
 1 file changed, 13 insertions(+), 7 deletions(-)

diff --git a/hw/scsi/esp.c b/hw/scsi/esp.c
index 62ba406149..ec9fcbeddf 100644
--- a/hw/scsi/esp.c
+++ b/hw/scsi/esp.c
@@ -487,8 +487,10 @@ static void esp_do_dma(ESPState *s)
     case STAT_MO:
         if (s->dma_memory_read) {
             len = MIN(len, fifo8_num_free(&s->cmdfifo));
-            s->dma_memory_read(s->dma_opaque, buf, len);
-            esp_set_tc(s, esp_get_tc(s) - len);
+            if (len) {
+                s->dma_memory_read(s->dma_opaque, buf, len);
+                esp_set_tc(s, esp_get_tc(s) - len);
+            }
         } else {
             len = esp_fifo_pop_buf(s, buf, fifo8_num_used(&s->fifo));
             len = MIN(fifo8_num_free(&s->cmdfifo), len);
@@ -541,9 +543,11 @@ static void esp_do_dma(ESPState *s)
         trace_esp_do_dma(cmdlen, len);
         if (s->dma_memory_read) {
             len = MIN(len, fifo8_num_free(&s->cmdfifo));
-            s->dma_memory_read(s->dma_opaque, buf, len);
-            fifo8_push_all(&s->cmdfifo, buf, len);
-            esp_set_tc(s, esp_get_tc(s) - len);
+            if (len) {
+                s->dma_memory_read(s->dma_opaque, buf, len);
+                fifo8_push_all(&s->cmdfifo, buf, len);
+                esp_set_tc(s, esp_get_tc(s) - len);
+            }
         } else {
             len = esp_fifo_pop_buf(s, buf, fifo8_num_used(&s->fifo));
             len = MIN(fifo8_num_free(&s->cmdfifo), len);
@@ -572,8 +576,10 @@ static void esp_do_dma(ESPState *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);
+                if (len) {
+                    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);
-- 
2.39.5