[PULL 15/17] esp.c: only call dma_memory_write function if transfer length is non-zero

["Philippe Mathieu-Daudé" <philmd@linaro.org>]

From: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>

In the cases where mixed DMA/non-DMA transfers are used or no data is
available, it is possible for the calculated transfer length to be zero.
Only call the dma_memory_write 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-ayland@ilande.co.uk>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Tested-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-ID: <20250711204636.542964-6-mark.cave-ayland@ilande.co.uk>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
---
 hw/scsi/esp.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/hw/scsi/esp.c b/hw/scsi/esp.c
index ec9fcbeddf4..1c7bad8fc02 100644
--- a/hw/scsi/esp.c
+++ b/hw/scsi/esp.c
@@ -631,7 +631,9 @@ static void esp_do_dma(ESPState *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);
+                if (len) {
+                    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));
@@ -681,6 +683,7 @@ static void esp_do_dma(ESPState *s)
                 buf[0] = s->status;
 
                 if (s->dma_memory_write) {
+                    /* Length already non-zero */
                     s->dma_memory_write(s->dma_opaque, buf, len);
                 } else {
                     esp_fifo_push_buf(s, buf, len);
@@ -715,6 +718,7 @@ static void esp_do_dma(ESPState *s)
                 buf[0] = 0;
 
                 if (s->dma_memory_write) {
+                    /* Length already non-zero */
                     s->dma_memory_write(s->dma_opaque, buf, len);
                 } else {
                     esp_fifo_push_buf(s, buf, len);
-- 
2.49.0