hw/dma/xlnx_csu_dma: don't throw guest errors when stopping the SRC DMA

UG1087 states for the source channel that: if SIZE is programmed to 0, and the
DMA is started, the interrupts DONE and MEM_DONE will be asserted.

This implies that it is allowed for the guest to stop the source DMA by writing
a size of 0 to the SIZE register, so remove the LOG_GUEST_ERROR in that case.

While at it remove the comment marking the SIZE register as write-only.

See: https://docs.xilinx.com/r/en-US/ug1087-zynq-ultrascale-registers/CSUDMA_SRC_SIZE-CSUDMA-Register

Signed-off-by: Frederic Konrad <fkonrad@amd.com>
Reviewed-by: Francisco Iglesias <francisco.iglesias@amd.com>
Message-id: 20231124143505.1493184-4-fkonrad@amd.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

diff --git a/hw/dma/xlnx_csu_dma.c b/hw/dma/xlnx_csu_dma.c
index 531013f..bc1505a 100644 (file)
--- a/hw/dma/xlnx_csu_dma.c
+++ b/hw/dma/xlnx_csu_dma.c
@@ -39,7 +39,7 @@
 REG32(ADDR, 0x0)
     FIELD(ADDR, ADDR, 2, 30) /* wo */
 REG32(SIZE, 0x4)
-    FIELD(SIZE, SIZE, 2, 27) /* wo */
+    FIELD(SIZE, SIZE, 2, 27)
     FIELD(SIZE, LAST_WORD, 0, 1) /* rw, only exists in SRC */
 REG32(STATUS, 0x8)
     FIELD(STATUS, DONE_CNT, 13, 3) /* wtc */
@@ -335,10 +335,14 @@ static uint64_t addr_pre_write(RegisterInfo *reg, uint64_t val)
 static uint64_t size_pre_write(RegisterInfo *reg, uint64_t val)
 {
     XlnxCSUDMA *s = XLNX_CSU_DMA(reg->opaque);
+    uint64_t size = val & R_SIZE_SIZE_MASK;
 
     if (s->regs[R_SIZE] != 0) {
-        qemu_log_mask(LOG_GUEST_ERROR,
-                      "%s: Starting DMA while already running.\n", __func__);
+        if (size || s->is_dst) {
+            qemu_log_mask(LOG_GUEST_ERROR,
+                          "%s: Starting DMA while already running.\n",
+                          __func__);
+        }
     }
 
     if (!s->is_dst) {
@@ -346,7 +350,7 @@ static uint64_t size_pre_write(RegisterInfo *reg, uint64_t val)
     }
 
     /* Size is word aligned */
-    return val & R_SIZE_SIZE_MASK;
+    return size;
 }
 
 static uint64_t size_post_read(RegisterInfo *reg, uint64_t val)