Per https://docs.xilinx.com/r/en-US/ug1085-zynq-ultrascale-trm/Message-Format

  Message Format

  The same message format is used for RXFIFO, TXFIFO, and TXHPB.
  Each message includes four words (16 bytes). Software must read
  and write all four words regardless of the actual number of data
  bytes and valid fields in the message.

There is no mention in this reference manual about what the
hardware does when not all four words are read. To fix the
reported underflow behavior, I choose to fill the 4 frame data
registers when the first register (ID) is accessed, which is how
I expect hardware would do.

Reported-by: Qiang Liu <cyruscyliu@gmail.com>
Fixes: 98e5d7a2b7 ("hw/net/can: Introduce Xilinx ZynqMP CAN controller")
Resolves: https://gitlab.com/qemu-project/qemu/-/issues/1427
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Francisco Iglesias <francisco.iglesias@amd.com>
Reviewed-by: Vikram Garhwal <vikram.garhwal@amd.com>
---
 hw/net/can/xlnx-zynqmp-can.c | 17 +++++++++--------
 1 file changed, 9 insertions(+), 8 deletions(-)

diff --git a/hw/net/can/xlnx-zynqmp-can.c b/hw/net/can/xlnx-zynqmp-can.c
index 1f1c686479..f60e480c3a 100644
--- a/hw/net/can/xlnx-zynqmp-can.c
+++ b/hw/net/can/xlnx-zynqmp-can.c
@@ -778,14 +778,18 @@ static void update_rx_fifo(XlnxZynqMPCANState *s, const qemu_can_frame *frame)
     }
 }
 
-static uint64_t can_rxfifo_pre_read(RegisterInfo *reg, uint64_t val)
+static uint64_t can_rxfifo_post_read_id(RegisterInfo *reg, uint64_t val)
 {
     XlnxZynqMPCANState *s = XLNX_ZYNQMP_CAN(reg->opaque);
+    unsigned used = fifo32_num_used(&s->rx_fifo);
 
-    if (!fifo32_is_empty(&s->rx_fifo)) {
-        val = fifo32_pop(&s->rx_fifo);
-    } else {
+    if (used < CAN_FRAME_SIZE) {
         ARRAY_FIELD_DP32(s->regs, INTERRUPT_STATUS_REGISTER, RXUFLW, 1);
+    } else {
+        val = s->regs[R_RXFIFO_ID] = fifo32_pop(&s->rx_fifo);
+        s->regs[R_RXFIFO_DLC] = fifo32_pop(&s->rx_fifo);
+        s->regs[R_RXFIFO_DATA1] = fifo32_pop(&s->rx_fifo);
+        s->regs[R_RXFIFO_DATA2] = fifo32_pop(&s->rx_fifo);
     }
 
     can_update_irq(s);
@@ -946,14 +950,11 @@ static const RegisterAccessInfo can_regs_info[] = {
         .post_write = can_tx_post_write,
     },{ .name = "RXFIFO_ID",  .addr = A_RXFIFO_ID,
         .ro = 0xffffffff,
-        .post_read = can_rxfifo_pre_read,
+        .post_read = can_rxfifo_post_read_id,
     },{ .name = "RXFIFO_DLC",  .addr = A_RXFIFO_DLC,
         .rsvd = 0xfff0000,
-        .post_read = can_rxfifo_pre_read,
     },{ .name = "RXFIFO_DATA1",  .addr = A_RXFIFO_DATA1,
-        .post_read = can_rxfifo_pre_read,
     },{ .name = "RXFIFO_DATA2",  .addr = A_RXFIFO_DATA2,
-        .post_read = can_rxfifo_pre_read,
     },{ .name = "AFR",  .addr = A_AFR,
         .rsvd = 0xfffffff0,
         .post_write = can_filter_enable_post_write,
-- 
2.41.0