From: Philippe Mathieu-Daudé <philmd@linaro.org>

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 written. To fix the
reported underflow behavior when DATA2 register is written,
I choose to fill the data with the previous content of the
ID / DLC / DATA1 registers, which is how I expect hardware
would do.

Note there is no hardware flag raised under such condition.

Reported-by: Qiang Liu <cyruscyliu@gmail.com>
Reviewed-by: Francisco Iglesias <francisco.iglesias@amd.com>
Reviewed-by: Vikram Garhwal <vikram.garhwal@amd.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-id: 20231124183325.95392-2-philmd@linaro.org
Fixes: 98e5d7a2b7 ("hw/net/can: Introduce Xilinx ZynqMP CAN controller")
Resolves: https://gitlab.com/qemu-project/qemu/-/issues/1425
Reviewed-by: Francisco Iglesias <francisco.iglesias@amd.com>
Reviewed-by: Vikram Garhwal <vikram.garhwal@amd.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
---
 hw/net/can/xlnx-zynqmp-can.c | 50 +++++++++++++++++++++++++++++++++---
 1 file changed, 47 insertions(+), 3 deletions(-)

diff --git a/hw/net/can/xlnx-zynqmp-can.c b/hw/net/can/xlnx-zynqmp-can.c
index e93e6c5e194..1f1c686479c 100644
--- a/hw/net/can/xlnx-zynqmp-can.c
+++ b/hw/net/can/xlnx-zynqmp-can.c
@@ -434,6 +434,52 @@ static bool tx_ready_check(XlnxZynqMPCANState *s)
     return true;
 }
 
+static void read_tx_frame(XlnxZynqMPCANState *s, Fifo32 *fifo, uint32_t *data)
+{
+    unsigned used = fifo32_num_used(fifo);
+    bool is_txhpb = fifo == &s->txhpb_fifo;
+
+    assert(used > 0);
+    used %= CAN_FRAME_SIZE;
+
+    /*
+     * Frame Message Format
+     *
+     * Each frame 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.
+     * If software misbehave (not writing all four words), we use the previous
+     * registers content to initialize each missing word.
+     *
+     * If used is 1 then ID, DLC and DATA1 are missing.
+     * if used is 2 then ID and DLC are missing.
+     * if used is 3 then only ID is missing.
+     */
+     if (used > 0) {
+        data[0] = s->regs[is_txhpb ? R_TXHPB_ID : R_TXFIFO_ID];
+    } else {
+        data[0] = fifo32_pop(fifo);
+    }
+    if (used == 1 || used == 2) {
+        data[1] = s->regs[is_txhpb ? R_TXHPB_DLC : R_TXFIFO_DLC];
+    } else {
+        data[1] = fifo32_pop(fifo);
+    }
+    if (used == 1) {
+        data[2] = s->regs[is_txhpb ? R_TXHPB_DATA1 : R_TXFIFO_DATA1];
+    } else {
+        data[2] = fifo32_pop(fifo);
+    }
+    /* DATA2 triggered the transfer thus is always available */
+    data[3] = fifo32_pop(fifo);
+
+    if (used) {
+        qemu_log_mask(LOG_GUEST_ERROR,
+                      "%s: Incomplete CAN frame (only %u/%u slots used)\n",
+                      TYPE_XLNX_ZYNQMP_CAN, used, CAN_FRAME_SIZE);
+    }
+}
+
 static void transfer_fifo(XlnxZynqMPCANState *s, Fifo32 *fifo)
 {
     qemu_can_frame frame;
@@ -451,9 +497,7 @@ static void transfer_fifo(XlnxZynqMPCANState *s, Fifo32 *fifo)
     }
 
     while (!fifo32_is_empty(fifo)) {
-        for (i = 0; i < CAN_FRAME_SIZE; i++) {
-            data[i] = fifo32_pop(fifo);
-        }
+        read_tx_frame(s, fifo, data);
 
         if (ARRAY_FIELD_EX32(s->regs, STATUS_REGISTER, LBACK)) {
             /*
-- 
2.34.1