Re: [PATCH 1/2] hw/display/xlnx_dp.c: Don't abort on AUX FIFO overrun/underrun

["Edgar E. Iglesias" <edgar.iglesias@gmail.com>]

On Thu, Nov 06, 2025 at 02:52:08PM +0000, Peter Maydell wrote:
> The documentation of the Xilinx DisplayPort subsystem at
> https://www.xilinx.com/support/documents/ip_documentation/v_dp_txss1/v3_1/pg299-v-dp-txss1.pdf
> doesn't say what happens if a guest tries to issue an AUX write
> command with a length greater than the amount of data in the AUX
> write FIFO, or tries to write more data to the write FIFO than it can
> hold, or issues multiple commands that put data into the AUX read
> FIFO without reading it such that it overflows.
> 
> Currently QEMU will abort() in these guest-error situations, either
> in xlnx_dp.c itself or in the fifo8 code.  Make these cases all be
> logged as guest errors instead.  We choose to ignore the new data on
> overflow, and return 0 on underflow. This is in line with how we handled
> the "read from empty RX FIFO" case in commit a09ef5040477.
> 
> Cc: qemu-stable@nongnu.org
> Resolves: https://gitlab.com/qemu-project/qemu/-/issues/1418
> Resolves: https://gitlab.com/qemu-project/qemu/-/issues/1419
> Resolves: https://gitlab.com/qemu-project/qemu/-/issues/1424
> Signed-off-by: Peter Maydell <peter.maydell@linaro.org>


Reviewed-by: Edgar E. Iglesias <edgar.iglesias@amd.com>



> ---
>  hw/display/xlnx_dp.c | 28 ++++++++++++++++++++++++++--
>  1 file changed, 26 insertions(+), 2 deletions(-)
> 
> diff --git a/hw/display/xlnx_dp.c b/hw/display/xlnx_dp.c
> index 96cbb1b3a7d..c2bf692e7b1 100644
> --- a/hw/display/xlnx_dp.c
> +++ b/hw/display/xlnx_dp.c
> @@ -435,7 +435,18 @@ static void xlnx_dp_aux_clear_rx_fifo(XlnxDPState *s)
>  
>  static void xlnx_dp_aux_push_rx_fifo(XlnxDPState *s, uint8_t *buf, size_t len)
>  {
> +    size_t avail = fifo8_num_free(&s->rx_fifo);
>      DPRINTF("Push %u data in rx_fifo\n", (unsigned)len);
> +    if (len > avail) {
> +        /*
> +         * Data sheet doesn't specify behaviour here: we choose to ignore
> +         * the excess data.
> +         */
> +        qemu_log_mask(LOG_GUEST_ERROR,
> +                      "%s: ignoring %zu bytes pushed to full RX_FIFO\n",
> +                      __func__, len - avail);
> +        len = avail;
> +    }
>      fifo8_push_all(&s->rx_fifo, buf, len);
>  }
>  
> @@ -466,7 +477,18 @@ static void xlnx_dp_aux_clear_tx_fifo(XlnxDPState *s)
>  
>  static void xlnx_dp_aux_push_tx_fifo(XlnxDPState *s, uint8_t *buf, size_t len)
>  {
> +    size_t avail = fifo8_num_free(&s->tx_fifo);
>      DPRINTF("Push %u data in tx_fifo\n", (unsigned)len);
> +    if (len > avail) {
> +        /*
> +         * Data sheet doesn't specify behaviour here: we choose to ignore
> +         * the excess data.
> +         */
> +        qemu_log_mask(LOG_GUEST_ERROR,
> +                      "%s: ignoring %zu bytes pushed to full TX_FIFO\n",
> +                      __func__, len - avail);
> +        len = avail;
> +    }
>      fifo8_push_all(&s->tx_fifo, buf, len);
>  }
>  
> @@ -475,8 +497,10 @@ static uint8_t xlnx_dp_aux_pop_tx_fifo(XlnxDPState *s)
>      uint8_t ret;
>  
>      if (fifo8_is_empty(&s->tx_fifo)) {
> -        error_report("%s: TX_FIFO underflow", __func__);
> -        abort();
> +        /* Data sheet doesn't specify behaviour here: we choose to return 0 */
> +        qemu_log_mask(LOG_GUEST_ERROR, "%s: attempt to read empty TX_FIFO\n",
> +                      __func__);
> +        return 0;
>      }
>      ret = fifo8_pop(&s->tx_fifo);
>      DPRINTF("pop 0x%2.2X from tx_fifo.\n", ret);
> -- 
> 2.43.0
>