[PATCH 3/4] serial: qcom-geni: fix mapping of empty DMA buffer

[Johan Hovold <johan+linaro@kernel.org>]

Make sure that there is data in the ring buffer before trying to set up
a zero-length DMA transfer.

This specifically fixes the following warning when unmapping the empty
buffer on the sc8280xp-crd:

   WARNING: CPU: 0 PID: 138 at drivers/iommu/dma-iommu.c:1046 iommu_dma_unmap_page+0xbc/0xd8
   ...
   Call trace:
    iommu_dma_unmap_page+0xbc/0xd8
    dma_unmap_page_attrs+0x30/0x1c8
    geni_se_tx_dma_unprep+0x28/0x38
    qcom_geni_serial_isr+0x358/0x75c

Fixes: 2aaa43c70778 ("tty: serial: qcom-geni-serial: add support for serial engine DMA")
Cc: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>
Signed-off-by: Johan Hovold <johan+linaro@kernel.org>
---
 drivers/tty/serial/qcom_geni_serial.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/drivers/tty/serial/qcom_geni_serial.c b/drivers/tty/serial/qcom_geni_serial.c
index 2aa3872e6283..9871225b2f9b 100644
--- a/drivers/tty/serial/qcom_geni_serial.c
+++ b/drivers/tty/serial/qcom_geni_serial.c
@@ -631,6 +631,9 @@ static void qcom_geni_serial_start_tx_dma(struct uart_port *uport)
 	if (port->tx_dma_addr)
 		return;
 
+	if (uart_circ_empty(xmit))
+		return;
+
 	xmit_size = uart_circ_chars_pending(xmit);
 	if (xmit_size < WAKEUP_CHARS)
 		uart_write_wakeup(uport);
-- 
2.39.2