[PATCH 2/2] tty: serial: qcom-geni-serial: check for valid size before starting dma

[Srinivas Kandagatla]

Check if there are valid length of bytes to transfer before starting dma.

without this check we can see below kernel warning when we try to map a zero size buffers.

------------[ cut here ]------------
WARNING: CPU: 0 PID: 0 at drivers/iommu/dma-iommu.c:1046 iommu_dma_unmap_page+0xe0/0xfc
Modules linked in:
CPU: 0 PID: 0 Comm: swapper/0 Tainted: G        W          6.3.0-rc1-dirty #347
Hardware name: Qualcomm Technologies, Inc. Robotics RB5 (DT)
pstate: 804000c5 (Nzcv daIF +PAN -UAO -TCO -DIT -SSBS BTYPE=--)
pc : iommu_dma_unmap_page+0xe0/0xfc
lr : iommu_dma_unmap_page+0x38/0xfc
...
Call trace:
 iommu_dma_unmap_page+0xe0/0xfc
 dma_unmap_page_attrs+0x30/0x1ec
 geni_se_tx_dma_unprep+0x58/0x80
 qcom_geni_serial_isr+0x350/0x750
 __handle_irq_event_percpu+0x58/0x148
 handle_irq_event_percpu+0x18/0x4c
 handle_irq_event+0x48/0x88
 handle_fasteoi_irq+0xb0/0x130
 generic_handle_domain_irq+0x2c/0x44
 gic_handle_irq+0xd4/0x140
 call_on_irq_stack+0x24/0x4c
 do_interrupt_handler+0x80/0x84
 el1_interrupt+0x38/0x6c
 el1h_64_irq_handler+0x18/0x24
 el1h_64_irq+0x64/0x68
 cpuidle_enter_state+0x1e4/0x310
 cpuidle_enter+0x3c/0x54
 call_cpuidle+0x1c/0x40
 do_idle+0x204/0x260
 cpu_startup_entry+0x28/0x2c
 kernel_init+0x0/0x12c
 arch_post_acpi_subsys_init+0x0/0x8
 start_kernel+0x3cc/0x74c
 __primary_switched+0xbc/0xc4

Fixes: 2aaa43c70778 ("tty: serial: qcom-geni-serial: add support for serial engine DMA")
Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@xxxxxxxxxx>
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@xxxxxxxxxx>
---
 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 5972b5c317d3..bb63a00f4c07 100644
--- a/drivers/tty/serial/qcom_geni_serial.c
+++ b/drivers/tty/serial/qcom_geni_serial.c
@@ -637,6 +637,9 @@ static void qcom_geni_serial_start_tx_dma(struct uart_port *uport)
 
 	xmit_size = CIRC_CNT_TO_END(xmit->head, xmit->tail, UART_XMIT_SIZE);
 
+	if (!xmit_size)
+		return;
+
 	qcom_geni_serial_setup_tx(uport, xmit_size);
 
 	ret = geni_se_tx_dma_prep(&port->se, &xmit->buf[xmit->tail],
-- 
2.21.0