Re: [PATCH] tty: serial: qcom-geni: re-arm RX DMA on spurious zero-length interrupt

[Greg KH <greg@kroah.com>]

On Wed, Apr 22, 2026 at 04:13:03PM +0000, Vynnie Von Diktus via B4 Relay wrote:
> From: Vynnie Von Diktus <vyndiktus@gmail.com>
> 
> qcom_geni_serial_handle_rx_dma() returns early when SE_DMA_RX_LEN_IN
> reads as zero, interpreting it as a spurious interrupt. The early return
> skips geni_se_rx_dma_prep(), leaving the RX DMA descriptor permanently
> unarmed. All subsequently received bytes are silently dropped until the
> port is closed and reopened.
> 
> On cold boots, chip startup transients on the UART lines can produce a
> genuine spurious DMA interrupt with a zero-length count. The bug is
> invisible on warm reboots (the UART stays powered and stable, so no
> spurious interrupt fires), which makes it appear as an intermittent
> failure only on power-cycle reboots.
> 
> Fix by restructuring the zero-length check to fall through to
> geni_se_rx_dma_prep() in all cases. Only the data processing
> (handle_rx_uart) is skipped when no bytes arrived.
> 
> Tested on SM8150 (Snapdragon 855) with a WCN3990 BT UART — Bluetooth
> firmware download now succeeds on every cold boot without
> "Frame reassembly failed" errors.
> 
> The same GENI serial IP block is used across SDM845, SM8150, SM8250,
> SM8350 and many other Snapdragon SoCs; the bug and fix are expected to
> apply to all of them.
> 
> Signed-off-by: Vynnie Von Diktus <vyndiktus@gmail.com>
> ---
>  drivers/tty/serial/qcom_geni_serial.c | 6 ++----
>  1 file changed, 2 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/tty/serial/qcom_geni_serial.c b/drivers/tty/serial/qcom_geni_serial.c
> index 69a632fef..3c950bdc0 100644
> --- a/drivers/tty/serial/qcom_geni_serial.c
> +++ b/drivers/tty/serial/qcom_geni_serial.c
> @@ -837,11 +837,9 @@ static void qcom_geni_serial_handle_rx_dma(struct uart_port *uport, bool drop)
>  	rx_in = readl(uport->membase + SE_DMA_RX_LEN_IN);
>  	if (!rx_in) {
>  		dev_warn(uport->dev, "serial engine reports 0 RX bytes in!\n");
> -		return;
> -	}
> -
> -	if (!drop)
> +	} else if (!drop) {
>  		handle_rx_uart(uport, rx_in, drop);
> +	}
>  
>  	ret = geni_se_rx_dma_prep(&port->se, port->rx_buf,
>  				  DMA_RX_BUF_SIZE,
> 
> ---
> base-commit: 4a8d8848356e9e4c41e22de9b1ef1507ea21734a
> change-id: 20260422-qcom-geni-uart-dma-rearm-a5df83e164ff
> 
> Best regards,
> --  
> Vynnie Von Diktus <vyndiktus@gmail.com>

Does not apply to the tty-next tree :(