Re: [PATCH v2 2/7] serial: qcom-geni: Fix the timeout in qcom_geni_serial_poll_bit()

["Ilpo Järvinen" <ilpo.jarvinen@linux.intel.com>]

On Thu, 30 May 2024, Douglas Anderson wrote:

> The qcom_geni_serial_poll_bit() is supposed to be able to be used to
> poll a bit that's will become set when a TX transfer finishes. Because
> of this it tries to set its timeout based on how long the UART will
> take to shift out all of the queued bytes. There are two problems
> here:
> 1. There appears to be a hidden extra word on the firmware side which
>    is the word that the firmware has already taken out of the FIFO and
>    is currently shifting out. We need to account for this.
> 2. The timeout calculation was assuming that it would only need 8 bits
>    on the wire to shift out 1 byte. This isn't true. Typically 10 bits
>    are used (8 data bits, 1 start and 1 stop bit), but as much as 13
>    bits could be used (14 if we allowed 9 bits per byte, which we
>    don't).
> 
> The too-short timeout was seen causing problems in a future patch
> which more properly waited for bytes to transfer out of the UART
> before cancelling.
> 
> Fixes: c4f528795d1a ("tty: serial: msm_geni_serial: Add serial driver support for GENI based QUP")
> Signed-off-by: Douglas Anderson <dianders@chromium.org>
> ---
> 
> Changes in v2:
> - New
> 
>  drivers/tty/serial/qcom_geni_serial.c | 32 ++++++++++++++++++++++++---
>  1 file changed, 29 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/tty/serial/qcom_geni_serial.c b/drivers/tty/serial/qcom_geni_serial.c
> index 2bd25afe0d92..32e025705f99 100644
> --- a/drivers/tty/serial/qcom_geni_serial.c
> +++ b/drivers/tty/serial/qcom_geni_serial.c
> @@ -271,7 +271,8 @@ static bool qcom_geni_serial_poll_bit(struct uart_port *uport,
>  	u32 reg;
>  	struct qcom_geni_serial_port *port;
>  	unsigned int baud;
> -	unsigned int fifo_bits;
> +	unsigned int max_queued_bytes;
> +	unsigned int max_queued_bits;
>  	unsigned long timeout_us = 20000;
>  	struct qcom_geni_private_data *private_data = uport->private_data;
>  
> @@ -280,12 +281,37 @@ static bool qcom_geni_serial_poll_bit(struct uart_port *uport,
>  		baud = port->baud;
>  		if (!baud)
>  			baud = 115200;
> -		fifo_bits = port->tx_fifo_depth * port->tx_fifo_width;
> +
> +		/*
> +		 * Add 1 to tx_fifo_depth to account for the hidden register
> +		 * on the firmware side that can hold a word.
> +		 */
> +		max_queued_bytes =
> +			DIV_ROUND_UP((port->tx_fifo_depth + 1) * port->tx_fifo_width,
> +				     BITS_PER_BYTE);
> +
> +		/*
> +		 * The maximum number of bits per byte on the wire is 13 from:
> +		 * - 1 start bit
> +		 * - 8 data bits
> +		 * - 1 parity bit
> +		 * - 3 stop bits
> +		 *
> +		 * While we could try count the actual bits per byte based on
> +		 * the port configuration, this is a rough timeout anyway so
> +		 * using the max is fine.
> +		 */
> +		max_queued_bits = max_queued_bytes * 13;
> +
>  		/*
>  		 * Total polling iterations based on FIFO worth of bytes to be
>  		 * sent at current baud. Add a little fluff to the wait.
> +		 *
> +		 * NOTE: this assumes that flow control isn't used, but with
> +		 * flow control we could wait indefinitely and that wouldn't
> +		 * be OK.
>  		 */
> -		timeout_us = ((fifo_bits * USEC_PER_SEC) / baud) + 500;
> +		timeout_us = ((max_queued_bits * USEC_PER_SEC) / baud) + 500;

You should try to generalize the existing uart_fifo_timeout() to suit what 
you're trying to do here instead of writing more variants of code with 
this same intent.

-- 
 i.