[v3,2/7] serial: qcom-geni: Fix the timeout in qcom_geni_serial_poll_bit()

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.

Rather than fix the calculation, replace it with the core-provided
uart_fifo_timeout() function.

NOTE: during earlycon, uart_fifo_timeout() has the same limitations
about not being able to figure out the exact timeout that the old
function did. Luckily uart_fifo_timeout() returns the same default
timeout of 20ms in this case. We'll add a comment about it, though, to
make it more obvious what's happening.

Fixes: c4f528795d1a ("tty: serial: msm_geni_serial: Add serial driver support for GENI based QUP")
Suggested-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Signed-off-by: Douglas Anderson <dianders@chromium.org>
---

Changes in v3:
- Use uart_fifo_timeout() for timeout.

Changes in v2:
- New

 drivers/tty/serial/qcom_geni_serial.c | 37 +++++++++++++--------------
 1 file changed, 18 insertions(+), 19 deletions(-)

On Tue, 4 Jun 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.
> 
> Rather than fix the calculation, replace it with the core-provided
> uart_fifo_timeout() function.
> 
> NOTE: during earlycon, uart_fifo_timeout() has the same limitations
> about not being able to figure out the exact timeout that the old
> function did. Luckily uart_fifo_timeout() returns the same default
> timeout of 20ms in this case. We'll add a comment about it, though, to
> make it more obvious what's happening.
> 
> Fixes: c4f528795d1a ("tty: serial: msm_geni_serial: Add serial driver support for GENI based QUP")
> Suggested-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
> Signed-off-by: Douglas Anderson <dianders@chromium.org>
> ---
> 
> Changes in v3:
> - Use uart_fifo_timeout() for timeout.
> 
> Changes in v2:
> - New
> 
>  drivers/tty/serial/qcom_geni_serial.c | 37 +++++++++++++--------------
>  1 file changed, 18 insertions(+), 19 deletions(-)
> 
> diff --git a/drivers/tty/serial/qcom_geni_serial.c b/drivers/tty/serial/qcom_geni_serial.c
> index 2bd25afe0d92..a48a15c2555e 100644
> --- a/drivers/tty/serial/qcom_geni_serial.c
> +++ b/drivers/tty/serial/qcom_geni_serial.c
> @@ -124,7 +124,6 @@ struct qcom_geni_serial_port {
>  	dma_addr_t tx_dma_addr;
>  	dma_addr_t rx_dma_addr;
>  	bool setup;
> -	unsigned int baud;
>  	unsigned long clk_rate;
>  	void *rx_buf;
>  	u32 loopback;
> @@ -269,24 +268,25 @@ static bool qcom_geni_serial_poll_bit(struct uart_port *uport,
>  				int offset, int field, bool set)
>  {
>  	u32 reg;
> -	struct qcom_geni_serial_port *port;
> -	unsigned int baud;
> -	unsigned int fifo_bits;
> -	unsigned long timeout_us = 20000;
> -	struct qcom_geni_private_data *private_data = uport->private_data;
> +	unsigned long timeout_us;
>  
> -	if (private_data->drv) {
> -		port = to_dev_port(uport);
> -		baud = port->baud;
> -		if (!baud)
> -			baud = 115200;
> -		fifo_bits = port->tx_fifo_depth * port->tx_fifo_width;
> -		/*
> -		 * Total polling iterations based on FIFO worth of bytes to be
> -		 * sent at current baud. Add a little fluff to the wait.
> -		 */
> -		timeout_us = ((fifo_bits * USEC_PER_SEC) / baud) + 500;
> -	}
> +	/*
> +	 * This function is used to poll bits, some of which (like CMD_DONE)
> +	 * might take as long as it takes for the FIFO plus the temp register
> +	 * on the geni side to drain. The Linux core calculates such a timeout
> +	 * for us and we can get it from uart_fifo_timeout().
> +	 *
> +	 * It should be noted that during earlycon the variables that
> +	 * uart_fifo_timeout() makes use of in "uport" may not be setup yet.
> +	 * It's difficult to set things up for earlycon since it can't
> +	 * necessarily figure out the baud rate and reading the FIFO depth
> +	 * from the wrapper means some extra MMIO maps that we don't get by
> +	 * default. This isn't a big problem, though, since uart_fifo_timeout()
> +	 * gives back its "slop" of 20ms as a minimum and that should be
> +	 * plenty of time for earlycon unless we're running at an extremely
> +	 * low baud rate.
> +	 */
> +	timeout_us = jiffies_to_usecs(uart_fifo_timeout(uport));

Hi,

While this is not exactly incorrect, the back and forth conversions nsecs 
-> jiffies -> usecs feels somewhat odd, perhaps reworking 
uart_fifo_timeout()'s return type from jiffies to e.g. usecs would be 
preferrable. As is, the jiffies as its return type seems a small obstacle 
for using uart_fifo_timeout() which has come up in other contexts too.

> @@ -1224,7 +1224,6 @@ static void qcom_geni_serial_set_termios(struct uart_port *uport,
>  	qcom_geni_serial_stop_rx(uport);
>  	/* baud rate */
>  	baud = uart_get_baud_rate(uport, termios, old, 300, 4000000);
> -	port->baud = baud;

It's always nice to see this kind of cache variable removed, good work. :-)