Re: [PATCH] tty: serial: msm_serial: Fix flow control

[Bjorn Andersson <bjorn.andersson@linaro.org>]

On Sat 19 Oct 14:06 PDT 2019, Jeffrey Hugo wrote:

> hci_qca interfaces to the wcn3990 via a uart_dm on the msm8998 mtp and
> Lenovo Miix 630 laptop.  As part of initializing the wcn3990, hci_qca
> disables flow, configures the uart baudrate, and then reenables flow - at
> which point an event is expected to be received over the uart from the
> wcn3990.  It is observed that this event comes after the baudrate change
> but before hci_qca re-enables flow. This is unexpected, and is a result of
> msm_reset() being broken.
> 
> According to the uart_dm hardware documentation, it is recommended that
> automatic hardware flow control be enabled by setting RX_RDY_CTL.  Auto
> hw flow control will manage RFR based on the configured watermark.  When
> there is space to receive data, the hw will assert RFR.  When the watermark
> is hit, the hw will de-assert RFR.
> 
> The hardware documentation indicates that RFR can me manually managed via
> CR when RX_RDY_CTL is not set.  SET_RFR asserts RFR, and RESET_RFR
> de-asserts RFR.
> 
> msm_reset() is broken because after resetting the hardware, it
> unconditionally asserts RFR via SET_RFR.  This enables flow regardless of
> the current configuration, and would undo a previous flow disable
> operation.  It should instead de-assert RFR via RESET_RFR to block flow
> until the hardware is reconfigured.  msm_serial should rely on the client
> to specify that flow should be enabled, either via mctrl() or the termios
> structure, and only assert RFR in response to those triggers.
> 

I traced msm_reset() and msm_set_mctrl() and I get the following:
       swapper/0-1     [000] d..1     3.091726: msm_set_mctrl: msm_set_mctrl() reset rfr
       swapper/0-1     [000] d..1     3.117046: msm_set_mctrl: msm_set_mctrl() reset rfr
       swapper/0-1     [000] d..1     3.125484: msm_set_termios: msm_reset() set rfr
       swapper/0-1     [003] d..1     4.491430: msm_set_termios: msm_reset() set rfr
       swapper/0-1     [003] d..1     4.491733: msm_set_mctrl: msm_set_mctrl() auto rfr
           login-313   [001] d..1    78.010785: msm_set_mctrl: msm_set_mctrl() reset rfr
           login-313   [001] d..1    78.011007: msm_set_termios: msm_reset() set rfr
           login-313   [001] d..1    78.011021: msm_set_mctrl: msm_set_mctrl() auto rfr
           login-313   [001] d..1    78.063330: msm_set_termios: msm_reset() set rfr
           login-313   [001] d..1    78.063641: msm_set_termios: msm_reset() set rfr

So while your change does make sense for BT, wouldn't this mean that
with your patch and a 4-pin UART for the console I would end this dance
with receive flow blocked?

Regards,
Bjorn

> Fixes: 04896a77a97b ("msm_serial: serial driver for MSM7K onboard serial peripheral.")
> Signed-off-by: Jeffrey Hugo <jeffrey.l.hugo@gmail.com>
> ---
>  drivers/tty/serial/msm_serial.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/tty/serial/msm_serial.c b/drivers/tty/serial/msm_serial.c
> index 3657a24913fc..aedabf7646f1 100644
> --- a/drivers/tty/serial/msm_serial.c
> +++ b/drivers/tty/serial/msm_serial.c
> @@ -987,7 +987,7 @@ static void msm_reset(struct uart_port *port)
>  	msm_write(port, UART_CR_CMD_RESET_ERR, UART_CR);
>  	msm_write(port, UART_CR_CMD_RESET_BREAK_INT, UART_CR);
>  	msm_write(port, UART_CR_CMD_RESET_CTS, UART_CR);
> -	msm_write(port, UART_CR_CMD_SET_RFR, UART_CR);
> +	msm_write(port, UART_CR_CMD_RESET_RFR, UART_CR);
>  
>  	/* Disable DM modes */
>  	if (msm_port->is_uartdm)
> -- 
> 2.17.1
>