On Mon 21 Oct 08:46 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.
>
> Fixes: 04896a77a97b ("msm_serial: serial driver for MSM7K onboard serial peripheral.")
> Signed-off-by: Jeffrey Hugo <jeffrey.l.hugo@xxxxxxxxx>
Looks good!
Reviewed-by: Bjorn Andersson <bjorn.andersson@xxxxxxxxxx>
> ---
>
> v2:
> -mask out RX_RDY_CTL in msm_reset() to close a small race window for RFR
>
> drivers/tty/serial/msm_serial.c | 6 +++++-
> 1 file changed, 5 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/tty/serial/msm_serial.c b/drivers/tty/serial/msm_serial.c
> index 3657a24913fc..00964b6e4ac1 100644
> --- a/drivers/tty/serial/msm_serial.c
> +++ b/drivers/tty/serial/msm_serial.c
> @@ -980,6 +980,7 @@ static unsigned int msm_get_mctrl(struct uart_port *port)
> static void msm_reset(struct uart_port *port)
> {
> struct msm_port *msm_port = UART_TO_MSM(port);
> + unsigned int mr;
>
> /* reset everything */
> msm_write(port, UART_CR_CMD_RESET_RX, UART_CR);
> @@ -987,7 +988,10 @@ 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);
> + mr = msm_read(port, UART_MR1);
> + mr &= ~UART_MR1_RX_RDY_CTL;
> + msm_write(port, mr, UART_MR1);
>
> /* Disable DM modes */
> if (msm_port->is_uartdm)
> --
> 2.17.1
>
