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

[Jeffrey Hugo]

On Sun, Oct 20, 2019 at 12:28 AM Bjorn Andersson
<bjorn.andersson@xxxxxxxxxx> wrote:
>
> 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?

No.  I don't think it occurred to you to consider how RX_RDY_CTL
factors into this.  RX_RDY_CTL allows the hardware to enable flow when
the hardware determines it is able to receive data.  RX_RDY_CTL gets
set in mctrl, and set_termios - essentially when the uart client has
indicated flow can be enabled.

Even though the console is a 2 wire console on msm8998, the driver
will go through the same flow.  I verified that RX_RDY_CTL is set on
the console uart after boot, so we won't end up with flow blocked.

However, this does bring up another issue.  RX_RDY_CTL doesn't get
unset in msm_reset.  RESET_RX will disable the rx path in the
hardware, and prevent RX_RDY_CTL from being active, but the rx path is
enabled in set_baud_rate() before RX_RDY_CTL can be masked out in
set_termios, so there may be a small window where flow can
inadvertently enabled.  I'll spin a v2 that also unsets RX_RDY_CTL in
msm_reset().

>
> Regards,
> Bjorn
>
> > Fixes: 04896a77a97b ("msm_serial: serial driver for MSM7K onboard serial peripheral.")
> > Signed-off-by: Jeffrey Hugo <jeffrey.l.hugo@xxxxxxxxx>
> > ---
> >  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
> >