Re: [PATCH 1/2] serial: port: Don't block system suspend even if bytes are left to xmit

[Stephen Boyd]

Quoting Douglas Anderson (2024-05-23 16:22:12)
> Recently, suspend testing on sc7180-trogdor based devices has started
> to sometimes fail with messages like this:
>
>   port a88000.serial:0.0: PM: calling pm_runtime_force_suspend+0x0/0xf8 @ 28934, parent: a88000.serial:0
>   port a88000.serial:0.0: PM: dpm_run_callback(): pm_runtime_force_suspend+0x0/0xf8 returns -16
>   port a88000.serial:0.0: PM: pm_runtime_force_suspend+0x0/0xf8 returned -16 after 33 usecs
>   port a88000.serial:0.0: PM: failed to suspend: error -16
>
> I could reproduce these problem by logging in via an agetty on the
> debug serial port (which was _not_ used for kernel console) and
> running:
>   cat /var/log/messages
> ...and then (via an SSH session) forcing a few suspend/resume cycles.
>
> Tracing through the code and doing some printf debugging shows that
> the -16 (-EBUSY) comes from the recently added
> serial_port_runtime_suspend().
>
> The idea of the serial_port_runtime_suspend() function is to prevent
> the port from being _runtime_ suspended if it still has bytes left to
> transmit. Having bytes left to transmit isn't a reason to block
> _system_ suspend, though.

Can you elaborate? I paused to think that maybe we would want to make
sure that everything that was transmitted had been transmitted but that
doesn't seem right because it's a problem for higher layers to solve,
e.g. serdev would want to make sure some sleep command sent over the
wire actually got sent.

> The DEFINE_RUNTIME_DEV_PM_OPS() used by the
> serial_port code means that the system suspend function will be
> pm_runtime_force_suspend(). In pm_runtime_force_suspend() we can see
> that before calling the runtime suspend function we'll call
> pm_runtime_disable(). This should be a reliable way to detect that
> we're called from system suspend and that we shouldn't look for
> busyness.
>
> Fixes: 43066e32227e ("serial: port: Don't suspend if the port is still busy")
> Signed-off-by: Douglas Anderson <dianders@xxxxxxxxxxxx>
> ---
>
>  drivers/tty/serial/serial_port.c | 10 ++++++++++
>  1 file changed, 10 insertions(+)
>
> diff --git a/drivers/tty/serial/serial_port.c b/drivers/tty/serial/serial_port.c
> index 91a338d3cb34..b781227cc996 100644
> --- a/drivers/tty/serial/serial_port.c
> +++ b/drivers/tty/serial/serial_port.c
> @@ -64,6 +64,16 @@ static int serial_port_runtime_suspend(struct device *dev)
>         if (port->flags & UPF_DEAD)
>                 return 0;
>
> +       /*
> +        * We only want to check the busyness of the port if PM Runtime is
> +        * enabled. Specifically PM Runtime will be disabled by
> +        * pm_runtime_force_suspend() during system suspend and we don't want
> +        * to block system suspend even if there is data still left to
> +        * transmit. We only want to block regulator PM Runtime transitions.

s/regulator/regular/

Is this a typo? Also, why is "runtime" capitalized?

> +        */
> +       if (!pm_runtime_enabled(dev))
> +               return 0;