Re: [PATCH 1/4] serial: core: Add wakeup() and start_pending_tx() for power management

Greg Kroah-Hartman
2021-10-13T12:33:44Z

On Thu, Sep 30, 2021 at 09:29:03AM +0300, Tony Lindgren wrote:
> If the serial driver implements PM runtime with autosuspend, the port may
> be powered down on TX. To wake up the port, let's add new wakeup() call
> for serial drivers to implement as needed. We can call wakeup() from
> __uart_start() and flow control related functions before attempting to
> write to the serial port registers.
>
> Let's keep track of the serial port with a new runtime_suspended flag
> that the device driver runtime PM suspend and resume can manage with
> atomic_set(). This is because only the device driver knows what the
> device runtime PM state as in Documentation/power/runtime_pm.rst
> under "9. Autosuspend, or automatically-delayed suspend" for locking.
>
> To allow the serial port drivers to send out pending tx on runtime PM
> resume, let's add start_pending_tx() as suggested by Johan Hovold
> <johan@kernel.org>.
>
> Suggested-by: Johan Hovold <johan@kernel.org>
> Signed-off-by: Tony Lindgren <tony@atomide.com>
> ---
> Documentation/driver-api/serial/driver.rst | 9 ++++
> drivers/tty/serial/serial_core.c | 56 +++++++++++++++++++++-
> include/linux/serial_core.h | 3 ++
> 3 files changed, 66 insertions(+), 2 deletions(-)
>
> diff --git a/Documentation/driver-api/serial/driver.rst b/Documentation/driver-api/serial/driver.rst
> --- a/Documentation/driver-api/serial/driver.rst
> +++ b/Documentation/driver-api/serial/driver.rst
> @@ -234,6 +234,15 @@ hardware.
>
> Interrupts: caller dependent.
>
> + wakeup(port)
> + Wake up port if it has been runtime PM suspended.
> +
> + Locking: port->lock taken.
> +
> + Interrupts: locally disabled.
> +
> + This call must not sleep
> +
> flush_buffer(port)
> Flush any write buffers, reset any DMA state and stop any
> ongoing DMA transfers.
> diff --git a/drivers/tty/serial/serial_core.c b/drivers/tty/serial/serial_core.c
> --- a/drivers/tty/serial/serial_core.c
> +++ b/drivers/tty/serial/serial_core.c
> @@ -91,6 +91,23 @@ static inline struct uart_port *uart_port_check(struct uart_state *state)
> return state->uart_port;
> }
>
> +/*
> + * This routine can be used before register access to wake up a serial
> + * port that has been runtime PM suspended by the serial port driver.
> + * Note that the runtime_suspended flag is managed by the serial port
> + * device driver runtime PM.
> + */
> +static int uart_port_wakeup(struct uart_port *port)
> +{
> + if (!atomic_read(&port->runtime_suspended))
> + return 0;

And if the value changes right after you read this?

Why not use a real lock here? Don't use an atomic if you don't need it.

thanks,

greg k-h