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

[Greg Kroah-Hartman]

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@xxxxxxxxxx>.
> 
> Suggested-by: Johan Hovold <johan@xxxxxxxxxx>
> Signed-off-by: Tony Lindgren <tony@xxxxxxxxxxx>
> ---
>  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