Re: [PATCH 2/4] serial: 8250: Implement wakeup for TX and use it for 8250_omap

On Thu, Sep 30, 2021 at 9:30 AM Tony Lindgren [off-list ref] wrote:

We can use the wakeup() and uart_start_pending_tx() calls to wake up an
idle serial port and send out the pending TX buffer on runtime PM resume.

This allows us to remove the depedency to pm_runtime_irq_safe() for

dependency

8250_omap driver in the following patches.

We manage the port runtime_suspended flag in the serial port driver as
only the driver knows when the hardware is runtime PM suspended. Note that
The current flag for rpm_tx_active cannot be used as it is TX specific
for 8250_port.

We already have serial8250_start_tx() call serial8250_rpm_get_tx(), and
serial8250_stop_tx() call serial8250_rpm_put_tx() to take care of the
runtime PM usage count for TX. To have the serial port driver call
uart_start_pending_tx() on runtime resume, we must now use just
pm_runtime_get() for serial8250_start_tx() instead of the sync version.

With these changes we must now also flip 8250_omap driver over to call
uart_start_pending_tx(). That's currently the only user of UART_CAP_RPM.

Do I understand the flow correctly:
 1) if we suspended, we request resume
 2) until resume is not fulfilled we return error code to user space
to try again
?

In this case we have no register access to the powered off device and
ACPI, for example, may have a chance to resume the device in a
non-atomic way. Is this the correct interpretation?

-- 
With Best Regards,
Andy Shevchenko