serial: port: Don't suspend if the port is still busy

From: Yicong Yang <yangyicong@hisilicon.com>

We accidently met the issue that the bash prompt is not shown after the
previous command done and until the next input if there's only one CPU
(In our issue other CPUs are isolated by isolcpus=). Further analysis
shows it's because the port entering runtime suspend even if there's
still pending chars in the buffer and the pending chars will only be
processed in next device resuming. We are using amba-pl011 and the
problematic flow is like below:

Bash                                         kworker
tty_write()
  file_tty_write()
    n_tty_write()
      uart_write()
        __uart_start()
          pm_runtime_get() // wakeup waker
            queue_work()
                                             pm_runtime_work()
                                               rpm_resume()
                                                status = RPM_RESUMING
                                                serial_port_runtime_resume()
                                                  port->ops->start_tx()
                                                    pl011_tx_chars()
                                                      uart_write_wakeup()
        […]
        __uart_start()
          pm_runtime_get() < 0 // because runtime status = RPM_RESUMING
                               // later data are not commit to the port driver
                                                status = RPM_ACTIVE
                                                rpm_idle() -> rpm_suspend()

This patch tries to fix this by checking the port busy before entering
runtime suspending. A runtime_suspend callback is added for the port
driver. When entering runtime suspend the callback is invoked, if there's
still pending chars in the buffer then request an runtime resume for
handling this.

Cc: Tony Lindgren <tony@atomide.com>
Signed-off-by: Yicong Yang <yangyicong@hisilicon.com>
---
 drivers/tty/serial/serial_port.c | 22 +++++++++++++++++++++-
 1 file changed, 21 insertions(+), 1 deletion(-)

On 2024/2/5 14:51, Tony Lindgren wrote:
> * Yicong Yang <yangyicong@huawei.com> [240204 03:24]:
>> From: Yicong Yang <yangyicong@hisilicon.com>
>>
>> We accidently met the issue that the bash prompt is not shown after the
>> previous command done and until the next input if there's only one CPU
>> (In our issue other CPUs are isolated by isolcpus=). Further analysis
>> shows it's because the port entering runtime suspend even if there's
>> still pending chars in the buffer and the pending chars will only be
>> processed in next device resuming. We are using amba-pl011 and the
>> problematic flow is like below:
>>
>> Bash                                         kworker
>> tty_write()
>>   file_tty_write()
>>     n_tty_write()
>>       uart_write()
>>         __uart_start()
>>           pm_runtime_get() // wakeup waker
>>             queue_work()
>>                                              pm_runtime_work()
>>                                                rpm_resume()
>>                                                 status = RPM_RESUMING
>>                                                 serial_port_runtime_resume()
>>                                                   port->ops->start_tx()
>>                                                     pl011_tx_chars()
>>                                                       uart_write_wakeup()
>>         […]
>>         __uart_start()
>>           pm_runtime_get() < 0 // because runtime status = RPM_RESUMING
>>                                // later data are not commit to the port driver
>>                                                 status = RPM_ACTIVE
>>                                                 rpm_idle() -> rpm_suspend()
> 
> Can you please confirm if this still happens also with commit 6f699743aebf
> ("serial: core: Fix runtime PM handling for pending tx")? It adds a check
> for -EINPROGRESS.

Tested nagetive on latest v6.8-rc3. Paste the current code snippet below in __uart_start():

	/* Increment the runtime PM usage count for the active check below */
	err = pm_runtime_get(&port_dev->dev);
	if (err < 0 && err != -EINPROGRESS) {
		pm_runtime_put_noidle(&port_dev->dev);
		return;
	}

	/*
	 * Start TX if enabled, and kick runtime PM. If the device is not
	 * enabled, serial_port_runtime_resume() calls start_tx() again
	 * after enabling the device.
	 */
	if (pm_runtime_active(&port_dev->dev))
		port->ops->start_tx(port);

In our issue case, the dev->power.runtime_status == RPM_RESUMING as analyzed in
commit. So we cannot pass the pm_runtime_active() check and the chars will still
be pending.

> 
>> This patch tries to fix this by checking the port busy before entering
>> runtime suspending. A runtime_suspend callback is added for the port
>> driver. When entering runtime suspend the callback is invoked, if there's
>> still pending chars in the buffer then request an runtime resume for
>> handling this.
> 
> OK. Let's look at this further after you have checked what happens with
> commit 6f699743aebf.
> 
> If needed, to me it seems that flushing tx and returning -EBUSY from
> serial_port_runtime_suspend() if busy might do the trick though.
> 

Do you mean something like below?

static int serial_port_runtime_suspend(struct device *dev)
{
	struct serial_port_device *port_dev = to_serial_base_port_device(dev);
	struct uart_port *port;
	unsigned long flags;
	int ret = 0;

	port = port_dev->port;

	if (port->flags & UPF_DEAD)
		return ret;

	spin_lock_irqsave(&port->lock, flags);
	if (__serial_port_busy(port)) {
		port->ops->start_tx(port);
		ret = -EBUSY;
	}
	spin_unlock_irqrestore(&port->lock, flags);

	return ret;
}

If so will the port fail to suspend after flushing the pending chars? Considering
underlay driver like amba-pl011 doesn't implement runtime power management, does
anyone will get the port into suspend routine later? I'm not quite sure about it.

In the patch's implementation the pending chars will be flushed in runtime_resume()
callback and rpm_resume() will try to call rpm_idle() later.

Thanks.

* Yicong Yang <yangyicong@huawei.com> [240205 08:55]:
> On 2024/2/5 14:51, Tony Lindgren wrote:
> > Can you please confirm if this still happens also with commit 6f699743aebf
> > ("serial: core: Fix runtime PM handling for pending tx")? It adds a check
> > for -EINPROGRESS.
> 
> Tested nagetive on latest v6.8-rc3. Paste the current code snippet below in __uart_start():

OK thanks for confirming it.

> In our issue case, the dev->power.runtime_status == RPM_RESUMING as analyzed in
> commit. So we cannot pass the pm_runtime_active() check and the chars will still
> be pending.

OK

> Do you mean something like below?
> 
> static int serial_port_runtime_suspend(struct device *dev)
> {
> 	struct serial_port_device *port_dev = to_serial_base_port_device(dev);
> 	struct uart_port *port;
> 	unsigned long flags;
> 	int ret = 0;
> 
> 	port = port_dev->port;
> 
> 	if (port->flags & UPF_DEAD)
> 		return ret;
> 
> 	spin_lock_irqsave(&port->lock, flags);
> 	if (__serial_port_busy(port)) {
> 		port->ops->start_tx(port);
> 		ret = -EBUSY;
> 	}
> 	spin_unlock_irqrestore(&port->lock, flags);
> 
> 	return ret;
> }

Yes the above should work.

> If so will the port fail to suspend after flushing the pending chars? Considering
> underlay driver like amba-pl011 doesn't implement runtime power management, does
> anyone will get the port into suspend routine later? I'm not quite sure about it.

Hmm yeah you may need to also call pm_runtime_mark_last_busy() to
ensure the port gets idled later on. Not sure if PM runtime core does that for
you on returning -EBUSY, worth checking it :)

The PM runtime hierarchy will block the serial port controller driver from
suspending, so the port drivers won't runtime suspend.

> In the patch's implementation the pending chars will be flushed in runtime_resume()
> callback and rpm_resume() will try to call rpm_idle() later.

On serial_port_runtime_suspend() the serial port controller will be active, so
you can call start_tx() directly.

Regards,

Tony

On 2024/2/5 17:07, Tony Lindgren wrote:
> * Yicong Yang <yangyicong@huawei.com> [240205 08:55]:
>> On 2024/2/5 14:51, Tony Lindgren wrote:
>>> Can you please confirm if this still happens also with commit 6f699743aebf
>>> ("serial: core: Fix runtime PM handling for pending tx")? It adds a check
>>> for -EINPROGRESS.
>>
>> Tested nagetive on latest v6.8-rc3. Paste the current code snippet below in __uart_start():
> 
> OK thanks for confirming it.
> 
>> In our issue case, the dev->power.runtime_status == RPM_RESUMING as analyzed in
>> commit. So we cannot pass the pm_runtime_active() check and the chars will still
>> be pending.
> 
> OK
> 
>> Do you mean something like below?
>>
>> static int serial_port_runtime_suspend(struct device *dev)
>> {
>> 	struct serial_port_device *port_dev = to_serial_base_port_device(dev);
>> 	struct uart_port *port;
>> 	unsigned long flags;
>> 	int ret = 0;
>>
>> 	port = port_dev->port;
>>
>> 	if (port->flags & UPF_DEAD)
>> 		return ret;
>>
>> 	spin_lock_irqsave(&port->lock, flags);
>> 	if (__serial_port_busy(port)) {
>> 		port->ops->start_tx(port);
>> 		ret = -EBUSY;
>> 	}
>> 	spin_unlock_irqrestore(&port->lock, flags);
>>
>> 	return ret;
>> }
> 
> Yes the above should work.
> 
>> If so will the port fail to suspend after flushing the pending chars? Considering
>> underlay driver like amba-pl011 doesn't implement runtime power management, does
>> anyone will get the port into suspend routine later? I'm not quite sure about it.
> 
> Hmm yeah you may need to also call pm_runtime_mark_last_busy() to
> ensure the port gets idled later on. Not sure if PM runtime core does that for
> you on returning -EBUSY, worth checking it :)

In this if the runtime_suspend() callback return -EBUSY, rpm core will try to
repeat to try to suspend the device. So this shall be ok.

So I respin a v2 as suggested:
https://lore.kernel.org/all/20240206073322.5560-1-yangyicong@huawei.com/

> 
> The PM runtime hierarchy will block the serial port controller driver from
> suspending, so the port drivers won't runtime suspend.
> 
>> In the patch's implementation the pending chars will be flushed in runtime_resume()
>> callback and rpm_resume() will try to call rpm_idle() later.
> 
> On serial_port_runtime_suspend() the serial port controller will be active, so
> you can call start_tx() directly.
> 
> Regards,
> 
> Tony
> 
> .
>