Re: [PATCH v3] tty: resolve loopback wait problem for aging serial port

Re: [PATCH v3] tty: resolve loopback wait problem for aging serial port

[Greg KH]

On Wed, Dec 30, 2020 at 09:45:29PM +0800, zhangqiumiao1@huawei.com wrote:
> From: Qiumiao Zhang <zhangqiumiao1@huawei.com>
> 
> Because serial port is low-speed device, serial port writing will be suspended
> when the buffer of driver layer is full due to serial port aging.

What do you mean here exactly?  What is "serial port aging"?

And since when is a serial port a low-speed device?  Compared to what?

> The concrete
> representation is n_tty_write blocking in the process of wait_woken, the process
> of writing serial port exists without feedback, and becomes a zombie process.

I do not understand this sentance, sorry.  Can you rephrase it?

> So for serial port, set the timeout value of wait_woken function to 60s. Wake up
> and flush the buffer after timeout.

Where did the 60 second number come from?  Why that value and not any
other specific value?


> 
> Signed-off-by: Qiumiao Zhang <zhangqiumiao1@huawei.com>
> ---
> v3:
>  add changes from v1 to v2
> 
> v2:
>  change to use "real name"
>  fix wrong expression in path description
>  remove config option
>  use driver type to judge tty device type
> 
>  drivers/tty/n_tty.c | 15 ++++++++++++++-
>  1 file changed, 14 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/tty/n_tty.c b/drivers/tty/n_tty.c
> index 319d68c..8e7591a 100644
> --- a/drivers/tty/n_tty.c
> +++ b/drivers/tty/n_tty.c
> @@ -87,6 +87,8 @@
>  # define n_tty_trace(f, args...)	no_printk(f, ##args)
>  #endif
> 
> +#define SIXTY_SEC_TIMEOUT (60 * HZ)

Is that define really needed?

> +
>  struct n_tty_data {
>  	/* producer-published */
>  	size_t read_head;
> @@ -2375,7 +2377,18 @@ static ssize_t n_tty_write(struct tty_struct *tty, struct file *file,
>  		}
>  		up_read(&tty->termios_rwsem);
> 
> -		wait_woken(&wait, TASK_INTERRUPTIBLE, MAX_SCHEDULE_TIMEOUT);
> +		/* Resolve the problem of loopback waiting when the serial port is aging seriously */
> +		if (tty->driver->type == TTY_DRIVER_TYPE_SERIAL) {
> +			if (wait_woken(&wait, TASK_INTERRUPTIBLE, SIXTY_SEC_TIMEOUT) == 0)
> +				if (tty && tty->ops->flush_buffer) {
> +					printk(KERN_INFO "n_tty_write flush buffer\n");

Why have you left debugging code in here?

> +					tty->ops->flush_buffer(tty);
> +				} else {
> +					printk(KERN_ERR "tty related structure not registered\n");

What can a user do with this?  How can it be hit?

And never use printk() in a driver, please use dev_err() and friends
instead.

thanks,

greg k-h

Re: [PATCH v3] tty: resolve loopback wait problem for aging serial port

[Johan Hovold]

On Wed, Dec 30, 2020 at 02:57:14PM +0100, Greg Kroah-Hartman wrote:
> On Wed, Dec 30, 2020 at 09:45:29PM +0800, zhangqiumiao1@huawei.com wrote:
> > From: Qiumiao Zhang <zhangqiumiao1@huawei.com>
> > 
> > Because serial port is low-speed device, serial port writing will be suspended
> > when the buffer of driver layer is full due to serial port aging.
> 
> What do you mean here exactly?  What is "serial port aging"?
> 
> And since when is a serial port a low-speed device?  Compared to what?
> 
> > The concrete
> > representation is n_tty_write blocking in the process of wait_woken, the process
> > of writing serial port exists without feedback, and becomes a zombie process.
> 
> I do not understand this sentance, sorry.  Can you rephrase it?
> 
> > So for serial port, set the timeout value of wait_woken function to 60s. Wake up
> > and flush the buffer after timeout.
> 
> Where did the 60 second number come from?  Why that value and not any
> other specific value?
> 
> 
> > 
> > Signed-off-by: Qiumiao Zhang <zhangqiumiao1@huawei.com>
> > ---
> > v3:
> >  add changes from v1 to v2
> > 
> > v2:
> >  change to use "real name"
> >  fix wrong expression in path description
> >  remove config option
> >  use driver type to judge tty device type

Note that no mails from zhangqiumiao1@huawei.com appears to make it to
the lists so anyone not in CC only sees your replies, Greg. 

Qiumiao Zhang, you need to check your mail setup so that you can post to
the lists.

Johan