Re: [RFC] With 8250 Designware UART, if writes to the LCR failed the kernel will hung up

[Peter Hurley <peter@hurleysoftware.com>]

Hi Zhang,

On 03/06/2015 04:11 AM, Zhang Zhen wrote:
> Hi,
> 
> 	I'm testing 4.0-rc1 kernel on my board with 8250 Designware UART.(ARM Cortex-a15 single core).
> 	
> 	I found if serial is busy and writes to the LCR failed after tried 1000 times.
> 	The kernel will hung up.
> 
> 	The system boot success after changed from:
> 
>  95 static void dw8250_serial_out(struct uart_port *p, int offset, int value)
>  96 {
>  97         struct dw8250_data *d = p->private_data;
>  98
> ...
> ...
> 112                         writeb(value, p->membase + (UART_LCR << p->regshift));
> 113                 }
> 114                 dev_err(p->dev, "Couldn't set LCR to %d\n", value);
> 115         }
> 116 }
> 
> 	to:
> 
>  95 static void dw8250_serial_out(struct uart_port *p, int offset, int value)
>  96 {
>  97         struct dw8250_data *d = p->private_data;
>  98
> ...
> ...
> 112                         writeb(value, p->membase + (UART_LCR << p->regshift));
> 113                 }
> 114                 dev_info(p->dev, "Couldn't set LCR to %d\n", value);            //changed here
> 115         }
> 116 }
> 
> 	The reason is serial8250_console_write can't get port->lock because serial8250_do_set_termios has
> 	got port->lock.
> 	So i think here we should change from dev_err to dev_info ?

That's not really going to help because this will still hang if the
console_loglevel is set to < KERN_INFO.

> 	Any suggestions are welcome.

Check that the port is not the uart_console() before logging the error,
like;

	if (!uart_console(p))
		dev_err(p->dev, "Couldn't .....");

Use a global flag to note the error and check it from other contexts.
Plus, find out why you can't write LCR there.

Also, consider re-designing how the 8250_dw driver implements that
"feature".

Regards,
Peter Hurley