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

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Message-ID: <54F9DACA.3020103@hurleysoftware.com>
Date:	Fri, 06 Mar 2015 11:50:18 -0500
From:	Peter Hurley <peter@...leysoftware.com>
To:	Zhang Zhen <zhenzhang.zhang@...wei.com>
CC:	linux-serial@...r.kernel.org, linux-kernel@...r.kernel.org,
	tim.kryger@...aro.org, gregkh@...uxfoundation.org,
	jamie@...ieiles.com, alan@...ux.intel.com, arnd@...db.de,
	shenjiangjiang@...wei.com, long.wanglong@...wei.com
Subject: Re: [RFC] With 8250 Designware UART, if writes to the LCR failed
 the kernel will hung up

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
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