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

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Date:	Tue, 10 Mar 2015 11:15:48 +0800
From:	Zhang Zhen <zhenzhang.zhang@...wei.com>
To:	Tim Kryger <tim.kryger@...il.com>
CC:	Alan Cox <alan@...ux.intel.com>,
	"long.wanglong" <long.wanglong@...wei.com>,
	Peter Hurley <peter@...leysoftware.com>,
	<linux-serial@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	<gregkh@...uxfoundation.org>, Jamie Iles <jamie@...ieiles.com>,
	Arnd Bergmann <arnd@...db.de>, <shenjiangjiang@...wei.com>,
	Wang Kai <morgan.wang@...wei.com>
Subject: Re: [RFC] With 8250 Designware UART, if writes to the LCR failed
 the kernel will hung up

On 2015/3/10 10:47, Tim Kryger wrote:
> On Mon, Mar 9, 2015 at 8:05 AM, Alan Cox <alan@...ux.intel.com> wrote:
> 
>> Ah no - I meant what is their official software workaround for existing
>> parts with the bug ? Presumably they have an errata document that
>> discusses this and the correct methods they recommend to avoid the
>> hang ?
> 
> As far as I know, the only advice they provided was rather naive.
> 
> The documentation I saw suggested stashing a copy of the LCR and then
> rewriting it when the special LCR write failed interrupt was raised.
> 
> That approach was not workable as the LCR might be written while the
> interrupt is masked causing the sequence of register writes to occur
> in an order other than what was desired.
> 
> Additionally, when the LCR needed to be re-written but the UART stayed
> busy, the interrupt would keep firing and the driver would starve out
> everything else on the CPU.
> 
> The current workaround of clearing fifos and retrying a fixed number
> of times isn't ideal but I'm not sure what else can be done given the
> way this hardware works.
> 
> Additional background is in c49436b657d0a56a6ad90d14a7c3041add7cf64d
> 
Hi,

If writes to the LCR failed and the dev_err in dw8250_serial_out lead to
the system hung up.
The root cause is that serial8250_console_write try to get port->lock which
has been got by serial8250_do_set_termios.

"Writes to the LCR are used to change the baud rate, parity, stop bit,
or data length so the data that may be lost is likely not important."
The dev_err in dw8250_serial_out must be needed? We can delete dev_err directly ?

Best regards!
> -Tim
> 
> .
> 


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