linux-kernel - Re: [PATCH 09/16] tty: serial: 8250_dma: Add a TX trigger workaround for AM33xx

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Message-ID: <54129F6C.7040508@linutronix.de>
Date:	Fri, 12 Sep 2014 09:23:24 +0200
From:	Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:	Frans Klaver <fransklaver@...il.com>,
	Frans Klaver <frans.klaver@...ns.com>
CC:	Heikki Krogerus <heikki.krogerus@...ux.intel.com>,
	Peter Hurley <peter@...leysoftware.com>, tony@...mide.com,
	gregkh@...uxfoundation.org, linux-kernel@...r.kernel.org,
	balbi@...com, linux-serial@...r.kernel.org,
	linux-omap@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	Alan Cox <alan@...ux.intel.com>
Subject: Re: [PATCH 09/16] tty: serial: 8250_dma: Add a TX trigger workaround
 for AM33xx

On 09/11/2014 07:04 PM, Frans Klaver wrote:
> On 11 September 2014 18:04:32 CEST, Sebastian Andrzej Siewior <bigeasy@...utronix.de> wrote:
>> On 09/11/2014 05:11 PM, Frans Klaver wrote:
>>
>>> I can still reproduce it on am335x. I can get out of it as soon as
>>> something else gets written to the console though.
>>>
>>> # echo "<3>something" >/dev/kmsg
>>
>> Is this to stall it or to get out of it?
> 
> This is to get out of it. I do this from an ssh connection after the console got stuck. The 3 kind of ensures the message is actually sent to ttyS0.

Interesting. This shouldn't do anything. If there is a TX operation in
progress then ->start_tx() will simply return and the xmit buffer will
be send once the current TX operation completes.

Is there anything I can do to reproduce this behavior?
This problem only pops-up if you use DMA. With disabled DMA you don't
see this, right?

Sebastian

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