linux-kernel - Re: [PATCH] tty: serial: 8250_omap: do not defer termios changes

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Date:	Thu, 14 Apr 2016 09:03:49 -0700
From:	Peter Hurley <peter@...leysoftware.com>
To:	Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
	John Ogness <john.ogness@...utronix.de>, nsekhar@...com
Cc:	Tony Lindgren <tony@...mide.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	linux-kernel@...r.kernel.org, linux-serial@...r.kernel.org,
	linux-omap@...r.kernel.org
Subject: Re: [PATCH] tty: serial: 8250_omap: do not defer termios changes

On 04/12/2016 11:42 AM, Peter Hurley wrote:
> On 04/12/2016 10:03 AM, Sebastian Andrzej Siewior wrote:
>> On 04/11/2016 10:10 PM, Peter Hurley wrote:
>>> On 04/11/2016 11:31 AM, Sebastian Andrzej Siewior wrote:
>>>> On 04/11/2016 07:53 PM, Peter Hurley wrote:
>>>>> On 04/11/2016 01:18 AM, John Ogness wrote:
>>>>>> On 2016-04-05, Peter Hurley <peter@...leysoftware.com> wrote:
>>>>>>> On 03/31/2016 01:41 AM, John Ogness wrote:
>>>>>>>> It has been observed that the TX-DMA can stall
>>>>>>>
>>>>>>> Does this happen on any other OMAP part besides am335x?
>>>>>>> I looked back over the LKML history of this and didn't see
>>>>>>> any other design implicated in this problem.
>>>>>>
>>>>>> I just ran the tests again using 4.6-rc2. I am able to reproduce the
>>>>>> dma-tx stall with am335x/edma and dra7/sdma.

What "test" is reproducing this problem?


>>>>> I thought we already established sdma was not to be used since
>>>>> the hardware does not actually support pausing without data loss.
>>>>
>>>> This workaround was not invented for sdma but for edma (with am335x).
>>>
>>> According to John above, dra7/sdma requires this workaround.
>>
>> It was reported by Frans Klaver against am335x
>>  http://lkml.kernel.org/r/20140908183353.GB4686@ci00147.xsens-tech.local
>>
>> and I managed to reproduce this with his yocto image on dra7 and am335x:
>>  http://lkml.kernel.org/r/20140921204100.GA10111@linutronix.de

I overlooked Sebastian's statement from that message:

This includes
changing the baudrate (not by yocto but the driver sets it to 0 and then
to the requested one) and this seems to be responsible for the "bad
bytes".

What is changing the baud rate to 0? How about get a stack trace for that?



> [...]
> 
>>> - hangs changing some unknown register if tx dma in progress
>>>   (ie., this termios change workaround)
>>
>> I think some registers are the baud-rate registers which pause engine.
> 
> Let's back up here and focus on just this problem for now.
> 
> Since this is observable on dra7/sdma, then it's not related to
> the OMAP_DMA_TX_KICK problem. IOW, dropping tx dma support for am335x
> does not make this go away, which is what I was asking.
> 
> Now, if the DLL/DLH/MDR1 register writes are causing dma to stall,
> then skipping those if they're unchanged should fix this, and then
> pause/terminating in-progress DMA if any of these registers are being
> written would be acceptable since some data loss is to be expected
> when changing the baud rate without waiting.
>