linux-kernel - Re: [PATCH 3/3] serial: 8250: omap: restore registers on shutdown

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Date:	Thu, 06 Aug 2015 14:31:27 +0200
From:	Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:	Peter Hurley <peter@...leysoftware.com>
CC:	John Ogness <john.ogness@...utronix.de>,
	linux-kernel@...r.kernel.org,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Tony Lindgren <tony@...mide.com>, linux-omap@...r.kernel.org,
	nsekhar@...com, nm@...com, linux-serial@...r.kernel.org,
	Heikki Krogerus <heikki.krogerus@...ux.intel.com>
Subject: Re: [PATCH 3/3] serial: 8250: omap: restore registers on shutdown

On 08/06/2015 02:27 PM, Peter Hurley wrote:
> Hi Sebastian,

Hi Peter,

> On 08/04/2015 07:58 AM, Sebastian Andrzej Siewior wrote:
>> On 08/03/2015 09:32 PM, Peter Hurley wrote:
>>
>>>> You mean a function in 8250-dma API which does what I did just here
>>>> with the wait_event() and the wake_up in the callback? That way I could
>>>> move the termios_wait into the dma struct instead of keeping in the
>>>> omap specific part. I am also not sure if OMAP is the only one that may
>>>> hang here or the other people just didn't notice it yet.
>>>
>>> Exactly; and we need to fix DMA wrt x_char anyway.
>>>
>>> Going back to the dmaengine api, I think something like this might work
>>> (as a first approximation):
>>>
>>> 	dma_sync_wait(dma->txchan, dma->tx_cookie);
>>> 	dmaengine_pause(dma->txchan);
>>>
>>> 	/* remainder of set_termios */
>>>
>>> 	dmaengine_resume(dma->txchan);
>>>
>>> We could require 8250 core dma to support pause/resume.
>>
>> I would prefer the waitqueue approach.
>> You can't do this while holding the port lock. The lock is taken with
>> irqs off so may not see the transfer completing.
>> Why do you pause the channel? It may not work without an active
>> descriptor and a start without "resume" should work. Also you must
>> ensure that DMA's complete callback does not start another transfer if
>> there is something queued up (that is why I had the tx_running dance).
>> I am not sure if a transfer that is active and then paused will not
>> trigger the hang bug if we change the termios in between.
> 
> I'll look at/test this this weekend, ok?

Sure. I'm currently re-spinning the patches so have everything in
proper pieces. While at it I will take a look at x_char.

> Regards,
> Peter Hurley
> 
Sebastian
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