linux-kernel - Re: [PATCH 15/15] tty: serial: 8250: omap: add dma support

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Date:	Fri, 29 Aug 2014 11:32:36 +0200
From:	Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:	Tony Lindgren <tony@...mide.com>
CC:	balbi@...com, linux-serial@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-omap@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org,
	Vinod Koul <vinod.koul@...el.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Paul Walmsley <paul@...an.com>
Subject: Re: [PATCH 15/15] tty: serial: 8250: omap: add dma support

On 08/29/2014 12:54 AM, Tony Lindgren wrote:
> * Sebastian Andrzej Siewior <bigeasy@...utronix.de> [140828 12:37]:
>> On 08/28/2014 06:46 PM, Tony Lindgren wrote:
>>>
>>> Sounds like there should be some way to clear that state.. I wonder
>>> if omap-serial.c had something before it's DMA support was removed?
>>
>> Its DMA was removed? Like there was DMA support?
> 
> Yeah see commit 494574304711a333386e7dd5fd3ebbc3b7024994...

Interesting. I've been browsing that file and checking other trees and
never noticed that it was there at some point. I only saw the pieces
which looked it was there.

> OK thanks, I'm seeing the same issue as you. And the idlest registers
> don't show any blockers. Looking at the errata docs, seems like
> "Usage Note 2.7" in sprz318f.pdf says:
> 
>  Details When configured for DMA operations using smartidle mode (SYSC[4:3].IDLEMODE =
>  0x2), the UART module will not acknowledge incoming idle requests. As a consequence,
>  it can prevent L4 from going to idle.
>  When there are additional expected transfers, the UART should be placed in force-idle
>  mode.

Ehm. So I haven't found an errata document for omap3630, there is
nothing like that in that one I have for am335x or dra7. The document
you mentioned is for AM3715. Interesting…

> So I've added also Paul to Cc, he may have better suggestions for the
> hwmod flags to use. The experimental patch below seems to allow idling
> for me, care to give it a try?

Yep, this one works. And I see DMA transfers (for RX side) after the
core hit idle so it seems to look good :)

> 
> Regards,
> 
> Tony

Sebastian
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