linux-kernel - Re: [PATCH] dma: omap-dma: add support for pause of non-cyclic transfers

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Message-ID: <55C4C4AE.9080004@hurleysoftware.com>
Date:	Fri, 07 Aug 2015 10:46:06 -0400
From:	Peter Hurley <peter@...leysoftware.com>
To:	Russell King - ARM Linux <linux@....linux.org.uk>
CC:	Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
	Vinod Koul <vinod.koul@...el.com>,
	Dan Williams <dan.j.williams@...el.com>,
	dmaengine@...r.kernel.org, linux-kernel@...r.kernel.org,
	nsekhar@...com, linux-omap@...r.kernel.org,
	linux-serial@...r.kernel.org, john.ogness@...utronix.de,
	Peter Ujfalusi <peter.ujfalusi@...com>
Subject: Re: [PATCH] dma: omap-dma: add support for pause of non-cyclic transfers

On 08/07/2015 09:25 AM, Russell King - ARM Linux wrote:
> On Fri, Aug 07, 2015 at 03:22:56PM +0200, Sebastian Andrzej Siewior wrote:

>> For TX-transfers, I would need to update the start-address so the
>> transfers begins where it stopped. However based on your concern I
>> can't really assume that the position reported by the HW is the correct
>> one.
> 
> Exactly - I don't believe that existing OMAP DMA hardware can ever support
> pausing a mem-to-device transfer without data loss as you have no idea
> how many bytes have been prefetched by the DMA, and therefore you have
> no idea how many bytes to unwind the hardware position.  It gets worse
> than that when you have to cross into the previous descriptor.  It's
> really not nice.
> 
> So, disallowing pause for mem-to-device is entirely reasonable given the
> data loss implications.
 
Thanks for adding your hard-won knowledge to this discussion, Russell.
This saves us a bunch of wasted effort trying to fix x_char with DMA
(and TCSANOW termios changes and throttling).

Regards,
Peter Hurley
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