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Message-ID: <20140814085753.GS30401@n2100.arm.linux.org.uk> Date: Thu, 14 Aug 2014 09:57:53 +0100 From: Russell King - ARM Linux <linux@....linux.org.uk> To: Ludovic Desroches <ludovic.desroches@...el.com> Cc: Maxime Ripard <maxime.ripard@...e-electrons.com>, Dan Williams <dan.j.williams@...el.com>, Vinod Koul <vinod.koul@...el.com>, linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org, dmaengine@...r.kernel.org, Arnd Bergmann <arnd@...db.de>, Antoine Ténart <antoine@...e-electrons.com>, Thomas Petazzoni <thomas@...e-electrons.com>, Alexandre Belloni <alexandre.belloni@...e-electrons.com>, Boris Brezillon <boris@...e-electrons.com>, Matt Porter <matt.porter@...aro.org>, laurent.pinchart@...asonboard.com, Gregory Clement <gregory.clement@...e-electrons.com>, Nicolas Ferre <nicolas.ferre@...el.com> Subject: Re: [PATCH] Documentation: dmaengine: Add a documentation for the dma controller API On Thu, Aug 14, 2014 at 10:53:01AM +0200, Ludovic Desroches wrote: > On Wed, Jul 30, 2014 at 06:03:13PM +0200, Maxime Ripard wrote: > > The dmaengine is neither trivial nor properly documented at the moment, which > > means a lot of trial and error development, which is not that good for such a > > central piece of the system. > > > > Attempt at making such a documentation. > > Good idea, many questions are asked when writing a new dmaengine driver. > > For instance I didn't find how to use the DMA_CTRL_ACK flags. > > - How this flag has to be managed? For instance, async_tx_ack is used in > dmaengine driver but also in some devices. > - Is it mandatory to deal with this flag? It seems some dmaengine > drivers don't care about it. This is one part of the DMA engine API that even I don't understand. It's got something to do with the async engine API, and seems to be something to do with whether a descriptor can have other transactions added to it, and whether a descriptor can be re-used (async-tx engines typically allocate a fixed set of descriptors and recycle them.) I was never able to get to the bottom of what that flag meant or how it should be used with the DMA slave API (and I suspect everyone else just gave up trying to understand it as well.) -- FTTC broadband for 0.8mile line: currently at 9.5Mbps down 400kbps up according to speedtest.net. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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