linux-kernel - Re: [PATCH] Documentation: dmaengine: Add a documentation for the dma controller API

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Message-ID: <20140805162545.GX8181@intel.com>
Date:	Tue, 5 Aug 2014 21:55:45 +0530
From:	Vinod Koul <vinod.koul@...el.com>
To:	Maxime Ripard <maxime.ripard@...e-electrons.com>
Cc:	Dan Williams <dan.j.williams@...el.com>,
	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	dmaengine@...r.kernel.org, Russell King <linux@....linux.org.uk>,
	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, ludovic.desroches@...el.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 Sat, Aug 02, 2014 at 04:49:25PM +0200, Maxime Ripard wrote:
> > - residue callculation, though situation is much better now but still lots
> >   of driver do it worng and folks do get it wrong
> 
> What mistake in often made regarding the residue calculation?
- residue is for the descriptor asked
- not for the current active one
- we can store the size locally and return that when not processing
- arg txstate can be null

> Another extra questions arose during starting this.
> 
> In the case of the call to device_control, especially in the
> DMA_SLAVE_CONFIG case, but that also applies to pause/resume, are the
> changes supposed to be immediates or can they happen later?
> 
> I actually have in mind the case where we would use a vchan, that
> might or might not be actually mapped to a physical channel at the
> moment where the DMA_SLAVE_CONFIG call is made. In the case where it's
> not mapped and not transfering anything, it's pretty trivial, to
> handle, but in the case where it's actually mapped to a physical
> channel, should we push the new configuration to the physical channel
> right away, or can it wait until the transfer ends ?
Ah no. The model doesn't assume the config will be written runtime. So the
descriptor which are already prepared doesn't change with DMA_SLAVE_CONFIG
call. Only new descriptors will take those values

But yes the pause/resume would be syncronous

HTH
-- 
~Vinod


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