linux-kernel - RE: [PATCHv4] DMAEngine: Define interleaved transfer request api

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Date:	Fri, 14 Oct 2011 10:50:41 -0700
From:	"Bounine, Alexandre" <Alexandre.Bounine@....com>
To:	"Williams, Dan J" <dan.j.williams@...el.com>,
	Jassi Brar <jaswinder.singh@...aro.org>
CC:	Vinod Koul <vinod.koul@...el.com>,
	Russell King <rmk@....linux.org.uk>,
	Barry Song <21cnbao@...il.com>, <linux-kernel@...r.kernel.org>,
	DL-SHA-WorkGroupLinux <workgroup.linux@....com>,
	Dave Jiang <dave.jiang@...el.com>
Subject: RE: [PATCHv4] DMAEngine: Define interleaved transfer request api

On Tue, Oct 11, 2011 at 12:45 PM, Williams, Dan J
<dan.j.williams@...el.com> wrote:

> Subject: Re: [PATCHv4] DMAEngine: Define interleaved transfer request
> api
> 
> [ Adding Alexandre ]
> 
> On Fri, Oct 7, 2011 at 4:27 AM, Jassi Brar
<jaswinder.singh@...aro.org>
> wrote:
> > On 7 October 2011 11:15, Vinod Koul <vinod.koul@...el.com> wrote:
> >
> >> Thru this patch Jassi gave a very good try at merging DMA_SLAVE and
> >> memcpy, but more we debate this, I am still not convinced about
> merging
> >> memcpy and DMA_SLAVE yet.
> >>
> > Nobody is merging memcpy and DMA_SLAVE right away.
> > The api's primary purpose is to support interleave transfers.
> > Possibility to merge other prepares into this is a side-effect.
> >
> >> I would still argue that if we split this on same lines as current
> >> mechanism, we have clean way to convey all details for both cases.
> >>
> > Do you mean to have separate interleaved transfer apis for Slave
> > and Mem->Mem ? Please clarify.
> >
> 
> This is a tangent, but it would be nice if this API extension also
> covered the needs of the incoming RapidIO case which wants to specify
> new device context information per operation (and not once at
> configuration time, like slave case).  Would it be enough if the
> transfer template included a (struct device *context) member at the
> end?  Most dma users could ignore it, but RapidIO could use it to do
> something like:
> 
>    struct rio_dev *rdev = container_of(context, typeof(*rdev),
device);
> 
> That might not be enough, but I'm concerned that making the context a
> (void *) is too flexible.  I'd rather have something like this than
> acquiring a lock in rio_dma_prep_slave_sg() and holding it over
> ->prep().  The alternative is to extend device_prep_slave_sg to take
> an extra parameter, but that impacts all other slave implementations
> with a dead parameter.
> 

Having context limited to the device structure will not be enough for
RapidIO because of 66-bit target address (dma_addr_t will not work
here).
Probably that range is out of practical use at this moment but it is
defined by RIO specification and I would prefer to deal with it now
instead of postponing it for future. Passing context using (void *) will
solve this.

Alex.

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