linux-kernel - RE: [PATCH] dmaengine: add dma_ctrl_cmd to pass buffer stride configuration

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Date:	Tue, 12 Jul 2011 17:01:23 +0530
From:	"Raju, Sundaram" <sundaram@...com>
To:	Jassi Brar <jassisinghbrar@...il.com>,
	Linus Walleij <linus.walleij@...aro.org>
CC:	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"davinci-linux-open-source@...ux.davincidsp.com" 
	<davinci-linux-open-source@...ux.davincidsp.com>,
	"linux@....linux.org.uk" <linux@....linux.org.uk>,
	"dan.j.williams@...el.com" <dan.j.williams@...el.com>,
	"linux-omap@...r.kernel.org" <linux-omap@...r.kernel.org>
Subject: RE: [PATCH] dmaengine: add dma_ctrl_cmd to pass buffer stride
 configuration

> -----Original Message-----
> From: Jassi Brar [mailto:jassisinghbrar@...il.com]
> Sent: Tuesday, July 12, 2011 4:51 PM
> To: Linus Walleij
> Cc: Raju, Sundaram; linux-arm-kernel@...ts.infradead.org; linux-
> kernel@...r.kernel.org; davinci-linux-open-source@...ux.davincidsp.com;
> linux@....linux.org.uk; dan.j.williams@...el.com; linux-omap@...r.kernel.org
> Subject: Re: [PATCH] dmaengine: add dma_ctrl_cmd to pass buffer stride
> configuration
> 
> On Tue, Jul 12, 2011 at 3:33 PM, Linus Walleij <linus.walleij@...aro.org>
> wrote:
> > On Tue, Jul 12, 2011 at 6:17 AM, Jassi Brar <jassisinghbrar@...il.com>
> wrote:
> >
> >> 1) Striding, in one form or other, is supported by other DMACs as well.
> >>   The number will only increase in future.
> >>   Are we to add  <VENDOR>_DMA_STRIDE_CONFIG for each case ?
> >
> > If we are sure about this and striding will work in a similar way on all
> > then let's have the enum named DMA_STRIDE_CONFIG and move the
> > passed-in struct to <linux/dmaengine.h) then?
> >
> > Would that be:
> >
> > struct dma_stride_config {
> >    u32 read_bytes;
> >    u32 skip_bytes;
> > };
> >
> > Or something more complex?
> Well, I am not sure if striding needs any special treatment at all.
> Why not have client drivers prepare and submit sg-list.
> Let the DMAC drivers interpret/parse the sg-list and program it
> as strides if the h/w supports it.
> If anything, we should make preparation and submission of sg-list
> as efficient as possible.
Jassi,

sg_lists describe only a bunch of disjoint buffers. But what if the
DMAC can skip and read the bytes within each of the buffers in
the sg_list? (like TI EDMAC and TI SDMAC)
How can that information be passed to the offload
engine driver from the client?

~Sundaram