linux-kernel - Re: [PATCH] dmaengine: add CSR SiRFprimaII DMAC driver

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Message-ID: <1315518676.6638.6.camel@vkoul-mobl4>
Date:	Fri, 09 Sep 2011 03:21:16 +0530
From:	Vinod Koul <vkoul@...radead.org>
To:	Barry Song <21cnbao@...il.com>
Cc:	vinod.koul@...el.com, Jassi Brar <jassisinghbrar@...il.com>,
	"arnd@...db.de" <arnd@...db.de>,
	"linus.walleij@...aro.org" <linus.walleij@...aro.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"workgroup.linux@....com" <workgroup.linux@....com>,
	"rongjun.ying@....com" <rongjun.ying@....com>,
	"Baohua.Song@....com" <Baohua.Song@....com>,
	"Williams, Dan J" <dan.j.williams@...el.com>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH] dmaengine: add CSR SiRFprimaII DMAC driver

On Thu, 2011-09-08 at 14:36 +0800, Barry Song wrote:
> 2011/9/8 Jassi Brar <jassisinghbrar@...il.com>:
> > On Thu, Sep 8, 2011 at 8:47 AM, Jassi Brar <jassisinghbrar@...il.com> wrote:
> >> On Thu, Sep 8, 2011 at 7:42 AM, Barry Song <21cnbao@...il.com> wrote:
> >>
> >>>>> it is much different with primacell based DMA like pl080, pl330.
> >>>>> prima2 has a self-defined DMAC IP. basically it is a 2D mode dma with
> >>>>> two scales X and Y and direct way to start and stop DMA.
> >>>>> every channel has fixed function to serve only one perpheral. so you
> >>>>> find we have a filter id.
> >>>> okay, what do you mean by 2D mode? Is it similar to what TI folks, Linus
> >>>> W and Jassi Brar posted RFC's on?
> >>>
> >>> In SiRFprimaII 2-D DMA, the system memory space is interpreted
> >>> as a 2-D layout instead of a linear 1-D layout. More specifically, the
> >>> system memory can be considered as
> >>> multiple data lines. The length of the data line is determined by the
> >>> user-selected DMA_WIDTH register.
> >>> The user can specify a data window that the user wants to access using
> >>> four parameters:
> >>> ■ Start address
> >>> ■ X length
> >>> ■ Y length
> >>> ■ Width
> >>>
> >>> The idea of a 2-D DMA is shown in figure 2d-dma.png attached.
> >>>
> >>> If you specifies the Y length as 0 or the X length equals to the DMA
> >>> width, then this 2-D DMA reduces to
> >>> 1-D. If the user configures the X length greater than the DMA width,
> >>> then the extra data is wrapped around
> >>> to the next data line, this may corrupt the DMA transfer for
> >>> multiple-line 2-D DMA. If this is a 1-D DMA, then
> >>> there is no issue. The attached diagram 2d-dma2.png shows the
> >>> wrap-around of the extra data in case the X length
> >>> greater than DMA width.
> >>
> >> Sorry, the role of DMA_WIDTH is not clear to me yet.
> >> In which case the user _must_ set {xlen > width} ?
> >>
> > Perhaps 2d-dma.PNG is inaccurate - it shouldn't depict any deltaX.
> > Doesn't xlen and width always start together ? If no, please don't read ahead.
> >
> > According to figures, {xlen > width} is to be set _only_ when a transfer
> > is divided into _exactly_ two chunks separated by gap _exactly_
> > equal to length of the second chunk (an extremely rare case).
> 
> Sorry i didn't list related full information in datasheet in my early reply.
> we don't have the case of xlen > dma_width.
What is intended usage of the 2D dma? 

In first diagram you sent, x length is first block length to transfer
and (dma_width - x_len) is length to skip, right?

So what is the second diagram about? Is this juts a case when notion of
xlen > dma_width? How much you transfer and how much you skip?

-- 
~Vinod Koul
Intel Corp.

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