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

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Message-ID: <CABb+yY1=P=D3GFFxMOUM0a2yWCiAHNqwVU0a--t+pJnUhqKHKA@mail.gmail.com>
Date:	Thu, 8 Sep 2011 08:47:02 +0530
From:	Jassi Brar <jassisinghbrar@...il.com>
To:	Barry Song <21cnbao@...il.com>
Cc:	"Koul, Vinod" <vinod.koul@...el.com>,
	"linus.walleij@...aro.org" <linus.walleij@...aro.org>,
	"Williams, Dan J" <dan.j.williams@...el.com>,
	"arnd@...db.de" <arnd@...db.de>,
	"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>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH] dmaengine: add CSR SiRFprimaII DMAC driver

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} ?

thanks
-jassi
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