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

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Date:	Thu, 8 Sep 2011 14:14:01 +0800
From:	Barry Song <21cnbao@...il.com>
To:	Jassi Brar <jassisinghbrar@...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

Hi Jassi,

2011/9/8 Jassi Brar <jassisinghbrar@...il.com>:
> 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} ?

DMA_WIDTH is the size address will increase after every DMA line.
For example:
if xlen = 4, dma_width=8, ylen=5;
Then DMA will run as(suppose DMA address start from 0):
0~3
8~11
16~19
24~27
...

This feature can help us to do DMA operation to non-continuous memory
area, for example, pick a part of a image.

The DMA X-length is in the 32-bit D-word boundary. This value
specifies the number of D-words transferred
in each line. The value should be less than or equal to the value of
DMA width register.

The DMA Y-length specifies the number of lines in DMA transfers. The
number of the lines in DMA transfer
is Y-length + 1.

Each channel has dedicated DMA Width registers. Each DMA channel
should not use other channel’s DMA
width.
To enable a correct 2-D DMA, the DMA Width register must be correctly
set. The value of the DMA Width
register must be greater than or equal to the X-length, otherwise, the
data is overlapped. DMA_WIDTH
should not be set to 0.


Thanks
barry
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