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

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Date:	Fri, 9 Sep 2011 10:52:38 +0800
From:	Barry Song <21cnbao@...il.com>
To:	Vinod Koul <vkoul@...radead.org>
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

2011/9/9 Barry Song <21cnbao@...il.com>:
> 2011/9/9 Vinod Koul <vkoul@...radead.org>:
>> 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?
>
> two cases:
> 1. continuous DMA:
> xlen = DMA_WIDTH
> 2. interleaved DMA:
> xlen < DMA_WIDTH
>
> 2 is for dma video/image or something like that.  For example, you
> might only copy 1/9 of a nine-square grid by it.
>
>>
>> In first diagram you sent, x length is first block length to transfer
>> and (dma_width - x_len) is length to skip, right?
>
> (dma_width - x_len) is length to skip
>
>>
>> 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?
>
> Actually, there is no real case for xlen > dma_width. this picture is
> only explaining what will happen if we set xlen > dma_width.
> when xlen > dma_width, for every line, dma will transfer only
> dma_width. Then the extra data is wrapped around
> to the next data line, it will corrupt the DMA transfer for
> multiple-line 2-D DMA since the extra data will overlap with the real
> next line.
>
> if we set ylen to 0, the DMA becomes 1D, the wrap-around of the extra
> data has no issue nobody will overlap with it.
>

For example, if we set xlen = 7, dma_width =4, ylen > 0
then dma will happen like:
0~6
4~10
8~14
12~18
....

But for ylen=0, it has no issue. there is no next line to overlap with
the extra data.

in case we set xlen = 0, something funny will happen:
i will copy the same memory again and again(ylen+1 times).

-barry
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