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

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Date:	Mon, 12 Sep 2011 08:13:24 +0800
From:	Barry Song <21cnbao@...il.com>
To:	Vinod Koul <vkoul@...radead.org>
Cc:	vinod.koul@...el.com, Jassi Brar <jassisinghbrar@...il.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

2011/9/11 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.
> But is it theoretically possible or just an error case?? Looks like its
> latter, right?

hardware can do since it doesn't require xlen must be less than or
equel with dma_width. but it is a wrong case in fact as prima2
datasheet has said.

it does exist a case cpu read same address (by volatile keyword or
memory barrier), for example, polling. it totally intends to control
some software execute  thread.
but it doesn't exist a case dma read overlapped address in just a dma
cycle since we bacically can't control when and where the dma will go
to hold bus. data transferred by dma is generically not related with
program switch and execution thread.
we do want the overlap and same memory is tranferred again, it should
be in another dma cycle with our instruction in software.

>
> --
> ~Vinod Koul
> Intel Corp.
>
>

-barry
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