linux-kernel - RE: [PATCH 1/3] i2c: add DMA support for freescale i2c driver

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Message-ID: <3d65fc9a0c5444b485c93de4087634e0@BL2PR03MB338.namprd03.prod.outlook.com>
Date:	Fri, 28 Feb 2014 11:36:01 +0000
From:	Yao Yuan <yao.yuan@...escale.com>
To:	Marek Vasut <marex@...x.de>
CC:	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"wsa@...-dreams.de" <wsa@...-dreams.de>,
	"mark.rutland@....com" <mark.rutland@....com>,
	"shawn.guo@...aro.org" <shawn.guo@...aro.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-i2c@...r.kernel.org" <linux-i2c@...r.kernel.org>
Subject: RE: [PATCH 1/3] i2c: add DMA support for freescale i2c driver

Hi Marek,
> On Friday, February 28, 2014 at 06:19:18 AM, Yao Yuan wrote:
> 
> [...]
> 
> > > > @@ -213,6 +238,7 @@ static struct imx_i2c_hwdata vf610_i2c_hwdata
> > > > = {
> > > >
> > > >  	.ndivs			= ARRAY_SIZE(vf610_i2c_clk_div),
> > > >  	.i2sr_clr_opcode	= I2SR_CLR_OPCODE_W1C,
> > > >  	.i2cr_ien_opcode	= I2CR_IEN_OPCODE_0,
> > > >
> > > > +	.has_dma_support	= true,
> > >
> > > So why exactly don't we have a DT prop for determining whether the
> > > controller has DMA support ?
> > >
> > > What about the other controllers, do they not support DMA for some
> > > specific reason? Please elaborate on that, thank you !
> >
> > Sorry for my fault. I will modify it.
> 
> I would prefer if you could explain why other controllers do have DMA
> disabled even if the hardware does support the DMA operation.
> 

Well, Because of the I2C in I.MX hardware don't support the DMA operation. 
But here I also think has_dma_support isn't necessary.

> > > Also, can the DMA not do full-duplex operation ? What I see here is
> > > just
> > > half- duplex operations , one for RX and the other one for TX .
> >
> > Yes, here have two dma channels, one for RX and the other one for TX.
> > When we request the channel we should determine it for TX or RX.
> 
> Sorry, I don't quite understand this. If you have two DMA channels, can
> you not use them both to do full-duplex SPI transfer ?
> 

Sorry, There are also hard for me. I don't understand what is full-duplex for dma? 
A DMA engine can only read or write at the same time. And a dma channel request for only DMA_MEM_TO_DEV or DMA_DEV_TO_MEM almost.
Also i2c is a half-duplex bus.