linux-kernel - Re: [PATCH] dma: sun4i: expose block size and wait cycle configuration to DMA users

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Date:	Mon, 7 Mar 2016 16:08:57 +0100
From:	Boris Brezillon <boris.brezillon@...e-electrons.com>
To:	Vinod Koul <vinod.koul@...el.com>
Cc:	Dan Williams <dan.j.williams@...el.com>, dmaengine@...r.kernel.org,
	Maxime Ripard <maxime.ripard@...e-electrons.com>,
	Chen-Yu Tsai <wens@...e.org>, linux-sunxi@...glegroups.com,
	Emilio López <emilio@...pez.com.ar>,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] dma: sun4i: expose block size and wait cycle
 configuration to DMA users

Hi Vinod,

On Mon, 7 Mar 2016 20:24:29 +0530
Vinod Koul <vinod.koul@...el.com> wrote:

> On Mon, Mar 07, 2016 at 10:59:31AM +0100, Boris Brezillon wrote:
> > +/* Dedicated DMA parameter register layout */
> > +#define SUN4I_DDMA_PARA_DST_DATA_BLK_SIZE(n)	(((n) - 1) << 24)
> > +#define SUN4I_DDMA_PARA_DST_WAIT_CYCLES(n)	(((n) - 1) << 16)
> > +#define SUN4I_DDMA_PARA_SRC_DATA_BLK_SIZE(n)	(((n) - 1) << 8)
> > +#define SUN4I_DDMA_PARA_SRC_WAIT_CYCLES(n)	(((n) - 1) << 0)
> > +
> > +/**
> > + * struct sun4i_dma_chan_config - DMA channel config
> > + *
> > + * @para: contains information about block size and time before checking
> > + *	  DRQ line. This is device specific and only applicable to dedicated
> > + *	  DMA channels
> 
> What information, can you elobrate.. And why can't you use existing
> dma_slave_config for this?

Block size is related to the device FIFO size. I guess it allows the
DMA channel to launch a transfer of X bytes without having to check the
DRQ line (the line telling the DMA engine it can transfer more data
to/from the device). The wait cycles information is apparently related
to the number of clks the engine should wait before polling/checking
the DRQ line status between each block transfer. I'm not sure what it
saves to put WAIT_CYCLES() to something != 1, but in their BSP,
Allwinner tweak that depending on the device.

Note that I'd be happy if the above configuration could go into the
generic dma_slave_config struct. This way we could avoid per-engine
specific APIs.

Best Regards,

Boris

-- 
Boris Brezillon, Free Electrons
Embedded Linux and Kernel engineering
http://free-electrons.com