Date: Fri, 11 Mar 2016 15:36:07 +0530
From: Vinod Koul <vinod.koul@intel.com>
To: Boris Brezillon <boris.brezillon@free-electrons.com>
Cc: Maxime Ripard <maxime.ripard@free-electrons.com>,
        Dan Williams <dan.j.williams@intel.com>, dmaengine@vger.kernel.org,
        Chen-Yu Tsai <wens@csie.org>, linux-sunxi@googlegroups.com,
        Emilio =?iso-8859-1?Q?L=F3pez?= <emilio@elopez.com.ar>,
        linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] dma: sun4i: expose block size and wait cycle
 configuration to DMA users
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On Fri, Mar 11, 2016 at 10:40:55AM +0100, Boris Brezillon wrote:
> On Fri, 11 Mar 2016 11:54:52 +0530
> Vinod Koul <vinod.koul@intel.com> wrote:
> 
> > On Wed, Mar 09, 2016 at 11:14:34AM +0100, Boris Brezillon wrote:
> > > > > > > > + * 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.
> > > > 
> > > > we already have block size aka src/dst_maxburst, why not use that one.
> > > 
> > > Okay, but then remains the question "how should we choose the real burst
> > > size?". The block size described in Allwinner datasheet is not the
> > > number of words you will transmit without being preempted by other
> > > master -> slave requests, it's the number of bytes that can be
> > > transmitted without checking the DRQ line.
> > > IOW, block_size = burst_size * X
> > 
> > Thats fine, API expects words for this and also a width value. Client shoudl
> > pass both and for programming you should use bytes converted from words and
> > width.
> > 
> 
> Not sure I get what you mean. Are you suggesting to add new fields to
> the dma_slave_config struct to describe this block concept, or should

No

> we pass it through ->xxx_burstsize, and try to guess the real burstsize?

Pass the real burstsize in words

> In the latter case, you still haven't answered my question: how should
> we choose the burstsize?

>From word value convert to bytes and program HW

burst(in bytes) = burst (in words ) * buswidth;

-- 
~Vinod