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

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Message-ID: <20160311104055.015472e9@bbrezillon>
Date:	Fri, 11 Mar 2016 10:40:55 +0100
From:	Boris Brezillon <boris.brezillon@...e-electrons.com>
To:	Vinod Koul <vinod.koul@...el.com>
Cc:	Maxime Ripard <maxime.ripard@...e-electrons.com>,
	Dan Williams <dan.j.williams@...el.com>,
	dmaengine@...r.kernel.org, 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

On Fri, 11 Mar 2016 11:54:52 +0530
Vinod Koul <vinod.koul@...el.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
we pass it through ->xxx_burstsize, and try to guess the real burstsize?
In the latter case, you still haven't answered my question: how should
we choose the burstsize?

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