Subject: [PATCH] dma: sun4i: expose block size and wait cycle configuration to DMA users

From: [email-masked] (Vinod Koul)
Date: Fri, 11 Mar 2016 16:51:02 +0530

On Fri, Mar 11, 2016 at 11:26:31AM +0100, Boris Brezillon wrote:
> On Fri, 11 Mar 2016 15:36:07 +0530
> Vinod Koul <[email-masked]> wrote:
> 
> > On Fri, Mar 11, 2016 at 10:40:55AM +0100, Boris Brezillon wrote:
> > > On Fri, 11 Mar 2016 11:54:52 +0530
> > > Vinod Koul <[email-masked]> 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;
> > 
> 
> 
> Except, as already explained, the blocksize and burstsize concepts are
> not exactly the same, and the sunxi engine expect both to be defined.
> So let's take a real example to illustrate my question:
> 
> For the NAND use case, here is my DMA channel setup:
> 
> buswidth (or wordsize) = 4 bytes
> burstsize = 4 words (32 bytes)
> blocksize = 128 bytes
> 
> Here, you can see that blocksize = 4 * burstsize, and again, burstsize
> and blocksize are not encoding the same thing. So, assuming we use
> ->src/dst_burstsize to encode the blocksize in our case, how should we
> deduce the real burstsize (which still needs to be configured in the
> engine).

Oh, i was somehow under the impression they are same! Then we can't use
blocksize here, pls pass burst and width properly.

How is block size calculated?

-- 
~Vinod