Re: [PATCH v1 1/1] spi: dw-mid: set DMA burst on memory side

[Andy Shevchenko <andriy.shevchenko@xxxxxxxxxxxxxxx>]

On Tue, 2016-04-12 at 19:32 +0530, Vinod Koul wrote:
> On Tue, Apr 12, 2016 at 02:56:35PM +0300, Andy Shevchenko wrote:
> > 
> > On Tue, 2016-04-12 at 01:34 +0100, Mark Brown wrote:
> > > 
> > > On Mon, Apr 11, 2016 at 07:30:12PM +0300, Andy Shevchenko wrote:

> > > To optimize amount of bus writes on memory side set burst to be
> > > > the
> > > > same amount
> > > > of data on both sides.
> > > > 
> > > > +	txconf.src_maxburst = 4 * dws->dma_width;
> > > >  	txconf.dst_maxburst = 16;
> > > This doesn't seem to do what the subject says (at least not
> > > always,
> > > it'll align for a dma_width of 4)?
> > Thanks you didn't apply the patch. 
> > 
> > I think the approach itself is wrong.
> > 
> > The peripheral drivers usually have no idea and shouldn't know about
> > DMA
> > engine memory side characteristics (bus width, bursts, etc).
> These are typically you system characterstics, like 32 bit or 64 bit
> bus to
> memory and rest (burst etc) should be maximum as the data will go
> from/to
> dmaengine FIFO to/from memory, so you would want to push as fast as
> possible
> 
> Said that, maximun burst with 32bit wide should be saner value in
> modern
> systems.

My point that peripheral driver does not and _should not_ care about
memory side of the transfer. This is property of DMAengine controller
and platform that has it installed.

Documentation tells nothing how clients should setup _memory side_ of
the transfer.

Thus, I propose to update documentation to tell that there are two sides
of the transfer in case of mem2dev, dev2mem, where one of them is
_memory side_, and it's DMAengine controller's responsibility to
rightfully set the transfer width and burst size.

I would make a patch if we would agree on this.

> 
> > 
> > 
> > This should be fixed in certain DMA engine drivers.
> > 
> > Also, as you may have noticed when we get maximum length of the
> > segment
> > we take into consideration what DMA device supports. Many of them
> > report
> > something like 2^n - 1, which is apparently unaligned and thus in
> > the
> > poorly written DMA driver leads to performance degradation.
> Which Intel controller supports 2^n - 1? AFAIK the dw and idma don't.

All three mentioned below takes block size as [0 .. 2 ^ number of bits
in the register - 1]. If transfer width is 1 byte (which is calculated
automatically now, the burst will be 1 byte on memory side!

>> Looks like all Intel related DMA drivers should be fixed (HSU,
> > iDMA64,
> > dw_dmac).


-- 
Andy Shevchenko <andriy.shevchenko@xxxxxxxxxxxxxxx>
Intel Finland Oy

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