[PATCH V4] MXS: Implement DMA support into mxs-i2c

[w.sang@pengutronix.de (Wolfram Sang)]

> > Unsurprisingly, I also couldn't get the mode switching between DMA and
> > PIOQUEUE to work :(
> 
> I'd say more work should be invested into this.

I ran out of ideas. Next thing I'd do is to ask FSL if this is possible
at all. Yet, I'd like to have some solution for 3.7.

> Besides, what about rather > trying PIO + DMA instead of PIOQ + DMA?

That could also be a solution.

> > However, I did come up with an idea how to select DMA or PIOQUEUE per
> > bus via devicetree. If we change the view from "I want PIOQUEUE" to "I
> > don't want DMA", we could simply override the default DMA configuration
> > in the devicetree with, e.g.:
> > 
> > 			i2c0: i2c at 80058000 {
> > 				...
> > 				fsl,i2c-dma-channel = <>;
> > 			};
> > 
> > So, no DMA channel set up will fall back to PIOQUEUE. This is more
> > generic than a custom binding, because if one doesn't want DMA, don't
> > configure it. That should also work with other devices which can fall
> > back to something. I think this is worth trying, another advantage is
> > that it doesn't expose something new to the user. So, there is no legacy
> > support needed in case there will be a better way of configuration.
> 
> Ewww ... that's such an ad-hoc hack.

Please give reasons.

> Besides, someone might simply forget to add the binding, you'd need to
> print a warning.

? It is "on" by default because of the entry in the dtsi. And the status
of DMA will be printed.

> > +	dev_info(dev, "registered. DMA: %s\n", i2c->dma_mode ? "on" : "off");
> 
> dev_debug() ?

Yup.

-- 
Pengutronix e.K.                           | Wolfram Sang                |
Industrial Linux Solutions                 | http://www.pengutronix.de/  |
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