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

[Marek Vasut]

Dear 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.

Why? You now have the module option solution.

> > 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.

It's abuse of proper configuration of hardware property.

> > 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.

Best regards,
Marek Vasut