Re: [PATCH v26 2/4] dt-bindings: i2c: ast2600-i2c.yaml: Add global-regs and transfer-mode properties

[Jeremy Kerr]

Hi Rob,

> > I don't think that really addresses Rob's point there.
> > 
> > The selection of mode is somewhat a driver implementation decision (and
> > so would not belong in a DT binding) - *except* that there are
> > considerations around the use of hardware DMA channels, as covered in
> > earlier review.
> 
> Am I supposed to go read the prior 25 versions?

Not at all - the next paragraph was my attempt at a recap of those, but
Ryan, please correct me if I am wrong on any of those points.

> > [My understanding is that the mode needs to be defined here to select
> > which i2c devices have a DMA channel allocated to them. I also
> > think that byte mode may be useful in some scenarios, but that
> > consideration certainly does not belong in the DT binding spec]
> 
> But this is selecting DMA for the bus, not specific devices. I would
> think the decision would be dynamic based on some xfer size. Certainly
> if the xfer is less than the buffer size (32bytes), then there is no
> advantage of DMA.
> 
> Or do you mean some instances of the I2C controllers have DMA and some
> do not?

Yes, some of the controllers may be allocated a DMA channel. A driver
implementation may (or may not) make use of that channel.

Since the hardware has a limited numbers of DMA channels, Ryan's aim
here is to specify which controllers may consume a channel.

(in relation to the 'bus' terminology though: there is a 1:1 mapping
between i2c buses and controllers here, plus a small area of global
registers which apply to all controller peripherals)

>  If so, then ...

[...]

> > In that case, we would just have a boolean property, like:
> > 
> >     aspeed,i2c-dma-enabled;
> 
> ... yes, this is fine.

Brilliant, thanks.

Ryan: I think this gives us a much cleaner approach to the binding.

Cheers,


Jeremy