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

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

Your understanding is correct; the byte and buffer mode is mostly the same.
And also mode should be decided before xfer, due to the controller/target
both use the same xfer mode, not decide by transfer time.
The original my submit is only buffer mode and dma mode, and use only one
Boolean property, aspeed,i2c-dma-enabled, but someone suggest add byte
mode select, so I start to add at v17. I can drop the byte mode, if this is confused.

byte mode request:
https://lore.kernel.org/all/[email protected]/

> 
> > > [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.

Yes, in AST2600 DMA is shared, so it only can enable 1 channel dma, others use
buffer mode, and in next generation AST2700, all i2c channel can use dma.

> 
> (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.
Thanks the feedback, do you mean, just one boolean property for mode selection,
Am I right?

> 
> Cheers,
> 
> 
> Jeremy