Re: [PATCH v6 1/2] dt-bindings: i2c: aspeed: support for AST2600-i2cv2

[Jeremy Kerr <jk@codeconstruct.com.au>]

Hi Ryan,

> Yes, I2C controller share the same dma engine. The original thought
> can be enable in all i2c channel. But in AST2600 have ERRATA "I2C DMA
> fails when DRAM bus is busy and it can not take DMA write data
> immediately", So it means only 1 i2c bus can be enable for DMA mode.

OK, this is a pretty important detail! I'd suggest putting it in the
binding document.

Anything in the cover letter will get lost after review. If there is
documentation that would be useful for a DTS author, I'd suggest putting
it in the binding.

> It means only 1 bus channel can be enable DMA for use case.
> That following example for board-specific selection.
> It is description in cover-letter.
> The following is board-specific design example.
> Board A                                         Board B
> -------------------------                       ------------------------
> > i2c bus#1(master/slave)  <===fingerprint ===> i2c bus#x (master/slave)|
> > i2c bus#2(master)-> tmp i2c device |        |                   |
> > i2c bus#3(master)-> adc i2c device |        |                   |
> -------------------------                       ------------------------
> 
> - in bus#1 situation, you should use DMA mode.
> Because bus#1 have trunk data needed for transfer, it can enable bus
> dma mode to reduce cpu utilized.

What is "trunk data" in this context? Is this just a statement about the
amount of expected transfers?

> - in bus#2/3 situation, you should use buffer/byte mode
> bus#2/3 is small package transmit, it can enable buffer mode or byte
> mode to reduce memory cache flush overhead.
> Buffer mode is better, because byte mode have interrupt
> overhead(interrupt per byte data transmit),
> 
> -But if you more bus#4 that still have trunk data needed for transfer
> (master/slave),
> it also use buffer mode to transmit. Because bus#1 have been use for
> DMA mode.

So, it sounds like:

 - there's no point in using byte mode, as buffer mode provides
   equivalent functionality with fewer drawbacks (ie, less interrupt
   load)

 - this just leaves the dma and buffer modes

 - only one controller can use dma mode

So: how about just a single boolean property to indicate "use DMA on
this controller"? Something like aspeed,enable-dma? Or if DT binding
experts can suggest something common that might be more suitable?

Cheers,


Jeremy