Re: [PATCH 2/5] i2c: sh_mobile: add DMA support

[Wolfram Sang <wsa@the-dreams.de>]

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On Thu, Dec 11, 2014 at 09:47:29AM +0200, Laurent Pinchart wrote:
> On Thursday 11 December 2014 08:37:27 Wolfram Sang wrote:
> > > I think this is a limitation of driver may not be for HW. The right model
> > > for dma_chan is to be viewed as SW channels and not the ones of HW (yes
> > > that is how most of the drivers use that, but we can improve upon)
> > > 
> > > If we rework the driver to view dma_chan as SW channels, then you can
> > > accept multiple channel requests and accept based on if we are able link
> > > the channel to that peripheral or not.
> > 
> > In my understanding, the DMA driver does exactly that.
> 
> Actually it doesn't at the moment, I should implement that.

OK, so I was misinterpreting the overcommitment. Thanks for the correction!

> > However, it is not even loaded at the time the I2C driver wants a channel,
> > so the dmaengine core defers the probe. That is the problem for optional DMA
> > channels: we can't know when deferring probe won't help anymore and don't
> > know when it is time to fall back to PIO.
> 
> This is true regardless of the whether the driver exposes HW or SW channels.

Yup.


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