spi: Provide core support for full duplex devices

[Mark Brown]

On Wed, Feb 19, 2014 at 02:32:28PM +0100, Geert Uytterhoeven wrote:
> On Wed, Feb 19, 2014 at 2:04 PM, Mark Brown <broonie at kernel.org> wrote:

> > This is a particularly thoughtless hardware design it has to be said.

> Do you mean the fact that you need to transmit dummy data on this particular
> hardware, or that you need to know the direction for Dual/Quad transfers,
> and don't need dummy data for the latter?

The combination of the two, clearly the hardware can actually cope with
an empty transmit FIFO.

> > That's readily fixable for DMA capable drivers, we can just repeatedly
> > transfer the same page (or whatever).  PIO drivers have big problems

> So let's hope all DMA-capable hardware has scatter/gather support, or an
> IOMMU ;-)

You can emulate scatter/gather well enough for most SPI controllers
anyway.
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