[PATCH 2/3] spi: mediatek: Add spi bus for Mediatek MT8173

[Eddie Huang]

On Wed, 2015-06-17 at 17:35 +0100, Mark Brown wrote:
> On Wed, Jun 17, 2015 at 10:10:51PM +0800, Eddie Huang wrote:
> 
> > Our hardware limitation is: we don't have separate dma tx, rx channel
> > with transfer finish interrupt, only have spi trigger operation.So the
> > mediatek SPI dma full duplex operation steps are:
> > 1. Set TX DMA address.
> > 2. Set RX DMA address.
> > 3. Set length (this step assume TX, RX are the same size).
> > 4. Set TX DMA enable, RX DMA enable bit in spi config register. (not
> > trigger DMA, just told spi use dma)
> > 5. Trigger spi operations.
> > 6. Wait spi operations finish interrupt.
> 
> Sure, that's what I understood.
> 
> > If tx scatterlist per list data size are 128, 4096, 256. rx scatterlist
> > per list data size are 128, 4096, 256. So we need to go through above
> > steps three times. If tx scatterlists per list data size are 128, 4096,
> > 256. rx scatterlists per list data size are 256, 4096, 128. If we start
> > sending first entry, tx size is 128, rx size is 256, this will cause
> > hardware malfunction because tx, rx data length are not the same.
> 
> > The solution I think is copy scatterlist data into one single buffer in
> > mediatek spi transfer function, but I think this is odd because
> > __spi_map_msg() map single buffer into scatterlist, then our driver map
> > scatterlist into single buffer again. I hope this explaination is more
> > clear than before.
> 
> To repeat what I said in my last mail: there's no need to use the
> scatterlists as-is, your driver can do whatever set of DMA transfers it
> likes to keep the lengths of each transfer the same.  Attempting to
> linearise the transfers in memory isn't going to work unless you
> allocate physically contiguous memory (which could get painful) and will
> add substantial overhead.
> 
> For example with your above example you could split the transfers up to
> be 128, 128, 3968, 128, 128.

This is a workable way.
Thanks your suggestion.We will try to implement this,

Eddie
Thanks