[PATCH v2] dmaengine: bcm2835: Add slave dma support

[Martin Sperl]

Tests with the initial (and incomplete) version of the spi-bcm2835 driver 
with DMA transfer support show that the dma-engine works as expected with 
this patch.

There is one one observation:

> On 18.04.2015, at 13:06, Noralf Trønnes <noralf at tronnes.org> wrote:
> +static struct dma_async_tx_descriptor *
> +bcm2835_dma_prep_slave_sg(struct dma_chan *chan,
> +			  struct scatterlist *sgl,
> +			  unsigned int sg_len,
> +			  enum dma_transfer_direction direction,
> +			  unsigned long flags, void *context)
> +{
...
> +			/* Enable */
> +			if (i == sg_len - 1 && len - j <= max_size)
> +				control_block->info |= BCM2835_DMA_INT_EN;

The observation is that an interrupt is always triggered - even in the case
where flags does NOT have DMA_PREP_INTERRUPT set.
This may not be necessary and avoid interrupts.

So maybe the above if clause should get extended by:
   && (flags & DMA_PREP_INTERRUPT)
to only trigger an interrupt when really requested.

I am not sure if there are any side-effects because of this besides having the
requirement on the client to run dmaengine_terminate_all() on that specific dma
channel without interrupts when the transfer is finished.

In the case of SPI we have TX feed the fifo - which finishes early - , but we
only need to the interrupt when RX finishes reading the fifo, which indicates
that the SPI-transfer is fully finished. 
So having an interrupt on TX is not necessary for the process.

The same observations may also apply to bcm2835_dma_prep_dma_cyclic (which is
outside of this patch provided by Noralf).

Martin