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

[Martin Sperl]

> On 08.05.2015, at 13:20, Jonathan Bell <jonathan at raspberrypi.org> wrote:
>> 
> I agree that the interrupt generated would be spurious - in the case where it is not required.
> 
> However if you do && (flags & DMA_PREP_INTERRUPT) then all users of this driver need to explicitly set interrupt flags when doing a scatter-gather transfer. As I understand it, currently the only upstream client of this driver is the I2S driver which only uses cyclic anyway.
> 
> Not requiring an interrupt on completion is a bit of an edge case - the default among other dmaengine drivers appears to be to enable interrupts unconditionally.

I have now submitted a patch for spi-bcm2835 to make use of dma,
so there is one candidate for this kind of behavior.
So please go forward with the merge.

Also note that with the spi-HW dma support of the bcm2835
it is necessary to do a RX transfer even if the data is not
used (similar for TX).

Right now we have to allocate a dummy buffer to run these kind
of “one-way” transfers where we need 2 DMA channels.

The bcm2835 dma hw supports such dummy-transfer modes via 
BCM2835_DMA_S_IGNORE and BCM2835_DMA_D_IGNORE.

So maybe we can add a “flag” to the dmaengine_prep_slave_sg
that will allow such kind of behavior to get implemented?

That is not a necessity, but would be a welcome improvement.

Tested-by: Martin Sperl <kernel at martin.sperl.org>