Re: [PATCH] of/platform: Initialise AMBA default DMA masks

[Christoph Hellwig <hch-jcswGhMUV9g@public.gmane.org>]

On Tue, Aug 28, 2018 at 03:14:14PM +0100, Russell King - ARM Linux wrote:
> But yes, the fundamental fact is that AMBA devices don't have any
> care about the differences between coherent and streaming DMA.  The
> distinction that we make in the kernel is purely a software one when
> it comes to these devices.
> 
> Most AMBA devices themselves are not DMA capable, as they are only
> connected to the APB (Amba peripheral bus) and they rely on a
> separate DMA engine for their DMA.  APB devices should not have DMA
> masks - their DMA capabilities are entirely down to the DMA controller.
> So, the majority of AMBA devices should not have any DMA masks.
> 
> Only those connected to a bus that they can master on (eg AXI) should
> have DMA masks - things like the PL08x DMA controllers, PL11x LCD
> controllers, etc.  As I've said above, there is no difference between
> streaming and coherent DMA for these devices.

So for now I plan to apply the patch from Linus to just set a dma
mask, as that gets back the previous behavior where dma did just
work (as it did without a mask).

But if Linus, you or someone else familiar with amba would like to
add an explicit opt-in into dma support eventually that would be
even better.