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

[Russell King - ARM Linux <linux-I+IVW8TIWO2tmTQ+vhA3Yw@public.gmane.org>]

On Wed, Aug 29, 2018 at 07:55:21AM +0200, Christoph Hellwig wrote:
> 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).

NAK on that at the moment.

> 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.

Well, as I've no idea what the issue is here, I can't do anything or
make any suggestions.  I wasn't copied on the initial part of the
thread.

-- 
RMK's Patch system: http://www.armlinux.org.uk/developer/patches/
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