Hi Rob,

On Tue, Apr 03, 2018 at 11:03:30AM -0500, Rob Herring wrote:
> On Tue, Apr 3, 2018 at 8:29 AM, Maxime Ripard <maxime.rip...@bootlin.com> 
> wrote:
> > Hi,
> >
> > We've had for quite some time to hack around in our drivers to take into
> > account the fact that our DMA accesses are not done through the parent
> > node, but through another bus with a different mapping than the CPU for the
> > RAM (0 instead of 0x40000000 for most SoCs).
> >
> > After some discussion after the submission of a camera device suffering of
> > the same hacks, I've decided to put together a serie that introduce a
> > property called dma-parent that allows to express the DMA relationship
> > between a master and its bus, even if they are not direct parents in the DT.
> Reading thru v6 of the camera driver, it seems like having
> intermediate buses would solve the problem in your case?

I guess it would yes, but I guess it wouldn't model the hardware
properly since this seems to be really a bus only meant to do DMA, and
you're not accessing the registers of the device through that bus.

And as far as I know, the DT implies that the topology is the one of
the "control" side of the devices.

We'll also need eventually to have retrieve the MBUS endpoints ID to
be able to support perf and PM QoS properly.

> As Arnd mentioned in that thread, something new needs to address all
> the deficiencies with dma-ranges and describing DMA bus topologies.
> This doesn't address the needs of describing bus interconnects.
> There's been some efforts by the QCom folks with an interconnect
> binding. They've mostly punted (for now at least) to not describing
> the whole interconnect in DT and keeping the details in a driver.

Is it that patch serie? https://lkml.org/lkml/2018/3/9/856

> On the flip side, this does mirror the established pattern used by
> interrupts, so maybe it's okay on it's own. I'll wait for others to
> comment.

We'll see how it turns out then :)


Maxime Ripard, Bootlin (formerly Free Electrons)
Embedded Linux and Kernel engineering

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