Re: [PATCH v2] of: Make of_dma_get_range() work on bus nodes

From: Rob Herring
Date: Tue Oct 08 2019 - 21:03:28 EST

On Tue, Oct 8, 2019 at 3:52 PM Nicolas Saenz Julienne
<nsaenzjulienne@xxxxxxx> wrote:
> Hi Rob/Robin,
> On Tue, 2019-10-08 at 14:52 -0500, Rob Herring wrote:
> > From: Robin Murphy <robin.murphy@xxxxxxx>
> >
> > Since the "dma-ranges" property is only valid for a node representing a
> > bus, of_dma_get_range() currently assumes the node passed in is a leaf
> > representing a device, and starts the walk from its parent. In cases
> > like PCI host controllers on typical FDT systems, however, where the PCI
> > endpoints are probed dynamically the initial leaf node represents the
> > 'bus' itself, and this logic means we fail to consider any "dma-ranges"
> > describing the host bridge itself. Rework the logic such that
> > of_dma_get_range() also works correctly starting from a bus node
> > containing "dma-ranges".
> >
> > While this does mean "dma-ranges" could incorrectly be in a device leaf
> > node, there isn't really any way in this function to ensure that a leaf
> > node is or isn't a bus node.
> Sorry, I'm not totally sure if this is what you're pointing out with the last
> sentence. But, what about the case of a bus configuring a device which also
> happens to be a memory mapped bus (say a PCI platform device). It'll get it's
> dma config based on its own dma-ranges which is not what we want.

What I was trying to say is we just can't tell if we should be looking
in the current node or the parent. 'dma-ranges' in a leaf node can be
correct or incorrect.

Your example is a bit different. I'm not sure that case is valid or
can ever work if it is. It's certainly valid that a PCI bridge's
parent has dma-ranges and now we'll actually handle any translation.
The bridge itself is not a DMA-capable device, but just passing thru
DMA. Do we ever need to know the parent's dma-ranges in that case? Or
to put it another way, is looking at anything other than leaf
dma-ranges useful?