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

[Nicolas Saenz Julienne]

On Tue, 2019-10-08 at 20:03 -0500, Rob Herring wrote:
> 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.

Yes, you're right, I hadn't thought of it from that perspective. Thanks!

> 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?

There is no need at all indeed.

With that,

Reviewed-by: Nicolas Saenz Julienne <nsaenzjulienne@xxxxxxx>

and

Tested-by: Nicolas Saenz Julienne <nsaenzjulienne@xxxxxxx>

On a Raspberry Pi 4 with pcie-brcmstb.c which is still work in progress but
depends on this.

Regards,
Nicolas

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