Re: Reparenting a platform device

On 04/05/2012 02:42 AM, Thierry Reding wrote:

Hi,

I have a device tree where I have a GART device and a DRM device which uses
the GART. The GART is implemented by an IOMMU driver (tegra-gart) and
requires the user device to be a child of the GART device (it explicitly
checks for this when the user device is attached).

Isn't this wrong?

I would expect the device parent/child relationship to reflect the
CPU-initiated register access bus topology.

A device's interaction with an IOMMU is an aspect of a device's
initiating accesses itself, not CPU-initiated register accesses.

I've tried two alternatives to achieve this: create the GART device in the
user driver's .probe() function and explicitly set the DRM device's parent
to the resulting platform device like so:

	gart = platform_device_alloc(...);
	...
	pdev->dev.parent = &gart->dev;

I guess that won't work when there's more than one device affected by
the IOMMU?

The alternative is to use the device tree to look up the GART device node and
resolve it to the corresponding struct device:

	gart_node = of_parse_phandle(drm->dev->of_node, "gart-parent", 0);

That seems more logical to me.

	gart = bus_find_device(drm->dev->bus, NULL, gart_node, match_of_node);

That part should probably be encapsulated into the IOMMU subsystem? In
fact, even the of_parse_phandle should perhaps be hidden inside some
IOMMU iommu_get() call, that can use DT as a data source, or some other
data structure set up by board files.