Reparenting a platform device

[Stephen Warren]

On 04/05/2012 12:08 PM, Thierry Reding wrote:
> * Thierry Reding wrote:
>> * Stephen Warren wrote:
>>> 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.
>>
>> Actually I have no idea why this was made a requirement. Maybe Hiroshi can
>> comment on this. The driver really only needs this to basically obtain a
>> pointer to itself. The MSM I/O MMU implementation does something similar,
>> though, and goes on to register actual child devices (they are instantiated
>> in arch/arm/mach-msm/devices-iommu.c). Each of those devices is then assigned
>> a specific memory area it seems.
> [...]
>> I don't think having more than one device using the IOMMU will work properly
>> anyway in the context of the Tegra GART driver because there is not means to
>> allocate specific regions of the GART aperture to individual devices. So
>> really the one and only client actually needs to manage the allocations from
>> the GART aperture.
> 
> I've been thinking about this some more and it occurred to me that maybe it
> would be best to add an additional layer between the GART and the clients
> which could manage the allocations. Such a virtual device could actually be
> registered as a child of the GART device and allow individual drivers to
> request mappings from it so that there is a single instance handing them out
> and keep them consistent.
> 
> How does that sound?

I must admit I'm not at all familiar with the IOMMU APIs, but isn't the
IOMMU driver/subsystem itself what is managing all the allocations and
handing them out to clients? And client drivers do things like asking
for N pages of memory mapped into their aperture? If that is true, I'm
not sure what the purpose is of the intermediate device you propose.
Sorry for being somewhat clueless.