Re: Raspberry Pi 4 DMA range fix

[Jared McNeill <jmcneill%invisible.ca@localhost>]

On May 3, 2022, at 6:49 PM, Jason Thorpe <thorpej%me.com@localhost> wrote:
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>> On May 3, 2022, at 2:46 PM, Jared McNeill <jmcneill%invisible.ca@localhost> wrote:
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>> That patch isn’t quite right because _DMA restrictions apply only to children of the bus that it appears on (ACPI 6.4 section 6.2.4). So yeah, we want _DMA to apply to PCI devices that are a child of the PCI0 device here, but this isn’t the right way to go about it.
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>> I think we need to special case this for acpipchb because we don’t really have a clean way of expressing the relationship between a PCI device and firmware tables.
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> Do you mean a PCI bus instance or a child device of the PCI bus?

In this case it’s the bus that needs to know about the restrictions that it has to apply to child devices. I got a bit ahead of myself here thinking about device properties and lumping those two problems together :)

I guess technically, you _could_ model a case where some child bus has restrictions that don’t exist on the parent. The hard handoff from “acpinodebus” to “pcibus” here kind of makes that challenging. Not sure how realistic that example is though, but you could imagine a similar situation on any other bus where we do the same acpi -> whatever handoff.