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Date:   Tue, 31 Mar 2020 14:21:11 -0400 (EDT)
From:   Alan Stern <stern@rowland.harvard.edu>
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To:     John Donnelly <john.p.donnelly@oracle.com>
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Subject: Re: ehci-pci : WARNING: CPU: 0 PID: 347 at kernel/dma/direct.c:34
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On Tue, 31 Mar 2020, John Donnelly wrote:

> Hi ,
> 
>  One more follow up question.
> 
> How does a X86 kernel know where to look for USB buses/control
> registers ?  I am aware PowerPC has a device-tree that is passed by
> reference from UBoot that contains known devices.

Some of the information comes from ACPI tables in the BIOS.  Other
parts of it come from the components on the motherboard (for example,
devices attached to a PCI bus can be discovered directly by the
system).

> For the BCM device :
> 
>    Bus 03.Port 1: Dev 1, Class=root_hub, Driver=uhci_hcd/2p, 12M
> 
> Is there a corresponding register set I can examine ?

Yes.  For example, you can look at the files under 
/sys/kernel/debug/usb/uhci/.  You can also use lspci and other related 
PCI utilities.

Alan Stern