[Skiboot] [ppc/pnv9] clarification on the dtb

[Frederic Barrat]

On 16/03/2022 17:20, Amol wrote:
> Hello,
> 
> Below is a part of the dtb file as passed on by the skiboot firmware to a
> ppc64 pnv9 VM.
> 
> The skiboot version is the one shipped with qemu's version
> "QEMU emulator version 6.2.50 (v6.2.0-1976-g6629bf78aa)"
> 
> / {
> 	....
> 	compatible = "qemu,powernv9\0ibm,powernv";
> 	model = "IBM PowerNV (emulated by qemu)";
> 	pciex at 600c3c0000000 {
> 		compatible = "ibm,power9-pciex\0ibm,ioda3-phb";
> 		device_type = "pciex";
> 		....
> 		#address-cells = <0x03>;
> 		#size-cells = <0x02>;
> 		#interrupt-cells = <0x01>;
> 		interrupt-parent = <0x804e>;
> 		ranges = <0x2000000 0x00 0x80000000 0x600c0 0x00 0x00
> 			0x7fff0000>;
> 		....                 /*  <-------- no interrupt-map HERE */
> 		pci at 0 {
> 			interrupts = <0x01>; /* <------------------ HERE */
> 			#address-cells = <0x03>;
> 			#size-cells = <0x02>;
> 			#interrupt-cells = <0x01>;
> 			interrupt-map-mask = <0x00 0x00 0x00 0x07>;
> 			interrupt-map =
> <0x00 0x00 0x00 0x01 0x804e 0xfeff8 0x01
>   0x00 0x00 0x00 0x02 0x804e 0xfeff9 0x01
>   0x00 0x00 0x00 0x03 0x804e 0xfeffa 0x01
>   0x00 0x00 0x00 0x04 0x804e 0xfeffb 0x01>;
> 			ranges = <0x2000000 0x00 0x00 0x2000000 0x00 0x00
> 				0xf0000000 0x00>;
> 		};
> 	};
> 
> Of interest is the "interrupts = <0x01>;" property of the pci at 0 node. It has
> "interrupts = <0x01>" property, which refers to the PCI #INTA. But its parent,
> the ioda3 node, does not have the corresponding interrupt-map facility to
> map the #INTA over to a descriptor understood/required by the interrupt
> controller.
> 
> I checked a dtb, linked at [2], seemingly captured from a hardware machine,
> and found no such violation at least among the pci nodes. In it, there is
> a pci at 0 node with "interrupts = <0x01>", but it does have a parent, another
> pci at 0 node, which possesses the interrupt-map mapping required for translation.
> 
> I checked the qemu's raw DTB output and found that it does not contain those
> pci-related nodes - they are being added by skiboot.
> 
> Is such a setup allowed under the dtb spec? The skiboot function which seems to
> add the nodes is pci_add_one_device_node or its peers.


That is a bug from the powernv model of qemu. It reuses a "generic" 
root port device which already existed in qemu and that generic device 
declares it wants a LSI (Interrupt Pin Register (Offset 3Dh) in the 
config space). That's where the following entry is coming from:

  		pci at 0 {
			interrupts = <0x01>; /* <-------- HERE

However skiboot doesn't add an interrupt-map property under 
"pciex at 600c3c0000000" to handle that LSI interrupt. Because, as you 
noticed, that doesn't make sense on real hardware.

So you're correct, the device tree seen under qemu is wrong.

Any specific reason why you're looking into it?

   Fred




> Thanks,
> Amol
> 
> [1] https://elinux.org/Device_Tree_Usage#Advanced_Interrupt_Mapping
> [2] https://people.freebsd.org/~jhibbits/p9_vga_out_devtree.txt
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