linux-kernel - Re: Using DT overlays for adding virtual hardware

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Date:	Tue, 21 Jun 2016 13:22:32 +0200
From:	Jan Kiszka <jan.kiszka@...mens.com>
To:	Pantelis Antoniou <pantelis.antoniou@...sulko.com>
Cc:	Mark Rutland <mark.rutland@....com>,
	devicetree <devicetree@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Jailhouse <jailhouse-dev@...glegroups.com>,
	Måns Rullgård <mans@...x.de>,
	Antonios Motakis <antonios.motakis@...wei.com>
Subject: Re: Using DT overlays for adding virtual hardware

On 2016-06-21 12:24, Pantelis Antoniou wrote:
> Hi Jan,
> 
>> On Jun 21, 2016, at 13:13 , Jan Kiszka <jan.kiszka@...mens.com> wrote:
>>
>> Hi Pantelis,
>>
>> coming back to this topic:
>>
>> On 2016-06-09 08:03, Jan Kiszka wrote:
>>> OK, trial and error, and some interesting insights: I've played with DT
>>> fragments and the overlay configfs patch of Pantelis [1] to have a
>>> convenient start. Interestingly, I wasn't able to load a fragment that
>>> followed the format specification for overlays ("Failed to resolve
>>> tree"). By chance, I got this one working:
>>>
>>> /dts-v1/;
>>> / {
>>> 	fragment {
>>> 		target-path = "/soc@...00000";
>>> 		__overlay__ {
>>> 			#address-cells = <2>;
>>> 			#size-cells = <2>;
>>>
>>> 			vpci@...000000 {
>>> 				compatible = "pci-host-cam-generic";
>>> 				device_type = "pci";
>>> 				#address-cells = <3>;
>>> 				#size-cells = <2>;
>>> 				reg = <0 0x2000000 0 0x1000000>;
>>> 				ranges =
>>> 					<0x02000000 0x00 0x10000000 0x00 0x10000000 0x00 0x30000000>;
>>> 			};
>>> 		};
>>> 	};
>>> };
>>>
>>> It successfully makes a BananaPi kernel add a pci host with the
>>> specified config space and MMIO window.
>>>
>>> [   81.619583] PCI host bridge /soc@...00000/vpci@...000000 ranges:
>>> [   81.619610]   No bus range found for /soc@...00000/vpci@...000000, using [bus 00-ff]
>>> [   81.619634]   MEM 0x10000000..0x3fffffff -> 0x10000000
>>> [   81.620482] pci-host-generic 2000000.vpci: ECAM at [mem 0x02000000-0x02ffffff] for [bus 00-ff]
>>> [   81.620779] pci-host-generic 2000000.vpci: PCI host bridge to bus 0000:00
>>> [   81.620801] pci_bus 0000:00: root bus resource [bus 00-ff]
>>> [   81.620814] pci_bus 0000:00: root bus resource [mem 0x10000000-0x3fffffff]
>>> [   81.620851] PCI: bus0: Fast back to back transfers enabled
>>>
>>> So, no /plugin/ statement, no phandles resolution. This format even
>>> builds with the in-kernel dtc. Any explanations? Does the code make
>>> sense (at least it builds without warnings)?
>>>
>>> Now I need to back this with some code in Jailhouse.
>>
>> Meanwhile I got a virtual PCI device recognized by Linux when running
>> over Jailhouse. However, my hack above doesn't get me to proper
>> interrupt mapping yet. This is what I was trying with upstream dtc:
>>
>> /dts-v1/;
>> / {
>> 	compatible = "lemaker,bananapi", "allwinner,sun7i-a20";
>>
>> 	fragment@0 {
>> 		target-path = "/soc@...00000";
>> 		__overlay__ {
>> 			#address-cells = <2>;
>> 			#size-cells = <2>;
>>
>> 			vpci@...0000 {
>> 				compatible = "pci-host-ecam-generic";
>> 				device_type = "pci";
>> 				bus-range = <0 0>;
>> 				#address-cells = <3>;
>> 				#size-cells = <2>;
>> 				#interrupt-cells = <1>;
>> 				interrupt-map-mask = <0 0 0 7>;
>> 				interrupt-map = <0 0 0 1 &gic 0 0 0 123 4>,
>> 						<0 0 0 2 &gic 0 0 0 124 4>,
>> 						<0 0 0 3 &gic 0 0 0 125 4>,
>> 						<0 0 0 4 &gic 0 0 0 126 4>;
>> 				reg = <0 0x2000000 0 0x100000>;
>> 				ranges =
>> 					<0x02000000 0x00 0x10000000 0x00 0x10000000 0x00 0x30000000>;
>> 			};
>> 		};
>> 	};
>>
>> 	gic: fragment@1 {
>> 		target-path = "/soc@...00000/interrupt-controller@...81000";
>> 		__overlay__ {
>> 		};
>> 	};
>> };
>>
> 
> ^ This is not going to work: You need the reference to the real gic not the empty fragment
> here that has a target there.
> 
> You need to compile with the correct dtc, and you also need to compile the base dts
> with dtc too, using the -@ flag. You can hack around it by adding something like
> 
> __symbols__ {
> 	gic = "/soc@...00000/interrupt-controller@...81000”;
> };
> 
> But you really need the __symbols__ node of the base dts generated by the dtc proper cause
> the above is a dirty hack.
> 

OK, re-building the kernel with DTC="/your/dtc -@", thus building the
base dtb with symbols, fixes proper overlay format loading.

However, no luck yet with the interrupt topic - maybe a different issue.
Digging deeper...

Thanks,
Jan

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