Re: [PATCH] drivers/of: add option to load a default Device Tree

[Jose Abreu <Jose.Abreu-HKixBCOQz3hWk0Htik3J/w@public.gmane.org>]

Hi Franck,

++ Carlos

On 24-06-2016 09:15, Franck Jullien wrote:
> Le 23/06/2016 à 20:27, Frank Rowand a écrit :
>> On 06/22/16 08:05, Franck Jullien wrote:
>>> Even if a platform doesn't use a device tree during its
>>> boot process it can be useful to enable CONFIG_OF and get
>>> an empty device tree.
>>>
>>> Then, devices can use device tree overlays to populate this
>>> default tree.
>>
>> Hi Franck,
>>
>> Can you elaborate on the problem that you are trying to
>> solve and the use cases that you are envisioning?
>>
>> Thanks,
>>
>> Frank
>>
> Hi,
>
> The idea behind this patch is to describe devices that are in a FPGA
> which is itself connected to the platform through PCIe. The platform
> might not use a devicetree for its own configuration. That's why we need
> this patch.

This sure looks interesting to us because we are using the
exactly same setup (x86_64 + FPGA through PCIe). As I said before
in the DMA mailing list our approach was different: we use
platform data in each driver and we register them using a PCI
driver which passes all the required parameters. Still, we were
wondering if your approach can save us development time in the
future. So, I have a few questions that I was hoping you could
answer.

>
> There is an ongoing  discussion about this subject [1].
>
> I have a working setup (on a x86_64) using this principle. I have
> created a pci resources proxy module which is parent of all devices on
> the FPGA. This is an example of binding:
>
> 	proxy: pci1337_0001 {
> 		#address-cells = <1>;
> 		#size-cells = <1>;
> 		#interrupt-cells = <1>;
> 		vendor-id = <0x00001337>;
> 		device-id = <0x00000001>;
> 		compatible = "pci1337,0001",
> 			     "generic,pci-proxy";
> 		interrupt-controller;
> 		/* Ranges will be overwritten during probe */
> 		ranges = <0 0 0xA0000000 0x100000     /* BAR 0 */
> 			  1 0 0xB0000000 0x100000     /* BAR 1 */
> 			  2 0 0xC0000000 0x100000>;   /* BAR 2 */
> 	};
>
> Ranges are dynamically updated on probe with regards to FPGA bars.
> This node is attached to the tree (an empty tree in my case) from the
> client module.

I am not fully understanding where does this binding is added.
You use your patches to create an empty device tree or a device
tree where the FPGA is declared?

>
> Then FPGA devices nodes are added using overlay. This is an example:
>
> /dts-v1/;
> / {
> 	#address-cells = <1>;
> 	#size-cells = <1>;
>
> 	fragment@0 {
> 		target-path = "/pci1337_0001";
> 		__overlay__ {
>
> 			#address-cells = <2>;
> 			#size-cells = <1>;
> 			interrupt-parent = <&proxy>;
>
> 			osc: oscillator {
> 				compatible = "fixed-clock";
> 				#clock-cells = <1>;
> 				clock-frequency  = <148500000>;
> 				clock-output-names = "osc";
> 			};
> 			enet0: ethoc {
> 				compatible = "opencores,ethoc";
> 				reg = <1 0x50000 0x100>;
> 				interrupts = <0>;
> 			};
> 			gpio0: gpio {
> 				#gpio-cells = <2>;
> 				compatible = "xlnx,xps-gpio-1.00.a";
> 				reg = <0 0x30000 0x10000>;
> 				gpio-controller;
> 			};
> 		};
> 	};
>
>
> };
>
> Interrupts property are dynamically modified before the overlay is
> applied. By the way, I had to do a little hack here in the pci-proxy.
> I did not declare a new interrupt domain. What I did is to get the PCI
> interrupt domain (IO-APIC on my platform):
>
> irq_data = irq_get_irq_data(priv->pci_dev->irq);
> domain_parent = irq_data->domain;
>
> and then set fwnode of IO-APIC to pci-proxy fwnode so
> irq_create_of_mapping get IO-APIC's domain:
>
> domain_parent->fwnode = &np->fwnode;

In my case we have an interrupt controller driver which is
embedded into the PCI driver so I also can't use the generic PCI
proxy without modifying it. Can this controller be declared in
the overlay device tree and then make the remaining drivers use
the domain created?

>
> There might be a better way to make things work.
>
> Beside this patch I needed to export of_attach_node and of_detach_node.
> This will be another patch.

There is still one more thing: By default CONFIG_OF is not
enabled in x86_64 so you need to enable this, right? Shouldn't a
"select OF" be added to your kconfig entry?

>
> Thank you for your interest.
>
> Franck.
>
> [1] http://www.spinics.net/lists/dmaengine/msg09869.html
>

Best regards,
Jose Miguel Abreu
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