Re: [PATCH] external references for device tree overlays

[Stefani Seibold]

Hi Pantelis,

On Wed, 2017-06-07 at 11:11 +0300, Pantelis Antoniou wrote:
> Hi Stefani,
> 
> On Tue, 2017-06-06 at 21:17 +0200, Stefani Seibold wrote:
> > Hi Pantelis,
> > 
> > thanks for the suggestion. This feature is not very well
> > documented. I
> > tried this on my rasp1 running 4.12.0-rc3 and it doesn't work. My
> > source is:
> > 
> > // rapsi example
> > /dts-v1/;
> > /plugin/;
> > 
> > / {
> > ÂÂÂÂcompatible = "brcm,bcm2835", "brcm,bcm2708", "brcm,bcm2709";
> > 
> > ÂÂÂÂfragment@0 {
> > ÂÂÂÂÂÂÂÂtarget-path = "/soc/i2s@7e203000";
> > ÂÂÂÂÂÂÂÂ__overlay__ {
> > ÂÂÂÂÂÂÂÂÂÂÂÂ#address-cells = <0x00000001>;
> > ÂÂÂÂÂÂÂÂÂÂÂÂ#size-cells = <0x00000001>;
> > ÂÂÂÂÂÂÂÂÂÂÂÂtest = "test";
> > ÂÂÂÂÂÂÂÂÂÂÂÂtimer = <&{/soc/timer@7e0030000}>;
> > ÂÂÂÂÂÂÂÂ};
> > ÂÂÂÂ};
> > };
> > 
> > 
> > The resulting overlay is (decompiled with fdtdump):
> > 
> > /dts-v1/;
> > // magic:		0xd00dfeed
> > // totalsize:		0x19a (410)
> > // off_dt_struct:	0x38
> > // off_dt_strings:	0x148
> > // off_mem_rsvmap:	0x28
> > // version:		17
> > // last_comp_version:	16
> > // boot_cpuid_phys:	0x0
> > // size_dt_strings:	0x52
> > // size_dt_struct:	0x110
> > 
> > / {
> > ÂÂÂÂcompatible = "brcm,bcm2835", "brcm,bcm2708", "brcm,bcm2709";
> > ÂÂÂÂfragment@0 {
> > ÂÂÂÂÂÂÂÂtarget-path = "/soc/i2s@7e203000";
> > ÂÂÂÂÂÂÂÂ__overlay__ {
> > ÂÂÂÂÂÂÂÂÂÂÂÂ#address-cells = <0x00000001>;
> > ÂÂÂÂÂÂÂÂÂÂÂÂ#size-cells = <0x00000001>;
> > ÂÂÂÂÂÂÂÂÂÂÂÂtest = "test";
> > ÂÂÂÂÂÂÂÂÂÂÂÂtimer = <0xdeadbeef>;
> > ÂÂÂÂÂÂÂÂ};
> > ÂÂÂÂ};
> > ÂÂÂÂ__fixups__ {
> > ÂÂÂÂÂÂÂÂ/soc/timer@7e0030000 = "/fragment@0/__overlay__:timer:0";
> > ÂÂÂÂ};
> > };
> > 
> > But this will not apply:
> > 
> > OF: resolver: overlay phandle fixup failed: -22
> > create_overlay: Failed to resolve tree
> > 
> > 
> 
> Yes, it will not work as it is; my point is that you don't need the
> magic __*__ node.
> 

The magic __fixups__ node was inserted by the device tree compiler. I
use the dtc from https://github.com/pantoniou/dtc at commit
d990b8013889b816ec054c7e07a77db59c56c400.

> You will need to modify the overlay application code to live insert a
> phandle (if it doesn't exist) when it encounters a /path fixup.
> 

That is part of my patch!

> > Anyway, the reason for my patch is that i can reference to nodes
> > which
> > lacks a phandle. The phandle will be created on the fly and also
> > destroyed when the overlay is unloaded.
> > 
> > I have a real use case for this patch:
> > 
> > I have a BIOS on some ARM64 servers which provides broken device
> > tree.
> > It also lacks some devices in this tree which needs references to
> > other
> > devices which lacks a phandle.
> > 
> > Since the BIOSes are closed source i need a way to work arround
> > this
> > problem without patching all the drivers involved to this devices.
> > 
> > Hope this helps to understand the reason for this patch.
> > 
> 
> FWIW your problem seems like something that would happen on the
> field.
> We can berate the vendor of not providing the correct device tree,
> but
> in the end workarounds for broken vendor things are common in the
> kernel.
> 

Yes, that is the way how linux do the things. Linux has a long history
to bypassing bugs of BIOSes, ACPI or broken devices.

Greetings,
Stefani