Re: Problem with commit to add overlay symbols to live device tree

[Frank Rowand <frowand.list-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>]

On 10/20/17 05:11, Alexander.Steffen-d0qZbvYSIPpWk0Htik3J/w@public.gmane.org wrote:
>> Hi Alexander,
>>
>> On 10/19/17 10:06, Alexander.Steffen-d0qZbvYSIPpWk0Htik3J/w@public.gmane.org wrote:
>>> Hi Frank, Rob,
>>>
>>> I ran some tests with kernel v4.14-rc2, where I came across an issue
>>> with your commit "of: overlay: add overlay symbols to live device
>>> tree" (d1651b03c2df75db8eda3fbcd3a07adb337ee8b0,
>>>
>> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/d
>> rivers/of?h=v4.14-rc5&id=d1651b03c2df75db8eda3fbcd3a07adb337ee8b0).
>>> I am not sure whether this is a problem with your change or whether I
>>> made a mistake somewhere.
>>>
>>> Without this commit, the following device tree overlay works fine on
>>> a Raspberry Pi:
>>>
>>> /dts-v1/;
>>> /plugin/;
>>>
>>> / {
>>> 	fragment@0 {
>>> 		target-path = "/soc/spi@7e204000";
>>> 		__overlay__ {
>>> 			#address-cells = <1>;
>>> 			#size-cells = <0>;
>>> 			status = "okay";
>>>
>>> 		        spidev1: spi@1 {
>>> 				compatible = "spidev";
>>> 				reg = <1>;
>>> 				spi-max-frequency = <5000000>;
>>> 			};
>>> 		};
>>> 	};
>>> };
>>>
>>> But with the commit, it is rejected:
>>>
>>> OF: overlay: no symbols in root of device tree.
>>> OF: overlay: of_build_overlay_info() failed for tree@/
>>> create_overlay: Failed to create overlay (err=-22)
>>>
>>> Only if I remove the spidev1 label, it continues to work with the
>>> commit (but this is not always possible in more complex overlays).
>>>
>>> I hope this does not make a difference, but I use the configfs
>>> interface to load the overlay, that is not part of mainline, but
>>> shipped by several distributions nonetheless (for example in the
>>> Raspberry Pi kernel,
>>>
>> https://github.com/raspberrypi/linux/commit/cccc24635da69799335566eb46
>> 4a4c9e1fb4a8ad).
>>>
>>>
>>> Could you tell me whether this change in behavior is intentional and
>>> whether I have to fix something in my usage?
>>
>> Yes, the change is intentional.
>>
>> If you are using overlays, the expectation is that the device tree
>> that was used to boot contains symbols (the paths of nodes that
>> have phandles) so that phandle references in the overlay can be
>> fixed up to match the values in the base devicetree.  Symbols
>> will be added to the device tree if if is compiled with the
>> "@" option.  This is the first method to avoid the overlay
>> load error.
> 
> Ah, thanks, this does solve the problem. I use nothing special to
> generate the base devicetree, just "make dtbs". Would it make sense
> to add the "@" option to the kernel Makefile, so that symbols are
> added by default? Or does this have any downside? I'd suspect that

Yes, there is a large downside.  Compiling with the "@" option will
increase the size of the DTB.  Most boards will not use overlays, 
and in those cases this overhead has no value.  So "@" should be
opt-in, not default.


> many more people will be hit by this change, since most tutorials for
> overlays that I have seen include the "@" option for the dtc calls.> 
>> If you compile the overlay with the "@" option then symbols
>> will be added to the overlay DTB.  Previous to commit
>> d1651b03c2df, these symbols would be ignored when an overlay
>> was loaded.  If you remove the "@" from the compile of
>> overlays, then everything should work the same way it did
>> before the commit.  This is the second way to avoid the
>> load overlay error.
>>
>>
>>> Thanks
>>> Alexander
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