Re: Moving ARM dts files

["Andreas Färber" <afaerber@suse.de>]

Am 05.12.18 um 05:17 schrieb Rob Herring:
> On Tue, Dec 4, 2018 at 7:22 PM Andreas Färber <afaerber@suse.de> wrote:
>>
>> Rob,
>>
>> Am 04.12.18 um 19:36 schrieb Rob Herring:
>>> I've put together a script to move the dts files and update the
>>> makefiles. It doesn't handle files not following a common prefix which
>>> isn't many and some includes within the dts files will need some fixups
>>> by hand.
>>>
>>> MAINTAINERS will also need updating.
>>>
>>> A few questions:
>>>
>>> Do we want to move absolutely everything to subdirs?
>>
>> This refactoring is a terrible idea!
> 
> How do you really feel?
> 
>> While it would've been nice to have more structure from the start,
>> bootloaders like U-Boot expect a flat structure for arm .dtb files now.
>> If you start installing them into subdirs instead, they won't find the
>> files anymore under the hardcoded name.
>>
>> Doing this only for new platforms would be much less invasive and allow
>> to prepare bootloaders accordingly.
> 
> That was my suggestion where this started for the new RDA platform.
> Olof preferred to move everything and that's my desire too.
> 
>> Alternatively, white-list which ones
>> are safe to move around.
> 
> I'd prefer to know which ones the distros don't want moved. That
> should be easier to figure out. We also need that anyways in context
> of what platforms we care about compatibility.
> 
> Another option is dtbs_install target could flatten the installed
> dtbs. That is the only part the distros should depend on.

I'd be okay with distinguishing source vs. install location. Due to the
issue I mention below (and more) we can't use install_dtbs for openSUSE
and had to reimplement it, which we'd need to (and can) adjust.

>> But don't just script a refactoring because it
>> looks nicer in the source tree, without testing what side effects this
>> can have for board/distro users of the compiled files in practice.
>> We already had that discussion for arm64 because Debian chose to ignore
>> the kernel-installed subdirectories and installed .dtb files into a flat
>> directory, which collided with openSUSE sticking to the kernel choice.
> 
> So everyone already deals with subdirs or not with arm and arm64
> already, seems like they can deal with this. I will raise the topic on
> the cross-distro list though.

Sounds like you're twisting words... The keyword was "hardcoded" paths -
one way or another (not "and") depending on the kernel choices being
flat for arm, vendor subdir for arm64.

>> This topic becomes even more important with EBBR: There is neither a
>> mechanism in place to sync .dts files into U-Boot or EDK2 source trees,
>> nor are capsule updates implemented in U-Boot for easily deploying such
>> bootloaders with new .dts sources or paths yet.
> 
> EBBR actually says firmware (including dtbs) goes in directories named
> by vendor.

Fine, but unrelated.

>> And I can assure you
>> that just getting users to dd the right bootloader can be difficult...
>> Since DT forward and backward compatibility is often being neglected,
>> for example with optional properties or renamed compatibles that break
>> booting with previous drivers, new kernel versions often need updated
>> Device Trees to make use of new/enhanced drivers. Therefore it is
>> unfortunately often enough a necessity to load newer kernel-based .dtb
>> files matching the kernel (as opposed to the dream of kernel-independent
>> hardware descriptions) when working with the latest -rc or -next kernels
>> at least. For examples of DTs needing updates, look no further than
>> Linaro's 96boards - in case of hikey960/EDK2 GRUB is another layer where
>> .dtb paths may be hardcoded, ditto for arm; and Armada was an example
>> where the upstream bindings for the network IP changed incompatibly.
> 
> Compatibility is an issue, yes, but that really has nothing to do with this.
> 
>> DT overlays are another topic that is not making any progress upstream
>> according to the ELCE BoF, so beyond the Raspberry Pi the only known
>> working way to apply them is to write a U-Boot boot.scr script, which
>> can either reuse $fdtcontroladdr DT or use the filename $fdtfile or
>> hardcode one, the latter two of which would break with your renaming.
> 
> DT overlays also have nothing to do with this as there aren't any in
> the kernel. I'm not inclined to take any either with a flat tree.
> We're already at 1800+ files.

Read again: a) Breaking DT changes and b) the desire to use Overlays
instead of replacing the bootloaders for each change are _reasons_ why
people depend on .dtb filenames from the kernel source tree for their
boot flow today. Nothing to do with downstream .dtbo files.

For example, remember when I reported that the kernel didn't compile DTs
with -@? No reaction except for Frank asking to be CC'ed - was it ever
fixed??? Do EDK2's or U-Boot's built-in DTs compile with -@ today?
Raspberry Pi overlays in U-Boot work because it switched to passing the
DT through from the proprietary firmware.
Point being that while it would be nice to get a current, compatible DT
via UEFI tables and ignore .dtb filenames outside of a few bootloaders,
in reality we're not quite there yet for all platforms.

I see no problem (except for naming choices) moving new targets like RDA
to subfolders because we can then hardcode it the new way; I also assume
deeply embedded targets like stm32f4 or targets with no mainline
bootloaders yet like owl-s500 could be refactored.
I do see problems refactoring widely used SBC targets like sunXi though.

Regards,
Andreas

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