21.11.2015 15:47, Andreas Färber пишет:
> Am 21.11.2015 um 06:49 schrieb Matwey V. Kornilov:
>> 21.11.2015 01:08, Andreas Färber пишет:
>>> Am 20.11.2015 um 20:09 schrieb Matwey V. Kornilov:
>>>> The flavor of dtb (dtb file) is to be specified in
>>>> /etc/sysconfig/bootloader
>>>
>>> That shouldn't be necessary, U-Boot is supposed to provide $fdtfile.
>>> In my testing I didn't need to specify a dtb file in extlinux.conf, only
>>> the dtb directory.
>>
>> That is great news, but I think it is better to provide optional
>> possibility to set the file manually. Let me describe use-case.
>>
>> BeagleBone Black supports the capes, and the kernel still doesn't have
>> an user-space API for overlays. So, to enable my specific hardware I
>> will modify am335x-beaglebone.dts and compile new version, say
>> am335x-beaglebone-with-geiger-counter-cap.dtb. Sure, when I will deploy
>> this to my production, I will make a package
>> dtb-am335x-with-geiger-counter-%{version}.rpm (Please, see above,
>> imagine, new kernel update has been issued and my build service has not
>> rebuild this package yet, so I'll receive an updated kernel and will not
>> receive and updated dtb package)
>>
>> How should u-boot guess what I want now? So, please, let the user some
>> freedom.
>
> Maybe your statement above was ambiguous - for me "is to be specified"
> reads as "must be specified". To that I objected.
>
> You're free to add an optional mechanism to override it, of course. But
> I would prefer to get the current pull sorted out and tested before we
> investigate such extra features.
Sure, I've missed that fdtdir and fdt are mutually exclusive, I will
handle this shortly and then update the pull-request.
>
> I do wonder whether there is a better way, changing fdtfile in U-Boot's
> environment rather than FDT in extlinux.conf, for instance.
> Unfortunately extlinux.conf is read before boot.scr, otherwise you
> could've overridden it there and called sysboot from the script. And
> uEnv.txt is only read by a handful of boards, so not a general solution,
> but I believe Matthias reported such an error on BBB?
Yes, it can be a great solution.
An environment should be fixed if we want to implement fallback booting
finally. Unfortunately, I've heard nothing about issues with uEnv.txt on
BBB.
> Do you actually need a different filename though? If you want to have a
> side-by-side installation you would really also need two extlinux.conf
> entries per kernel to switch easily. Otherwise you could just leave the
> .dtb filename the same for your modified package, needing no
> modifications to extlinux.conf.
Why can't I have two entries with the same kernel and different dtb's?
There are cases when editing of dts is safe and does not require
recompiling the kernel. For instance, enabling some disabled-by-default
node.
> BTW I thought that months ago we had enabled DT overlays in the kernel
> on your request. What's missing to make overlays work? I don't have any
> capes/hats/shields/etc.
DT overlays is a technique to change device tree on the fly, it is great
that it is compiled and does not crashes. Unfortunately, there is
currently still no interface to ask the kernel: "ok, lets change the
device tree in the specific manner".
See https://lkml.org/lkml/2015/5/13/157 for instance
>
> Regards,
> Andreas
>
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