[LEDE-DEV] [PATCH] kernel: make fix extending dtb cmd with bootloader

[Martin Blumenstingl]

Hi John, Hi Hauke,

On Fri, Dec 9, 2016 at 8:08 AM, John Crispin <john at phrozen.org> wrote:
>
>
> On 09/12/2016 00:21, Hauke Mehrtens wrote:
>> This was introduced in kernel 4.4, but broken there and fixed in 4.5.
>> I would like to activate it, but I am scared about the boot loader
>> around giving us all sorts of command lines.
>>
>> Signed-off-by: Hauke Mehrtens <hauke at hauke-m.de>
>
> we had previously used a syntax of prepending the cmdline with +/- to
> indicate whether this should replace/extend the bootloader cmdline. the
> biggest problem is that the bootloaders will pass console= and root= and
> thus breaking boot. merging/activating this as is will most likely lead
> to 50% of lantiq boards not booting anymore. imho this feature should be
> an opt-in based on the specific board
Mathias just pointed me at a bug report [0] where I caused the exact
problem you described (with [1]).
When /chosen/bootargs is not defined the kernel falls back to using
the cmdline passed from the bootloader - breaking at least the console
in that specific case.

maybe we should consider the following use-cases:
1) bootloader passed bogus root=/console= parameters
2) Hauke wants to specify root= himself in the bootloader (where Hauke
stands for "developer who wants to use NFS rootfs)
3) bootloader passes some valid parameters (for example ethaddr) -
could be the same bootloader as #1

maybe we can brainstorm potential solutions which work in all cases:
- if we simply revert my patch we're fixing #1 but that would still
leave #2 and #3 unsolved
- checking with upstream why the bootloader cmdline is used
(CONFIG_MIPS_CMDLINE_FROM_BOOTLOADER is not set in our kernel config),
but that would only fix #1
- not reverting my patch and enabling CONFIG_MIPS_CMDLINE_DTB_EXTEND
would work for #2 and #3 but leave #1 broken
- reverting my patch and enabling CONFIG_MIPS_CMDLINE_DTB_EXTEND would
fix #2 and #3 while #1 is broken again
- not reverting my patch, enabling CONFIG_MIPS_CMDLINE_DTB_EXTEND and
adding a "black- or white-listed bootargs from bootloader" per device
might work for all 3 (John, I guess this is what you meant with this
opt-in specific behavior?)

I'm open for more ideas. I can also have a look at the implementation
of a patch once we decided on a solution.


Regards,
Martin


[0] https://bugs.lede-project.org/index.php?do=details&task_id=350
[1] https://git.lede-project.org/?p=source.git;a=commitdiff;h=6b94234a6598b855573a6516494de8e7d755e944