linux-kernel - Re: [PATCH] arm: dt: Initialize boot_command_line from CONFIG_CMDLINE in case DT does not provide /chosen/bootargs

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Date:   Thu, 15 Dec 2016 00:18:08 +0000
From:   Robin Murphy <robin.murphy@....com>
To:     Pali Rohár <pali.rohar@...il.com>
CC:     Russell King - ARM Linux <linux@...linux.org.uk>,
        Arnd Bergmann <arnd@...db.de>,
        Linus Walleij <linus.walleij@...aro.org>,
        Ben Dooks <ben-linux@...ff.org>,
        Tony Lindgren <tony@...mide.com>,
        Ivaylo Dimitrov <ivo.g.dimitrov.75@...il.com>,
        Sebastian Reichel <sre@...nel.org>,
        "Aaro Koskinen" <aaro.koskinen@....fi>,
        Pavel Machek <pavel@....cz>,
        <linux-arm-kernel@...ts.infradead.org>,
        <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] arm: dt: Initialize boot_command_line from
 CONFIG_CMDLINE in case DT does not provide /chosen/bootargs

On Thu, 15 Dec 2016 01:09:20 +0100
Pali Rohár <pali.rohar@...il.com> wrote:

> On Thursday 15 December 2016 00:52:24 Russell King - ARM Linux wrote:
> > On Wed, Dec 14, 2016 at 10:12:43PM +0100, Pali Rohár wrote:
> > > Commit 008a2ebcd677 ("ARM: dts: omap3: Remove skeleton.dtsi
> > > usage") broke support for setting cmdline on Nokia N900 via
> > > CONFIG_CMDLINE.
> > >
> > > It is because arm code booted in DT mode parse cmdline only via
> > > function early_init_dt_scan_chosen() and that function does not
> > > fill variable boot_command_line when DTB does not contain /chosen
> > > entry. It is called from function early_init_dt_scan_nodes() in
> > > setup_machine_fdt().
> > >
> > > This patch fixes it by explicitly filling boot_command_line in
> > > function setup_machine_fdt() after calling
> > > early_init_dt_scan_nodes() in case boot_command_line still remains
> > > empty.
> >
> > This looks like a hack.
> >
> > First, the matter of the ATAGs compatibility.  The decompressor
> > relies on there being a pre-existing /chosen node to insert the
> > command line and other parameters into.  If we've dropped it (by
> > dropping skeleton.dtsi) then we've just regressed more than just
> > N900 - the decompressor won't be able to merge the ATAGs into the
> > concatenated FDT.
>
> Hm... I did not think about it. But right this can be broken too...
>
> > Second, CONFIG_CMDLINE has never been in place on DT systems - it's
> > something atags_parse.c has been dealing with which isn't involved
> > on DT.  For DT, we expect the command line to be passed in.
>
> If cmdline is not in DT, but /chosen exists, then function
> early_init_dt_scan_chosen() use cmdline from CONFIG_CMDLINE.
>
> > Now, given that, I find your commit message rather fishy.  You seem
> > to be claiming that CONFIG_CMDLINE used to work, but the fact is,
> > we've never had it working on device tree systems.
>
> Looks like that CONFIG_CMDLINE really did not worked (correctly) on
> DT booted systems... We have already discussed about it on #armlinux
>
> > Instead, what I think was going on is that the bug you're seeing is
> > that the removal of skeleton.dtsi results in the /chosen node being
> > absent, which in turn prevents the decompressor picking the various
> > parameters out of the ATAGs and dropping them into the DT blob.
>
> In my case, bootloader did not provided any cmdline in ATAG, so
> decompressor did not parsed any cmdline...
>
> > That, to me, sounds like a regression, and the fix is not to hack
> > support for an unrelated feature, but to fix the original problem -
> > and I think in this case, it's reasonable to ask for the offending
> > commit to be reverted.
>
> I will let decision to others, who created and accepted that patch
> 008a2ebcd677. What I need is working CONFIG_CMDLINE as I had it
> before as bootloader for Nokia N900 does not pass any cmdline -- and
> without it I cannot boot userspace.

Isn't that what CONFIG_CMDLINE_FORCE is for?

Or you could, y'know, just add a chosen node to the N900 DT to put it
back in the same state as before...

Robin.

> But still, I would like to see working CONFIG_CMDLINE support for DT
> systems. Other systems (e.g. arm ATAG based or x86) have it.
>
> What is reason that CONFIG_CMDLINE is not supported for DT?
>

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