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:   Fri, 16 Dec 2016 13:32:36 +0100
From:   Pali Rohár <pali.rohar@...il.com>
To:     Javier Martinez Canillas <javier@....samsung.com>
Cc:     Tony Lindgren <tony@...mide.com>,
        "Russell King - ARM Linux" <linux@...linux.org.uk>,
        Arnd Bergmann <arnd@...db.de>,
        Robin Murphy <robin.murphy@....com>,
        Linus Walleij <linus.walleij@...aro.org>,
        Ben Dooks <ben-linux@...ff.org>,
        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

Hi!

On Friday 16 December 2016 13:13:34 Javier Martinez Canillas wrote:
> Hello Pali,
> 
> On 12/16/2016 08:46 AM, Pali Rohár wrote:
> > On Thursday 15 December 2016 01:09:20 Pali Rohár 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...
> > 
> > Tony, Javier: are you aware of above ↑↑↑ problem?
> > 
> > It looks like commit 008a2ebcd677 ("ARM: dts: omap3: Remove
> > skeleton.dtsi usage") should be really reverted.
> 
> I don't think reverting the mentioned commit is the correct fix for
> your problem. We are trying to get rid of skeleton.dtsi for many
> reasons, see commit commit ("3ebee5a2e141 arm64: dts: kill
> skeleton.dtsi").

$ git show 3ebee5a2e141

* The default empty /chosen and /aliases are somewhat useless...

That is not truth, they are not useless as Russell King wrote -- 
removing them break ATAG support.

(But that commit is for arm64 which probably is not using ATAGs... But I 
do not know. At least it is not truth for 32bit arm.)

> Also, the chosen node is mentioned to be optional in the ePAPR
> document and u-boot creates a chosen node if isn't found [0] so this
> issue is only present in boards that don't use u-boot like the
> N900/N950/N9 phones.

Linux arm decompressor does not propagate ATAGs when /chosen is missing. 
Sorry, but if for Linux /chosen is required (and without it is broken!) 
then some ePARP document does not apply there. Either Linux code needs 
to be fixed (so /chosen will be really only optional) or /chosen stay in 
Linux required. There is no other option.

And I hope that U-boot is not the only one bootloader which Linux kernel 
supports. I thought that I can use *any* bootloader to boot Linux kernel 
not just U-Boot which is doing some magic...

With this step you are basically going to break booting Linux kernel 
with all others bootloaders... And personally I really dislike this 
idea.

> So if NOLO doesn't do the same than u-boot and the kernel expects a
> chosen node, I suggest to add an empty chosen node in the
> omap3-n900.dts and omap3-n950-n9.dtsi device tree source files.

That would fix a problem for N900, N950 and N9. But not for all other 
ARM devices which bootloader pass some ATAGs.

IIRC rule of kernel is not to break compatibility and that commit 
008a2ebcd677 really did it.

Note: I'm not saying if 008a2ebcd677 is good or bad. I'm just saying 
that it cause problems which need to be properly fixed. And if fixing 
them is harder and will take more time, then correct option is to revert 
008a2ebcd677 due to breaking support for more devices.

> [0]:
> http://git.denx.de/?p=u-boot.git;a=blob;f=common/fdt_support.c;h=c9f
> 7019e38e8de1469f506cdd57353fd27d8e134;hb=HEAD#l226
> 
> Best regards,

-- 
Pali Rohár
pali.rohar@...il.com

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