Message-Id: <201612161348.35917@pali>
Date:   Fri, 16 Dec 2016 13:48:35 +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

On Friday 16 December 2016 13:38:34 Javier Martinez Canillas wrote:
> Hello Pali,
> 
> On 12/16/2016 09:32 AM, Pali Rohár wrote:
> > 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.
> 
> If you think that others boards may have the same issue, then you
> could add an empty chosen node to omap3.dtsi. As I said I think that
> in practice this will only be needed for the machines using NOLO but
> you are right that in theory you could boot them using other
> bootloaders and having an empty node doesn't cause any harm anyway.

Should not be it part of any arm board? IIRC ATAG support is (or was) 
not omap3 specified.

> >> [0]:
> >> http://git.denx.de/?p=u-boot.git;a=blob;f=common/fdt_support.c;h=c
> >> 9f 7019e38e8de1469f506cdd57353fd27d8e134;hb=HEAD#l226
> >> 
> >> Best regards,
> 
> Best regards,

-- 
Pali Rohár
pali.rohar@...il.com

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