linux-kernel - Re: [PATCH 5/5] arm: boot: store ATAGs structure into DT "/chosen/linux,atags" entry

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Message-ID: <20150713131902.GH26485@atomide.com>
Date:	Mon, 13 Jul 2015 06:19:02 -0700
From:	Tony Lindgren <tony@...mide.com>
To:	Pali Rohár <pali.rohar@...il.com>
Cc:	Russell King - ARM Linux <linux@....linux.org.uk>,
	Laura Abbott <lauraa@...eaurora.org>,
	Grant Likely <grant.likely@...aro.org>,
	Rob Herring <robherring2@...il.com>,
	Will Deacon <will.deacon@....com>,
	Ivaylo Dimitrov <ivo.g.dimitrov.75@...il.com>,
	Sebastian Reichel <sre@...ian.org>,
	Pavel Machek <pavel@....cz>,
	Andreas Färber <afaerber@...e.de>,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	linux-omap@...r.kernel.org
Subject: Re: [PATCH 5/5] arm: boot: store ATAGs structure into DT
 "/chosen/linux,atags" entry

* Pali Rohár <pali.rohar@...il.com> [150707 05:00]:
> On Tuesday 07 July 2015 12:32:13 Russell King - ARM Linux wrote:
> > On Mon, Jul 06, 2015 at 10:26:13PM +0200, Pali Rohár wrote:
> > > Legacy bootloaders can pass additional information for kernel or legacy
> > > userspace applications. When booting DT kernel then ATAGs structure is not
> > > more visible after running kernel uncompress code. This patch stores full
> > > ATAGs structure into DT "/chosen/linux,atags" entry, so kernel can later
> > > reuse it and export via /proc/atags to userspace.
> > 
> > I think you need to go through your commit messages and improve them,
> > especially the ones with "TODO" in them.  As long as there's still things
> > to be done, they're obviously not ready for merging.
> > 
> 
> I know, in cover letter email I wrote that documentation is not ready...
> I send patches for review and comments (like yours). I think it is still
> better to send something and mark it as incomplete. It could prevent to
> work on something which will be again rewritten...
> 
> > Moreover, exporting the ATAGS is questionable, even _if_ there are non-
> > kexec programs making use of this.  The ATAGs have _never_ been exported
> > to userspace when kexec disabled is the kernel - it was introduced for
> > kexec, and has always had this:
> > 
> > config ATAGS_PROC
> >         bool "Export atags in procfs"
> >         depends on ATAGS && KEXEC
> >         default y
> > 
> > Now, the fact that someone decided to start using it is pretty sad,
> > because it means that if you disable KEXEC, userspace breaks.  That's
> > not a kernel regression in any shape or form, because /proc/atags has
> > never been there without KEXEC enabled.  That's a userspace bug, plain
> > and simple.
> > 
> > Given that, I'm in two minds about whether to accept the last two
> > patches which make this more than just "for KEXEC use to enable a KEXEC
> > kernel to be booted."
> > 
> > Had it been provided without the KEXEC conditional, then I don't have
> > a problem with these two patches.
> > 
> 
> I understand it. Nokia originally invented their own entries in /proc/
> which export needed ATAGs from kernel in human-readable form, but all
> those entries were non-standard and specific for Nokia's kernels.
> 
> Do you have some other idea how to provide ATAGs information created
> dynamically by legacy closed proprietary bootloader to userspace from DT
> booted kernel?
> 
> Anyway, for supporting kexec (with passing ATAGs) it is needed to have
> working /proc/atags file, right?

Yeah I think that since we already have it in /proc, we should just
support it. And keep it behind CONFIG_KEXEC and CONFIG_ARM_APPENDED_DTB
and hope we don't find other users for it.. Then reconsider the Kconfig
dependencies if we do find other users.
 
> > It also sets a precedent: by adding this into DT, it is creating a new
> > DT ABI as well, and we'll end up seeing dts files with an ATAG block
> > patched into them.
> > 
> > Are the ATAGs at a fixed address on the N900?
> 
> Yes, in board-rx51.c is:
> 
> .atag_offset	= 0x100
> 
> and Nokia Bootloader (proprietary) store them to that address.
>
> > Can that be handled in
> > some kind of legacy file for the N900 which calls save_atags() on it, so
> > we don't end up introducing yet more stuff that we have to maintain into
> > the distant future?  If not, what about copying a known working atag
> > structure into a legacy file for the N900?
>
> I already asked question if it is possible to read ATAGs from DT booted
> kernel. And somebody (do not remember who) wrote to ML, that it is not
> possible and it can be done in that uncompress code.

I guess the other option would be to keep the raw ATAG area reserved,
and only initialize /proc/atags from a board specific initcall.
But I think that would complicate the already fragile uncompress
relocation code even further?

Regards,

Tony
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