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

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Date:	Tue, 7 Jul 2015 13:58:19 +0200
From:	Pali Rohár <pali.rohar@...il.com>
To:	Russell King - ARM Linux <linux@....linux.org.uk>
Cc:	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>, Tony Lindgren <tony@...mide.com>,
	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

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?

> 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.

-- 
Pali Rohár
pali.rohar@...il.com
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