[PATCH 5/5] arm: boot: store ATAGs structure into DT "/chosen/linux,atags" entry
tony at atomide.com
Mon Oct 12 13:16:40 PDT 2015
* Tony Lindgren <tony at atomide.com> [150713 06:21]:
> * Pali Rohár <pali.rohar at gmail.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?
Pali, any news on posting an updated series with the comments addressed
in this thread? It seems that we all pretty much agree what needs to
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