linux-kernel - Re: [PATCH v4 2/8] OF: Introduce DT overlay support.

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Date:	Tue, 20 May 2014 09:38:49 +0200
From:	Geert Uytterhoeven <geert@...ux-m68k.org>
To:	Grant Likely <grant.likely@...retlab.ca>
Cc:	Pantelis Antoniou <pantelis.antoniou@...sulko.com>,
	Rob Herring <robherring2@...il.com>,
	Stephen Warren <swarren@...dotorg.org>,
	Matt Porter <matt.porter@...aro.org>,
	Koen Kooi <koen@...inion.thruhere.net>,
	Alison Chaiken <Alison_Chaiken@...tor.com>,
	Dinh Nguyen <dinh.linux@...il.com>,
	Jan Lubbe <jluebbe@...net.de>,
	Alexander Sverdlin <alexander.sverdlin@....com>,
	Michael Stickel <ms@...able.de>,
	Guenter Roeck <linux@...ck-us.net>,
	Dirk Behme <dirk.behme@...il.com>,
	Alan Tull <delicious.quinoa@...il.com>,
	Sascha Hauer <s.hauer@...gutronix.de>,
	Michael Bohan <mbohan@...eaurora.org>,
	Ionut Nicu <ioan.nicu.ext@....com>,
	Michal Simek <monstr@...str.eu>,
	Matt Ranostay <mranostay@...il.com>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Pete Popov <pete.popov@...sulko.com>,
	Dan Malek <dan.malek@...sulko.com>,
	Georgi Vlaev <georgi.vlaev@...sulko.com>
Subject: Re: [PATCH v4 2/8] OF: Introduce DT overlay support.

Hi Grant,

On Tue, May 20, 2014 at 7:50 AM, Grant Likely <grant.likely@...retlab.ca> wrote:
>> Why has the overlay system been designed for plugging and unpluging whole
>> overlays?
>> That means the kernel has to remember the full stack, causing issues with
>> e.g. kexec.
>
> Mostly so that drivers don't see any difference in the livetree data
> structure. It also means that userspace sees a single representation of
> the hardware at any given time.

Sorry, I don't follow the argument about the "single representation of the
hardware".

>> Why not allowing the addition of removal of subtrees of the full device
>> tree?
>
> Overlays is more than just a subtree. A single overlay can make
> manipulations of multiple subtrees that should be handled as logically
> atomic.

Sure, it's more complicated due to the atomicity of multiple changes.

>> This is similar to other hotpluggable subsystems (which are not necessarily
>> DT-based), like PCI Express. That way the kernel can pass a
>> DT-representation of the actual current device tree to a kexec'ed kernel.
>
> I'm not following you argument. Hardware hotplug systems like PCIe don't
> manipulate the firmware data. The kernel detects the new device and
> populates the Linux device model directly. Firmware provided data (ACPI
> or FDT) isn't involved.

I mean the kernel doesn't remember the exact order in a stack, to reverse
operations. E.g. I can add hotplug a PCIe bridge with multiple devices
behind it, and unplug a single device later. It's still one subtree, though.

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds
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