linux-kernel - Re: [PATCH] firmware: Add device tree binding for coreboot

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Message-ID: <CAODwPW9tRM8kxBVg3apkec2K1rahuENk_b_BD_0jvXON7JzeaA@mail.gmail.com>
Date:	Fri, 13 Jun 2014 14:58:51 -0700
From:	Julius Werner <jwerner@...omium.org>
To:	Rob Herring <robherring2@...il.com>
Cc:	Julius Werner <jwerner@...omium.org>,
	Rob Herring <robh+dt@...nel.org>,
	Pawel Moll <pawel.moll@....com>,
	Mark Rutland <mark.rutland@....com>,
	Ian Campbell <ijc+devicetree@...lion.org.uk>,
	Kumar Gala <galak@...eaurora.org>,
	Stephen Warren <stephen.r.warren@...il.com>,
	Doug Anderson <dianders@...omium.org>,
	Olof Johansson <olofj@...omium.org>,
	Stefan Reinauer <stefan.reinauer@...eboot.org>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] firmware: Add device tree binding for coreboot

> This is just to export a fixed log to userspace (like a DMI table) or
> the kernel will actually use the data in some way? Based on the link,
> it looks like the former to me.

I could imagine both. The link is an in-kernel driver that exposes a
log through a sysfs node (in a way that has already been established
on x86 systems, which find the location through EBDA or ACPI entries
instead). We are also using a user-space tool that reads the address
from /proc/device-tree and accesses it through /dev/mem. The areas can
contain many interesting entries (like the location of an early
framebuffer set up by the firmware), so I could also imagine use cases
where the kernel makes use of it directly.

> Don't you need need to keep the kernel from allocating this memory by
> using one of the reserved memory mechanisms? The recently added one
> should be able to specific what the memory is reserved for IIRC.

Our bootloader is carving the location out of the /memory node and
adding it to the device tree reserve map. As far as I know, that only
contains a list of raw start and size entries. At any rate, I think
it's useful (and in line with other bindings) to add a more explicit
node like this (if only to make it easier accessible through
/proc/device-tree).

> /firmware is already used IIRC. What if you have other firmware such
> as Trustzone?

I'm not quite sure how Trusted Foundations works and whether it would
even make sense to use it in parallel to coreboot, but it seems to be
using the /firmware/trusted-foundations subnode so that should be
fine. "firmware" seems to be used by other firmware implementations
(like "samsung,secure-firmware") which are similar in nature to and
mutually exclusive with coreboot, so I thought the node makes sense.
(The kernel should use the compatible string to find it anyway, so a
future name clash would not be world-ending.)
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