Re: [PATCH 0/8] Basic ARM device tree support

From: Rob Herring
Date: Wed Jan 26 2011 - 17:33:04 EST

On 01/26/2011 02:04 PM, John Linn wrote:
-----Original Message-----
From: Grant Likely [mailto:glikely@xxxxxxxxxxxx] On Behalf Of Grant Likely
Sent: Tuesday, January 18, 2011 1:29 PM
To: linux-arm-kernel@xxxxxxxxxxxxxxxxxxx
Cc: Nicolas Pitre; Russell King; Catalin Marinas; devicetree-discuss@xxxxxxxxxxxxxxxx; linux-
kernel@xxxxxxxxxxxxxxx; Olof Johansson; Jeremy Kerr; John Linn; Lennert Buijtenhek
Subject: [PATCH 0/8] Basic ARM device tree support

It's been a while since these patches have been circulated for review.
Much has changed since the last posting and there are no longer any
ugly hacks or workarounds to it working. From my viewpoint, these
patches are complete and ready to be used.

This patch series allows CONFIG_OF to be enabled on ARM and allows the
kernel to accept a dtb pointer from boot firmware instead of atags.
If a dtb is passed, then the kernel will use the root 'compatible'
property to find a matching machine_desc, and it will use it to obtain
the memory layout, initrd location and kernel parameters string.

Only limited device tree support is enabled here. It does not perform
any kind of device registration from device tree data. That support
will be enabled in a later patch set. Right now those patches depend
on the common struct clk patches that Jeremy is working on, so I'm not
going to post them yet. Rather, this series is a stable base that
other engineers can use to start working on device tree support for
their platforms.

Tested on Versatile (qemu) and nVidia Tegra Harmony. Kernel compiles
an boot with CONFIG_OF both on and off, both with and without passing
a device tree blob.

Tested on Xilinx platform. Did see an issue with loading the device
tree blob into higher memory. At address 0 there was no issue.

I have a fix for this. The issue is the early mmu page table needs to map the dtb. I later realized this is also may be problem for ATAGs as well, so I planned to re-work to make it unconditional and work for ATAGs. Or perhaps ATAGs only get accessed with the MMU on later than the dtb and everything is okay?

This applies to the DT patch set prior to the one Grant sent out
this week, so it may need updates.

From: Rob Herring <rob.herring@xxxxxxxxxxx>
Date: Tue, 5 Oct 2010 10:32:51 -0500
Subject: [PATCH] arm/dt: map devtree blob in initial page table

If the DTB is not in 1st 1MB of RAM, it will not be mapped before
memblock is setup, so map it early. We assume the dtb is less than
1MB and map 2MB so it can be at any alignment.

Some assembly optimizations from Nicolas Pitre.

Signed-off-by: Rob Herring <rob.herring@xxxxxxxxxxx>
arch/arm/kernel/head.S | 13 +++++++++++++
1 files changed, 13 insertions(+), 0 deletions(-)

diff --git a/arch/arm/kernel/head.S b/arch/arm/kernel/head.S
index 0480f25..1fcb962 100644
--- a/arch/arm/kernel/head.S
+++ b/arch/arm/kernel/head.S
@@ -228,6 +228,19 @@ __create_page_tables:
str r6, [r0]

+ /* Make sure DT blob is mapped */
+ adr r0, __devtree_pointer
+ ldr r0, [r0]
+ mov r0, r0, lsr #20
+ mov r0, r0, lsl #20
+ add r3, r0, #PAGE_OFFSET
+ add r3, r4, r3, lsr #18
+ orr r6, r7, r0
+ str r6, [r3]
+ add r6, r6, #0x100000
+ str r6, [r3, #4]
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