[PATCH] arm: update advice on kernel and FDT load address.

Ian Campbell ian.campbell at citrix.com
Mon Jul 29 08:56:18 EDT 2013

Based on the discussion in [0]:
  - Recommend that the kernel be placed under 128MiB ("for automatic RAM
    location to work", says Nico)
  - Recommend that DTB larger than ~16KiB be placed after the 128MiB boundary
    (in order to avoid conflicting with the kernel decompressor, kernel is at
    most 32MiB) and within the lowmem region. Somewhat arbitrarily recommend
    placement below the 256MiB boundary, the real lowmem limit is likely up
    around 600-800MiB but varies.

[0] http://article.gmane.org/gmane.linux.ports.arm.kernel/254538

Signed-off-by: Ian Campbell <ian.campbell at citrix.com>
Cc: Russell King <linux at arm.linux.org.uk>
Cc: Nicolas Pitre <nicolas.pitre at linaro.org>
Cc: Will Deacon <will.deacon at arm.com>
Cc: Dave Martin <dave.martin at linaro.org>
Cc: Grant Likely <grant.likely at secretlab.ca>
Cc: linux-arm-kernel at lists.infradead.org
 Documentation/arm/Booting |   14 +++++++++++---
 1 files changed, 11 insertions(+), 3 deletions(-)

diff --git a/Documentation/arm/Booting b/Documentation/arm/Booting
index 0c1f475..72c035e 100644
--- a/Documentation/arm/Booting
+++ b/Documentation/arm/Booting
@@ -125,6 +125,13 @@ with the caveat that it may not be located at physical address 0 since
 the kernel interprets a value of 0 in r2 to mean neither a tagged list
 nor a dtb were passed.
+If the DTB is larger than ~16KiB, such that placement in the first
+16KiB would lead to a conflict with the kernel loaded at 32KiB (as
+recommended below) which uses the memory between 16KiB-32KiB itself,
+then the DTB should be placed after the first 128MiB of RAM in order
+to avoid conflicting with the kernel decompressor. The recommnded
+placement in this case just below the 256MiB boundary.
 5. Calling the kernel image
@@ -136,9 +143,10 @@ is stored in flash, and is linked correctly to be run from flash,
 then it is legal for the boot loader to call the zImage in flash
-The zImage may also be placed in system RAM (at any location) and
-called there.  Note that the kernel uses 16K of RAM below the image
-to store page tables.  The recommended placement is 32KiB into RAM.
+The zImage may also be placed in system RAM and called there.  The
+kernel should be placed in the first 128MiB of RAM. Note that the
+kernel uses 16KiB of RAM below the image to store page tables.  The
+recommended placement is 32KiB into RAM.
 In either case, the following conditions must be met:

More information about the linux-arm-kernel mailing list