[PATCH V2] Add arm description to Documentation/kdump/kdump.txt

[HuKeping]

----------------------------------------

Add arm specific parts to kdump kernel documentation.

It seem that we prefer to use nr_cpus=1 instead of maxcpus=1. but nr_cpus
does not work fine on arm.
There will be a warning when dump-caputre kernel booting if use nr_cpus:

"[    0.000000] DT/cpu 2nodes greater than max cores 1, capping them"

This is from arch/arm/kernel/devtree.c, in function
void __init arm_dt_init_cpu_maps(void)

145    if (WARN(cpuidx > nr_cpu_ids, "DT /cpu %u nodes greater than "
146                "max cores %u, capping them\n",
147                cpuidx, nr_cpu_ids)) {
148        cpuidx = nr_cpu_ids;
149        break;
150    }

Since we have already using nr_cpus to specify the number of cpus we
want to be online, kdump should not to make the comparison with dtb
(which is strongly recommended on arm-32bit ) again, but it does, so the
warning out.
Meanwhile the maxcpus works fine.

v1 -> v2:
- Add description about dump-capture kernel config options(Arch Dependent, arm).

- Add description about the defference between arm and the others.
  on extended crashkernel syntax

Signed-off-by: Hu Keping <hukeping at huawei.com>
---
 Documentation/kdump/kdump.txt | 37 ++++++++++++++++++++++++++++++++++---
 1 file changed, 34 insertions(+), 3 deletions(-)

diff --git a/Documentation/kdump/kdump.txt b/Documentation/kdump/kdump.txt
index 88d5a86..d07b902 100644
--- a/Documentation/kdump/kdump.txt
+++ b/Documentation/kdump/kdump.txt
@@ -18,7 +18,7 @@ memory image to a dump file on the local disk, or across the network to
 a remote system.
 
 Kdump and kexec are currently supported on the x86, x86_64, ppc64, ia64,
-and s390x architectures.
+s390x and arm architectures.
 
 When the system kernel boots, it reserves a small section of memory for
 the dump-capture kernel. This ensures that ongoing Direct Memory Access
@@ -112,7 +112,7 @@ There are two possible methods of using Kdump.
 2) Or use the system kernel binary itself as dump-capture kernel and there is
    no need to build a separate dump-capture kernel. This is possible
    only with the architectures which support a relocatable kernel. As
-   of today, i386, x86_64, ppc64 and ia64 architectures support relocatable
+   of today, i386, x86_64, ppc64, ia64 and arm architectures support relocatable
    kernel.
 
 Building a relocatable kernel is advantageous from the point of view that
@@ -241,6 +241,14 @@ Dump-capture kernel config options (Arch Dependent, ia64)
   kernel will be aligned to 64Mb, so if the start address is not then
   any space below the alignment point will be wasted.
 
+Dump-capture kernel config options (Arch Dependent, arm)
+----------------------------------------------------------
+
+-   To use a relocatable kernel,
+    Enable "AUTO_ZRELADDR" support under "Boot" options:
+
+    AUTO_ZRELADDR=y
+
 
 Extended crashkernel syntax
 ===========================
@@ -256,6 +264,10 @@ The syntax is:
     crashkernel=<range1>:<size1>[,<range2>:<size2>,...][@offset]
     range=start-[end]
 
+Plesse note, on arm, the offset is required.
+    crashkernel=<range1>:<size1>[,<range2>:<size2>,...]@offset
+    range=start-[end]
+
     'start' is inclusive and 'end' is exclusive.
 
 For example:
@@ -296,6 +308,12 @@ Boot into System Kernel
    on the memory consumption of the kdump system. In general this is not
    dependent on the memory size of the production system.
 
+   On arm, use "crashkernel=Y at X". Note that the start address of the kernel
+   will be aligned to 128MiB (0x08000000), so if the start address is not then
+   any space blow the alignment point may be overwrited by the dump-caputre kernel,
+   which means it is possible that the vmcore is not that precise as expected.
+
+
 Load the Dump-capture Kernel
 ============================
 
@@ -315,7 +333,8 @@ For ia64:
 	- Use vmlinux or vmlinuz.gz
 For s390x:
 	- Use image or bzImage
-
+For arm:
+	- Use zImage
 
 If you are using a uncompressed vmlinux image then use following command
 to load dump-capture kernel.
@@ -331,6 +350,15 @@ to load dump-capture kernel.
    --initrd=<initrd-for-dump-capture-kernel> \
    --append="root=<root-dev> <arch-specific-options>"
 
+If you are using a compressed zImage, then use following command
+to load dump-capture kernel.
+
+   kexec --type zImage -p <dump-capture-kernel-bzImage> \
+   --initrd=<initrd-for-dump-capture-kernel> \
+   --dtb=<dtb-for-dump-capture-kernel> \
+   --append="root=<root-dev> <arch-specific-options>"
+
+
 Please note, that --args-linux does not need to be specified for ia64.
 It is planned to make this a no-op on that architecture, but for now
 it should be omitted
@@ -347,6 +375,9 @@ For ppc64:
 For s390x:
 	"1 maxcpus=1 cgroup_disable=memory"
 
+For arm:
+	"1 maxcpus=1 reset_devices"
+
 Notes on loading the dump-capture kernel:
 
 * By default, the ELF headers are stored in ELF64 format to support
-- 
1.8.5.5