Add documentation for extended crashkernel syntax

This adds the documentation for the extended crashkernel syntax into
Documentation/kdump/kdump.txt.

Signed-off-by: Bernhard Walle <bwalle@suse.de>
Cc: Andi Kleen <ak@suse.de>
Cc: "Luck, Tony" <tony.luck@intel.com>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Paul Mundt <lethal@linux-sh.org>
Cc: Vivek Goyal <vgoyal@in.ibm.com>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

2 files changed, 33 insertions, 0 deletions

diff --git a/Documentation/kdump/kdump.txt b/Documentation/kdump/kdump.txt
index 1b37b28cc234..d0ac72cc19ff 100644
--- a/Documentation/kdump/kdump.txt
+++ b/Documentation/kdump/kdump.txt
@@ -231,6 +231,32 @@ Dump-capture kernel config options (Arch Dependent, ia64)
   any space below the alignment point will be wasted.
 
 
+Extended crashkernel syntax
+===========================
+
+While the "crashkernel=size[@offset]" syntax is sufficient for most
+configurations, sometimes it's handy to have the reserved memory dependent
+on the value of System RAM -- that's mostly for distributors that pre-setup
+the kernel command line to avoid a unbootable system after some memory has
+been removed from the machine.
+
+The syntax is:
+
+    crashkernel=<range1>:<size1>[,<range2>:<size2>,...][@offset]
+    range=start-[end]
+
+For example:
+
+    crashkernel=512M-2G:64M,2G-:128M
+
+This would mean:
+
+    1) if the RAM is smaller than 512M, then don't reserve anything
+       (this is the "rescue" case)
+    2) if the RAM size is between 512M and 2G, then reserve 64M
+    3) if the RAM size is larger than 2G, then reserve 128M
+
+
 Boot into System Kernel
 =======================
 
diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt
index 189df0bcab99..0a3fed445249 100644
--- a/Documentation/kernel-parameters.txt
+++ b/Documentation/kernel-parameters.txt
@@ -497,6 +497,13 @@ and is between 256 and 4096 characters. It is defined in the file
                         [KNL] Reserve a chunk of physical memory to
                         hold a kernel to switch to with kexec on panic.
 
+        crashkernel=range1:size1[,range2:size2,...][@offset]
+                        [KNL] Same as above, but depends on the memory
+                        in the running system. The syntax of range is
+                        start-[end] where start and end are both
+                        a memory unit (amount[KMG]). See also
+                        Documentation/kdump/kdump.txt for a example.
+
         cs4232=         [HW,OSS]
                         Format: <io>,<irq>,<dma>,<dma2>,<mpuio>,<mpuirq>