[PATCH v9 5/5] kdump: update Documentation about crashkernel on arm64

[Chen Zhou <chenzhou10@huawei.com>]

Now we support crashkernel=X,[low] on arm64, update the Documentation.
We could use parameters "crashkernel=X crashkernel=Y,low" to reserve
memory above 4G.

Signed-off-by: Chen Zhou <chenzhou10@huawei.com>
Tested-by: John Donnelly <John.p.donnelly@oracle.com>
Tested-by: Prabhakar Kushwaha <pkushwaha@marvell.com>
---
 Documentation/admin-guide/kdump/kdump.rst       | 13 +++++++++++--
 Documentation/admin-guide/kernel-parameters.txt | 17 +++++++++++++++--
 2 files changed, 26 insertions(+), 4 deletions(-)

diff --git a/Documentation/admin-guide/kdump/kdump.rst b/Documentation/admin-guide/kdump/kdump.rst
index 2da65fef2a1c..6ba294d425c9 100644
--- a/Documentation/admin-guide/kdump/kdump.rst
+++ b/Documentation/admin-guide/kdump/kdump.rst
@@ -299,7 +299,13 @@ Boot into System Kernel
    "crashkernel=64M@16M" tells the system kernel to reserve 64 MB of memory
    starting at physical address 0x01000000 (16MB) for the dump-capture kernel.
 
-   On x86 and x86_64, use "crashkernel=64M@16M".
+   On x86 use "crashkernel=64M@16M".
+
+   On x86_64, use "crashkernel=Y[@X]" to select a region under 4G first, and
+   fall back to reserve region above 4G when '@offset' hasn't been specified.
+   We can also use "crashkernel=X,high" to select a region above 4G, which
+   also tries to allocate at least 256M below 4G automatically and
+   "crashkernel=Y,low" can be used to allocate specified size low memory.
 
    On ppc64, use "crashkernel=128M@32M".
 
@@ -316,8 +322,11 @@ Boot into System Kernel
    kernel will automatically locate the crash kernel image within the
    first 512MB of RAM if X is not given.
 
-   On arm64, use "crashkernel=Y[@X]".  Note that the start address of
+   On arm64, use "crashkernel=Y[@X]". Note that the start address of
    the kernel, X if explicitly specified, must be aligned to 2MiB (0x200000).
+   If crashkernel=Z,low is specified simultaneously, reserve spcified size
+   low memory for crash kdump kernel devices firstly and then reserve memory
+   above 4G.
 
 Load the Dump-capture Kernel
 ============================
diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index fb95fad81c79..335431a351c0 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -722,6 +722,9 @@
 			[KNL, x86_64] select a region under 4G first, and
 			fall back to reserve region above 4G when '@offset'
 			hasn't been specified.
+			[KNL, arm64] If crashkernel=X,low is specified, reserve
+			spcified size low memory for crash kdump kernel devices
+			firstly, and then reserve memory above 4G.
 			See Documentation/admin-guide/kdump/kdump.rst for further details.
 
 	crashkernel=range1:size1[,range2:size2,...][@offset]
@@ -746,13 +749,23 @@
 			requires at least 64M+32K low memory, also enough extra
 			low memory is needed to make sure DMA buffers for 32-bit
 			devices won't run out. Kernel would try to allocate at
-			at least 256M below 4G automatically.
+			least 256M below 4G automatically.
 			This one let user to specify own low range under 4G
 			for second kernel instead.
 			0: to disable low allocation.
 			It will be ignored when crashkernel=X,high is not used
 			or memory reserved is below 4G.
-
+			[KNL, arm64] range under 4G.
+			This one let user to specify own low range under 4G
+			for crash dump kernel instead.
+			Different with x86_64, kernel allocates specified size
+			physical memory region only when this parameter is specified
+			instead of trying to allocate at least 256M below 4G
+			automatically.
+			This parameter is used along with crashkernel=X when we
+			want to reserve crashkernel above 4G. If there are devices
+			need to use ZONE_DMA in crash dump kernel, it is also
+			a good choice.
 	cryptomgr.notests
 			[KNL] Disable crypto self-tests
 
-- 
2.20.1


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