[PATCH v12 9/9] kdump: update Documentation about crashkernel

[Chen Zhou]

For arm64, the behavior of crashkernel=X has been changed, which
tries low allocation in DMA zone, and fall back to high allocation
if it fails.
We can also use "crashkernel=X,high" to select a high region above
DMA zone, which also tries to allocate at least 256M low memory in
DMA zone automatically.
"crashkernel=Y,low" can be used to allocate specified size low memory
in DMA zone.
For non-RPi4 platforms, change DMA zone memtioned above to DMA32 zone.

For x86 and arm64, we introduce threshold for the required memory.
if required size X is too large and leads to very little free low
memory after low allocation, the system may not work well.
So add a threshold and go for high allocation directly if the required
size is too large. The threshold is set as the half of low memory.

So update the Documentation.

Signed-off-by: Chen Zhou <chenzhou10@xxxxxxxxxx>
---
 Documentation/admin-guide/kdump/kdump.rst     | 25 ++++++++++++++++---
 .../admin-guide/kernel-parameters.txt         | 13 ++++++++--
 2 files changed, 33 insertions(+), 5 deletions(-)

diff --git a/Documentation/admin-guide/kdump/kdump.rst b/Documentation/admin-guide/kdump/kdump.rst
index 2da65fef2a1c..549611abc581 100644
--- a/Documentation/admin-guide/kdump/kdump.rst
+++ b/Documentation/admin-guide/kdump/kdump.rst
@@ -299,7 +299,16 @@ 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=X" to select a region under 4G first, and
+   fall back to reserve region above 4G. And go for high allocation
+   directly if the required size is too large.
+   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.
+   Use "crashkernel=Y@X" if you really have to reserve memory from specified
+   start address X.
 
    On ppc64, use "crashkernel=128M@32M".
 
@@ -316,8 +325,18 @@ 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
-   the kernel, X if explicitly specified, must be aligned to 2MiB (0x200000).
+   On arm64, use "crashkernel=X" to try low allocation in DMA zone, and
+   fall back to high allocation if it fails. And go for high allocation
+   directly if the required size is too large.
+   We can also use "crashkernel=X,high" to select a high region above
+   DMA zone, which also tries to allocate at least 256M low memory in
+   DMA zone automatically.
+   "crashkernel=Y,low" can be used to allocate specified size low memory
+   in DMA zone.
+   For non-RPi4 platforms, change DMA zone memtioned above to DMA32 zone.
+   Use "crashkernel=Y@X" if you really have to reserve memory from
+   specified start address X. Note that the start address of the kernel,
+   X if explicitly specified, must be aligned to 2MiB (0x200000).
 
 Load the Dump-capture Kernel
 ============================
diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index a1068742a6df..f7df572d8f64 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -727,6 +727,10 @@
 			[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] Try low allocation in DMA zone, fall back
+			to high allocation if it fails when '@offset' hasn't been
+			specified. For non-RPi4 platforms, change DMA zone to
+			DMA32 zone.
 			See Documentation/admin-guide/kdump/kdump.rst for further details.
 
 	crashkernel=range1:size1[,range2:size2,...][@offset]
@@ -743,6 +747,8 @@
 			Otherwise memory region will be allocated below 4G, if
 			available.
 			It will be ignored if crashkernel=X is specified.
+			[KNL, arm64] range in high memory.
+			Allow kernel to allocate physical memory region from top.
 	crashkernel=size[KMG],low
 			[KNL, X86-64] range under 4G. When crashkernel=X,high
 			is passed, kernel could allocate physical memory region
@@ -751,13 +757,16 @@
 			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 in low memory.
+			This one let user to specify a low range in DMA zone for
+			crash dump kernel. For non-RPi4 platforms, change DMA zone
+			to DMA32 zone.
 	cryptomgr.notests
 			[KNL] Disable crypto self-tests
 
-- 
2.20.1