[PATCH] docs/kernel-parameters: update description of mem=

[Mike Rapoport <rppt@kernel.org>]

From: Mike Rapoport <rppt@linux.ibm.com>

The existing description of mem= does not cover all the cases and
differences between how architectures treat it.

Extend the description to match the code.

Signed-off-by: Mike Rapoport <rppt@linux.ibm.com>
---

This is in a way a followup for the discussion of mem= usage on MIPS:

https://lore.kernel.org/all/1646461289-31992-1-git-send-email-yangtiezhu@loongson.cn

 .../admin-guide/kernel-parameters.txt         | 20 +++++++++++++++++++
 1 file changed, 20 insertions(+)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index f5a27f067db9..f3597841a031 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -2834,6 +2834,15 @@
 			2 when the kernel is not able to see the whole system memory;
 			3 memory that lies after 'mem=' boundary is excluded from
 			 the hypervisor, then assigned to KVM guests.
+			4 to limit the memory available for kdump kernel.
+
+			[ARC,MICROBLAZE] - the limit applies only to low memory,
+			high memory is not affected.
+
+			[ARM64] - only limits memory covered by the linear
+			mapping. The NOMAP regions are not affected.
+
+			[HEXAGON] - must be use to set the memory size, there is
 
 			[X86] Work as limiting max address. Use together
 			with memmap= to avoid physical address space collisions.
@@ -2844,6 +2853,17 @@
 			in above case 3, memory may need be hot added after boot
 			if system memory of hypervisor is not sufficient.
 
+	mem=nn[KMG]@ss[KMG]
+			[ARM,MIPS] - override the memory layout reported by
+			firmware.
+			Define a memory region of size nn[KMG] starting at
+			ss[KMG].
+			Multiple different regions can be specified with
+			multiple mem= parameters on the command line.
+
+	mem=nn[KMG]	[HEXAGON] Set the memory size.
+			Must be specified, otherwise memory size will be 0.
+
 	mem=nopentium	[BUGS=X86-32] Disable usage of 4MB pages for kernel
 			memory.
 
-- 
2.34.1