[PATCH 11/11] Documentation: admin-guide: kdump: document linux,no-dump DT property

[Chen Wandun <chenwandun1@gmail.com>]

Describe the new 'linux,no-dump' reserved-memory device tree property
and the automatic exclusion of /memreserve/ entries from the kdump
vmcore.

The section covers:

 - The two mechanisms that exclude reserved memory from the vmcore
   (firmware /memreserve/ entries and linux,no-dump child nodes).
 - Intended use cases (firmware-owned GPU, DSP and modem carveouts).
 - Interaction with the existing 'no-map' and 'reusable' flags, with
   the silent-ignore precedence implemented by the kernel.
 - Architectures honouring the hint (arm64, riscv, loongarch).
 - An illustrative reserved-memory DTS snippet.

The DT binding for the property itself is maintained in dt-schema.

Signed-off-by: Chen Wandun <chenwandun@lixiang.com>
---
 Documentation/admin-guide/kdump/kdump.rst | 59 +++++++++++++++++++++++
 1 file changed, 59 insertions(+)

diff --git a/Documentation/admin-guide/kdump/kdump.rst b/Documentation/admin-guide/kdump/kdump.rst
index 7587caadbae1..c2246888e84d 100644
--- a/Documentation/admin-guide/kdump/kdump.rst
+++ b/Documentation/admin-guide/kdump/kdump.rst
@@ -600,6 +600,65 @@ with /sys/kernel/config/crash_dm_crypt_keys for setup,
 3. After the dump-capture kerne get booted, restore the keys to user keyring
    echo yes > /sys/kernel/crash_dm_crypt_keys/restore
 
+Excluding reserved memory regions from the vmcore (device tree)
+===============================================================
+
+On architectures that boot from a device tree and use kexec_file for
+kdump (arm64, riscv, loongarch), specific reserved memory regions can
+be excluded from the ELF PT_LOAD segments of the crash dump.
+
+Two mechanisms contribute to the exclusion:
+
+1) /memreserve/ entries from the FDT header.
+
+   These are firmware-level memory reservations with no associated
+   device tree node and therefore no driver-level description. Their
+   contents are typically firmware scratch areas that carry no value
+   for kernel crash analysis, so they are excluded from the vmcore
+   automatically.
+
+2) Reserved-memory nodes carrying the 'linux,no-dump' property.
+
+   Device tree authors can add this boolean hint to any
+   /reserved-memory child node to request that the kernel skip that
+   region when constructing the elfcorehdr. This is intended for
+   firmware-owned carveouts such as GPU, DSP and modem memory, whose
+   contents tend to significantly inflate the vmcore without aiding
+   kernel crash analysis.
+
+   Example::
+
+       reserved-memory {
+               #address-cells = <2>;
+               #size-cells = <2>;
+               ranges;
+
+               gpu_fw@a0000000 {
+                       reg = <0x0 0xa0000000 0x0 0x01000000>;
+                       no-map;
+                       linux,no-dump;
+               };
+
+               modem_fw@b0000000 {
+                       reg = <0x0 0xb0000000 0x0 0x02000000>;
+                       linux,no-dump;
+               };
+       };
+
+Interaction with other reserved-memory flags:
+
+- 'no-map': the region is already absent from the kernel linear map,
+  so it does not appear in the vmcore to begin with. Combining
+  'linux,no-dump' with 'no-map' is harmless but redundant.
+
+- 'reusable': the region is actively used by the kernel for movable
+  page allocations (CMA) and its contents are relevant to crash
+  analysis. 'linux,no-dump' is silently ignored on a reusable region.
+
+The property is an operating-system hint; DTBs that do not set it
+retain the legacy behaviour (all memory is dumped). Architectures
+that do not honour the hint simply ignore it.
+
 Contact
 =======
 
-- 
2.43.0