[PATCH v4] arm64: dts: mediatek: mt7988a-bananapi-bpi-r4: add ramoops region

[Martino Dell'Ambrogio <tillo@tillo.ch>]

Reserve 1 MiB of RAM just below the ARM Trusted Firmware secmon region
(0x42f00000-0x43000000) for persistent kernel log storage via pstore/ramoops,
allowing post-panic console output and oops dumps to be recovered after a
reboot. Without it, kernel crash logs on this board are lost when the SoC
warm-resets and the on-chip console buffer is reinitialised.

With record-size=128 KiB, console-size=256 KiB, ftrace-size=64 KiB and
pmsg-size=64 KiB, ramoops_probe() carves the post-console remainder
(640 KiB) into five 128 KiB kmsg records, with the requested ecc-size=16
reserving a small Reed-Solomon parity block from each zone's own
allocation (per persistent_ram_new()). The ECC lets pstore recover dumps
even when the panic path truncates writes mid-record.

The no-map property is required so the reserved region is kept out of the
kernel linear map. ramoops remaps the carve-out write-combine via
ioremap_wc(); on arm64, leaving the same physical RAM mapped cacheable in
the linear map at the same time is an attribute-mismatch and risks losing
panic data to dirty cache evictions from the linear alias.

The carve-out sits immediately below the ATF region already declared at
0x43000000 in mt7988a.dtsi, so no other reserved-memory child is moved or
resized. BPI-R4 ships with at least 4 GiB of DRAM starting at 0x40000000,
so the region is well within installed memory on every variant.

For the carve-out to actually preserve content across a reset, the boot
loader must also avoid touching this region on warm reset; on standard
BPI-R4 boards with the stock OpenWrt U-Boot fork this already holds.

Signed-off-by: Martino Dell'Ambrogio <tillo@tillo.ch>
---
Changes in v4:
 - Use a plain reserved-memory node inside the root node instead of the
   &{/reserved-memory} path reference, as AngeloGioacchino pointed out. dtc
   merges it with the node from mt7988a.dtsi, so the resulting DTB is
   identical; this is a syntax change only. Same change made to the BPI-R3
   patch.

No rush on my account for the earlier silence - the v2 postings were
cross-threaded into each other's v1, so they were easy to miss.

For what it is worth, this layout is in use on a BPI-R4 here: the kernel
brings it up as
  OF: reserved mem: 0x42f00000..0x42ffffff (1024 KiB) nomap non-reusable
  pstore: Registered ramoops as persistent store backend
  ramoops: using 0x100000@0x42f00000, ecc: 16
(that build carries the same node via the path-reference form, so the DTB is
the one this patch produces). I have not yet confirmed a panic dump survives
end to end on it.

v3: https://lore.kernel.org/all/20260915064254.1594808-1-tillo@tillo.ch/
v2: https://lore.kernel.org/all/20260528123645.2650085-1-tillo@tillo.ch/
v1: https://lore.kernel.org/all/20260528092807.1936177-1-tillo@tillo.ch/

 .../boot/dts/mediatek/mt7988a-bananapi-bpi-r4.dtsi  | 13 +++++++++++++
 1 file changed, 13 insertions(+)

diff --git a/arch/arm64/boot/dts/mediatek/mt7988a-bananapi-bpi-r4.dtsi b/arch/arm64/boot/dts/mediatek/mt7988a-bananapi-bpi-r4.dtsi
index 0ff69da..75f5940 100644
--- a/arch/arm64/boot/dts/mediatek/mt7988a-bananapi-bpi-r4.dtsi
+++ b/arch/arm64/boot/dts/mediatek/mt7988a-bananapi-bpi-r4.dtsi
@@ -66,6 +66,19 @@
 		regulator-always-on;
 	};
 
+	reserved-memory {
+		ramoops@42f00000 {
+			compatible = "ramoops";
+			reg = <0 0x42f00000 0 0x100000>;
+			no-map;
+			record-size = <0x20000>;
+			console-size = <0x40000>;
+			ftrace-size = <0x10000>;
+			pmsg-size = <0x10000>;
+			ecc-size = <16>;
+		};
+	};
+
 	/* SFP1 cage (WAN) */
 	sfp1: sfp1 {
 		compatible = "sff,sfp";
-- 
2.47.3