[PATCH v2] arm64: dts: mediatek: mt7986a-bananapi-bpi-r3: add ramoops region

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

Reserve 64 KiB of RAM just below the ARM Trusted Firmware secmon region
(0x42ff0000-0x43000000) for persistent kernel log storage via pstore/ramoops,
allowing post-panic console output and oops dumps to be recovered after a
warm reset. Without it, kernel crash logs on this board are lost when the
SoC reboots.

The zone sizes (record-size=8 KiB, console-size=32 KiB, ftrace-size=8 KiB,
pmsg-size=8 KiB) consume the full 64 KiB carve-out. The requested ecc-size=16
reserves a small Reed-Solomon parity block from each zone's own allocation
in persistent_ram_new(), which 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 region sits immediately below the ATF block already declared at
0x43000000 in mt7986a.dtsi, so no other reserved-memory child is moved or
resized. BPI-R3 ships with 2 GiB of DRAM starting at 0x40000000, well above
0x43000000, so the region is always within installed memory.

For the carve-out to actually preserve content across a reset, the boot
loader must avoid touching this region on warm reset; on standard BPI-R3
boards with the stock OpenWrt U-Boot fork this already holds.

Signed-off-by: Martino Dell'Ambrogio <tillo@tillo.ch>
---
Changes in v2:
 - Add no-map; to keep the carve-out out of the kernel linear map and
   avoid the cacheable/write-combine attribute mismatch on arm64.
   (sashiko-bot, gemini-3.1-pro)
 - Rewrite the ECC paragraph in the commit log: the zone sizes already
   fill the 64 KiB region, so there is no remainder; ecc-size is carved
   from each zone in persistent_ram_new(). (sashiko-bot, gemini-3.1-pro)

v1: https://lore.kernel.org/all/20260528092807.1936177-1-tillo@tillo.ch/

 .../boot/dts/mediatek/mt7986a-bananapi-bpi-r3.dts    | 13 +++++++++++++
 1 file changed, 13 insertions(+)

diff --git a/arch/arm64/boot/dts/mediatek/mt7986a-bananapi-bpi-r3.dts b/arch/arm64/boot/dts/mediatek/mt7986a-bananapi-bpi-r3.dts
index 19f538d..31ee189 100644
--- a/arch/arm64/boot/dts/mediatek/mt7986a-bananapi-bpi-r3.dts
+++ b/arch/arm64/boot/dts/mediatek/mt7986a-bananapi-bpi-r3.dts
@@ -140,6 +140,19 @@ sfp2: sfp-2 {
 	};
 };

+&{/reserved-memory} {
+	ramoops@42ff0000 {
+		compatible = "ramoops";
+		reg = <0 0x42ff0000 0 0x10000>;
+		no-map;
+		record-size = <0x2000>;
+		console-size = <0x8000>;
+		ftrace-size = <0x2000>;
+		pmsg-size = <0x2000>;
+		ecc-size = <16>;
+	};
+};
+
 &cpu_thermal {
 	cooling-maps {
 		map-cpu-active-high {
--
2.47.3