[PATCH v2] 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 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: ramoops carves ecc-size
   from each zone's own allocation in persistent_ram_new() and the
   post-console remainder is split into record-size'd kmsg records, not
   one record plus a separate ECC pool. (sashiko-bot, gemini-3.1-pro)

v1: https://lore.kernel.org/all/20260528093038.1945245-1-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..f7d4944 100644
--- a/arch/arm64/boot/dts/mediatek/mt7988a-bananapi-bpi-r4.dtsi
+++ b/arch/arm64/boot/dts/mediatek/mt7988a-bananapi-bpi-r4.dtsi
@@ -80,6 +80,19 @@ sfp1: sfp1 {
 	};
 };

+&{/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>;
+	};
+};
+
 &cci {
 	proc-supply = <&rt5190_buck3>;
 };
--
2.47.3