[PATCH v4] 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 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 mt7986a.dtsi, so the resulting DTB is
   identical; this is a syntax change only. Same change made to the BPI-R4
   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.

v3: https://lore.kernel.org/all/20260915064302.1595204-1-tillo@tillo.ch/
v2: https://lore.kernel.org/all/20260528123655.2650868-1-tillo@tillo.ch/
v1: https://lore.kernel.org/all/20260528093038.1945245-1-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 637e556..150b794 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 @@
 		tx-disable-gpios = <&pio 15 GPIO_ACTIVE_HIGH>;
 		tx-fault-gpios = <&pio 48 GPIO_ACTIVE_HIGH>;
 	};
+
+	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>;
+		};
+	};
 };
 
 &cpu_thermal {
-- 
2.47.3