ipq40xx: mikrotik: kernel: pet watchdog during kernel uncompress

kernel 6.9 removed the KConfig entry our RouterBOOT watchdog pet hack was relying on:
Linux df59427a1122 ("ARM: qcom: merge remaining subplatforms into sensible Kconfig entry")

Introduce a new specific KConfig entry for this hack,
and enable it for Mikrotik ipq40xx kernel.
CONFIG_ARCH_QCOM_IPQ40XX_BOOT_COMPRESSED_PET_WATCHDOG_EARLY

With appropriate DEBUG_LL and DEBUG_UNCOMPRESS, this watchdog reset
can be typically seen on console as a reset before "Uncompressing Linux..."
reaches " done, booting the kernel."

RouterBOOT

loading kernel... OK
setting up elf image... OK
jumping to kernel code
Jumping to kernel
DTB:0x80381A60 (0x000048C4)
C:0x800000E0-0x80386420->0x80FAB500-0x81331840
DTB:0x8132CE80 (0x000049B8)
Uncompressing Linux...
Format: Log Type - Time(microsec) - Message - Optional Info
Log Type: B - Since Boot(Power On Reset),  D - Delta,  S - Statistic
S - QC_IMAGE_VERSION_STRING=BOOT.BF.3.1.1-00096

versus:

Uncompressing Linux... done, booting the kernel.
[    0.000000] Booting Linux on physical CPU 0x0

On Mikrotik RouterBOOT devices, this is complicated by some RouterBOOT
versions successfully loading the same kernel that other RouterBOOT versions fail. Example:
RouterBOOT backup booter 6.45.9 fine, RouterBOOT booter 7.16 fail

Fixes: openwrt#19841

Signed-off-by: John Thomson <git@johnthomson.fastmail.com.au>
Link: https://github.com/openwrt/openwrt/pull/20305
Signed-off-by: Robert Marko <robimarko@gmail.com>

target/linux/ipq40xx/config-6.12

@@ -11,6 +11,7 @@ CONFIG_ARCH_MULTI_V7=y
 CONFIG_ARCH_OPTIONAL_KERNEL_RWX=y
 CONFIG_ARCH_OPTIONAL_KERNEL_RWX_DEFAULT=y
 CONFIG_ARCH_QCOM=y
+# CONFIG_ARCH_QCOM_IPQ40XX_BOOT_COMPRESSED_PET_WATCHDOG_EARLY is not set
 CONFIG_ARCH_QCOM_RESERVE_SMEM=y
 CONFIG_ARCH_SELECT_MEMORY_MODEL=y
 CONFIG_ARCH_SPARSEMEM_ENABLE=y

target/linux/ipq40xx/mikrotik/config-default

@@ -1,3 +1,4 @@
+CONFIG_ARCH_QCOM_IPQ40XX_BOOT_COMPRESSED_PET_WATCHDOG_EARLY=y
 CONFIG_CRYPTO_HASH_INFO=y
 CONFIG_MIKROTIK=y
 CONFIG_MIKROTIK_RB_SYSFS=y

target/linux/ipq40xx/patches-6.12/302-arm-compressed-set-ipq40xx-watchdog-to-allow-boot.patch

@@ -32,7 +32,7 @@ Signed-off-by: John Thomson <git@johnthomson.fastmail.com.au>
 + * data and functionality from drivers/watchdog/qcom-wdt.c
 + * address from arch/arm/boot/dts/qcom-ipq4019.dtsi
 + */
-+#ifdef CONFIG_ARCH_IPQ40XX
++#ifdef CONFIG_ARCH_QCOM_IPQ40XX_BOOT_COMPRESSED_PET_WATCHDOG_EARLY
 +watchdog_set:
 +		/* offsets:
 +		 * 0x04 reset	(=1 resets countdown)
@@ -59,8 +59,23 @@ Signed-off-by: John Thomson <git@johnthomson.fastmail.com.au>
 +		mov r1, #1
 +		str r1, [r0, #0x08]	@Enable the watchdog
 +watchdog_finished:
-+#endif /* CONFIG_ARCH_IPQ40XX */
++#endif /* CONFIG_ARCH_QCOM_IPQ40XX_BOOT_COMPRESSED_PET_WATCHDOG_EARLY */
 +
  /*
   * The C runtime environment should now be setup sufficiently.
   * Set up some pointers, and start decompressing.
+--- a/arch/arm/mach-qcom/Kconfig
++++ b/arch/arm/mach-qcom/Kconfig
+@@ -21,4 +21,12 @@ config ARCH_QCOM_RESERVE_SMEM
+ 	  Reserve 2MB at the beginning of the System RAM for shared mem.
+ 	  This is required on IPQ40xx, MSM8x60 and MSM8960 platforms.
+ 
++config ARCH_QCOM_IPQ40XX_BOOT_COMPRESSED_PET_WATCHDOG_EARLY
++	bool "Pet ipq40xx watchdog early in boot uncompression"
++	help
++	  Pet the IPQ40xx watchdog very early in boot uncompress stage.
++	  Use where bootloader sets a watchdog timeout that bites the
++	  device before the Linux watchdog device driver can manage it.
++	  Required for Mikrotik RouterBOOT on ipq40xx devices
++
+ endif