general: drop f2fs and modify documentation

BoardConfig.mk

@@ -1,3 +1,10 @@
+# BoardConfig for rubenslte (SM-T365 msm8226) TWRP builds.
+#
+# The kernel is built from source (kernel/samsung/rubenslte, Samsung msm8226
+# stock tree) and shipped as a separate device tree blob (dt.img) that the
+# bootloader uses to pick the right DTB per hardware variant. All boot-image
+# work (mkbootimg, dt.img, ramdisk trim) happens in mkbootimg.mk.
+
 DEVICE_TREE := device/samsung/rubenslte
 
 # Architecture
@@ -30,6 +37,12 @@ KERNEL_TOOLCHAIN_PREFIX := arm-eabi-
 # Boot image with separate device tree blob (dt.img via dtbtool)
 BOARD_CUSTOM_BOOTIMG_MK := $(DEVICE_TREE)/mkbootimg.mk
 BOARD_KERNEL_SEPARATED_DT := true
+
+# NOTE: these two must stay *recursive* ("=", not ":="). BoardConfig.mk is
+# parsed by build/envsetup.mk before PRODUCT_OUT/KERNEL_OUT are defined, while
+# mkbootimg.mk is included from build/core/Makefile:993 -- before
+# build/core/tasks/kernel.mk (included at :1942) defines KERNEL_OUT.
+# ":=" would freeze empty values here and break the dt.img rule.
 INSTALLED_DTIMAGE_TARGET = $(PRODUCT_OUT)/dt.img
 
 BOARD_KERNEL_CMDLINE := console=null androidboot.console=null androidboot.hardware=qcom user_debug=23 androidboot.selinux=permissive
@@ -40,7 +53,9 @@ BOARD_MKBOOTIMG_ARGS = --kernel_offset 0x00008000 --ramdisk_offset 0x02000000 --
 # Filesystems
 BOARD_HAS_LARGE_FILESYSTEM := true
 TARGET_USERIMAGES_USE_EXT4 := true
-TARGET_USERIMAGES_USE_F2FS := true
+# F2FS intentionally NOT enabled: the rubenslte kernel has no F2FS driver,
+# so offering "Data (F2FS)" in the TWRP format menu would only produce an
+# unreadable /data partition.
 BOARD_SUPPRESS_SECURE_ERASE := true
 
 # Partitions

README.md

@@ -0,0 +1,53 @@
+# twrp_device_samsung_rubenslte
+
+TWRP device tree for the Samsung Galaxy Tab Active **SM-T365** (msm8226,
+codename `rubenslte`), targeting omni's `twrp-5.1` manifest.
+
+```text
+Android.mk            includes subdirs only when TARGET_DEVICE == rubenslte
+AndroidProducts.mk    product makefile = omni_rubenslte.mk
+BoardConfig.mk        architecture, kernel-from-source, partitions, TWRP opts
+mkbootimg.mk          custom boot/recovery image rules (dt.img, LZMA ramdisk)
+dtbtool/              dtbToolLineage sources (packs .dtb files into dt.img)
+```
+
+## How it fits together
+
+- The kernel is **built from source** from `rubenslte/android_kernel_samsung_rubenslte`
+  (base defconfig + rubenslte variant + selinux defconfigs, see the `TARGET_KERNEL_*`
+  variables in `BoardConfig.mk`).
+- `mkbootimg.mk` is pulled in through `BOARD_CUSTOM_BOOTIMG_MK`. Two things it
+  does that the core build does not:
+  1. builds `dt.img` from the kernel's compiled `.dtb` files and passes it to
+     `mkbootimg` (`--dt`), because Samsung bootloaders validate `dt.img`;
+  2. appends the **`SEANDROIDENFORCE`** trailer to boot/recovery images, which
+     the stock bootloader also requires.
+
+## Known gotchas (read before touching anything)
+
+- `board_config.mk`: `INSTALLED_DTIMAGE_TARGET` and `BOARD_MKBOOTIMG_ARGS` must
+  stay **recursive** (`=`, not `:=`). This file is parsed before `PRODUCT_OUT`
+  exists and `mkbootimg.mk` is included from `build/core/Makefile:993` — before
+  `tasks/kernel.mk` (included at `:1942`) defines `KERNEL_OUT`. The dt.img rule
+  therefore depends on `INSTALLED_KERNEL_TARGET` (defined at `core/Makefile:480`),
+  not on `TARGET_PREBUILT_INT_KERNEL`.
+- The recovery partition is only 10,485,248 bytes; the stock gzip ramdisk
+  overflows it. `mkbootimg.mk` overrides the core ramdisk recipes (our file is
+  included after `core/Makefile`, so these win) to drop tzdata, keep only
+  `en.xml`/`pl.xml` languages, and compress with **LZMA** (the kernel has
+  `CONFIG_RD_LZMA=y`). Do not revert these rules without rechecking the size.
+- **F2FS is intentionally disabled** (`TARGET_USERIMAGES_USE_F2FS` absent):
+  the kernel ships no F2FS driver, so offering an F2FS format in TWRP would
+  only produce an unreadable `/data`.
+- The variant defconfig enables only `CONFIG_MACH_RUBENSLTE_OPEN`, so **dt.img
+  carries just the EUR DTBs** (r02/r03). SM-T365Y/AUS, SM-T365M/KOR units need
+  the matching `CONFIG_MACH_RUBENSLTE_*` entries and a rebuild.
+
+## Building
+
+Do not build here directly: the driver is `builders_rubenslte/build_twrp.sh`,
+which clones this tree into the omni workspace and builds `recovery.img`.
+
+```text
+./build_twrp.sh          # in builders_rubenslte; produces recovery.img
+```

mkbootimg.mk

@@ -1,3 +1,19 @@
+# Custom boot/recovery image rules for rubenslte.
+#
+# This file is picked up via BOARD_CUSTOM_BOOTIMG_MK and included from
+# build/core/Makefile:993, *before* build/core/tasks/kernel.mk (included at
+# :1942) defines KERNEL_OUT. This is why everything kernel-related below uses
+# recursive variables and why the dt.img rule depends on INSTALLED_KERNEL_TARGET
+# (defined earlier, at core/Makefile:480) rather than on
+# TARGET_PREBUILT_INT_KERNEL.
+#
+# Rules defined here override the core ones because our file is included after
+# core/Makefile, so these recipe bodies win. That is used deliberately at the
+# bottom: the recovery ramdisk recipes get replaced to fix an oversize image.
+#
+# Samsung bootloaders also validate a "SEANDROIDENFORCE" trailer on boot images,
+# which the stock rules do not add.
+
 LOCAL_PATH := $(call my-dir)
 
 # Build dt.img from the kernel's compiled .dtb files using dtbToolLineage