rk27xx: add the Scheme B flash translation layer

Scheme B is the self-describing NAND format of the HiFiMAN HM-601 and
similar players: every block carries its logical number and a version
in its metadata, so the mapping is rebuilt by a scan at mount, and
small writes go through a 16-page RAM cache journalled to flash.

ftl-scheme-b.c is a reimplementation from reverse engineering. The
format and the behaviour were worked out by analysing the machine code
of the HM-601's NAND bootloader and of a compiled Rockchip FTL object
from the rk2808 platform, which handles the same format, and checked
against dumps of the media; no source code was used. Where the two
binaries differ the HM-601 is followed: 16-bit versions compared
across wrap, plain 0xF200/0xF100 tags, a copy that stamps one header
on every page. The number of open exchange blocks is configurable - 8
on the HM-601, whose mount recovers no more.

Checked by running the compiled object under qemu over a NAND
simulator, side by side with this code, on a 4 GiB HM-601 dump: the
same state after mount, identical reads of all 3958 logical blocks,
and flash programs and erases identical one for one - over 600 random
writes on each of three seeds and at every power-cut point of three
sweeps, 1435 points - with 0 wrong sectors.

Not built yet: no target selects it.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Change-Id: Icb935ea78d716c1e51f1453fc3ab3f47f3467027

firmware/target/arm/rk27xx/ftl-scheme-b.h

@@ -0,0 +1,87 @@
+/***************************************************************************
+ *             __________               __   ___.
+ *   Open      \______   \ ____   ____ |  | _\_ |__   _______  ___
+ *   Source     |       _//  _ \_/ ___\|  |/ /| __ \ /  _ \  \/  /
+ *   Jukebox    |    |   (  <_> )  \___|    < | \_\ (  <_> > <  <
+ *   Firmware   |____|_  /\____/ \___  >__|_ \|___  /\____/__/\_ \
+ *                     \/            \/     \/    \/            \/
+ *
+ * Copyright (C) 2026 by Marcin Bukat
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License
+ * as published by the Free Software Foundation; either version 2
+ * of the License, or (at your option) any later version.
+ *
+ * This software is distributed on an "AS IS" basis, WITHOUT WARRANTY OF ANY
+ * KIND, either express or implied.
+ *
+ ****************************************************************************/
+
+/* The "Scheme B" flash translation layer of rk27xx devices - the
+ * self-describing on-flash format the original firmware of the HiFiMAN
+ * HM-601 and similar players uses, read and written compatibly so that the
+ * original firmware keeps working on the same media. A reimplementation
+ * from reverse engineering; see ftl-scheme-b.c for how it works. */
+
+#ifndef __FTL_SCHEME_B_H__
+#define __FTL_SCHEME_B_H__
+
+#include <stdbool.h>
+#include <stdint.h>
+
+struct ftl_b_config
+{
+    /* The first block the FTL owns; the boot area is everything before it.
+     * Not recorded in the FTL's own structures. */
+    uint16_t first_block;
+    /* Exchange blocks open at once. The original firmware's own count must
+     * not be exceeded: its mount recovers no more than that many, and loses
+     * the data of the rest. 8 on the HM-601. */
+    uint8_t  exch_blocks;
+    /* Never write, not even the repairs a mount normally makes. */
+    bool     read_only;
+};
+
+/* Why a mount failed */
+enum ftl_b_error
+{
+    FTL_B_OK = 0,
+    FTL_B_ERR_GEOMETRY,         /* the chip is not one this FTL can map */
+    FTL_B_ERR_CONFIG,           /* an unusable configuration */
+    FTL_B_ERR_NO_TABLE,         /* no bad-block table: not formatted */
+    FTL_B_ERR_TABLE,            /* the bad-block table is damaged */
+    FTL_B_ERR_VERSION,          /* a format generation not supported */
+    FTL_B_ERR_TOO_BIG,          /* more logical blocks than fit in RAM */
+};
+
+struct ftl_b_status
+{
+    enum ftl_b_error error;
+    bool     writable;
+    uint16_t format_version;    /* as recorded in the bad-block table */
+    uint16_t logical_blocks;
+    uint16_t bad_blocks;
+    uint16_t free_blocks;       /* erased or erasable, queued for use */
+    uint8_t  open_exch;         /* exchange blocks in progress */
+    uint8_t  cached_pages;      /* pages held in the write cache */
+};
+
+/* Mount the media. Returns FTL_B_OK or the reason it failed. */
+enum ftl_b_error ftl_b_mount(const struct ftl_b_config *config);
+void ftl_b_get_status(struct ftl_b_status *status);
+
+/* Sectors in the logical space; 0 when not mounted */
+uint32_t ftl_b_capacity(void);
+
+/* Return 0, or non-zero if the request was refused or a sector could not be
+ * read reliably. Sectors are 512 bytes. */
+int ftl_b_read(uint32_t sector, void *buf, uint32_t count);
+int ftl_b_write(uint32_t sector, const void *buf, uint32_t count);
+
+/* Every write is on the flash when ftl_b_write() returns - the write cache
+ * is journalled as it changes - so there is nothing to write out. Kept for
+ * the storage layer's interface. */
+void ftl_b_sync(void);
+
+#endif /* __FTL_SCHEME_B_H__ */