tashaboot: arm64 bootloader

A small arm64 bootloader. No board code, no device tree porting, the
architecture manual is the whole story: exception vectors in the
fixed 16 slot layout (Table D1-7), ESR_ELx decoded by exception class
(D1-2172), EL entry and eret chains per the programmers model
(D1-2146), cache maintenance by set/way over the CLIDR_EL1 levels,
semihosting for console and file io per DUI 0203, and the A64 boot
protocol from Documentation/arch/arm64/booting.rst.

The loader boots a stock mainline Image end to end on the qemu virt
machine. Boot receipt with 7.3-rc3 (42MB Image):

  tashaboot 0.1
  loaded 43450368 bytes at 40200000, entry 40200000
  jumping
  [    0.000000] Booting Linux on physical CPU 0x0000000000 [0x411fd070]
  [    0.000000] Linux version 7.3.0-rc3
  [    0.000000] Machine model: linux,dummy-virt
  [    0.000000] earlycon: pl11 MMIO32:0x0000000009000000
  ...
  ---[ end Kernel panic - not syncing: VFS: Unable to mount root fs ]---

The panic is the expected end state, no root filesystem is handed
over yet.

The boot chain, state per stage, start to payload:

+-----------+-----+--------------+----------------------------------+
| stage     | EL  | state        | work                             |
+-----------+-----+--------------+----------------------------------+
| firmware  | any | MMU maybe on | x0 = dtb, jump in               |
+-----------+-----+--------------+----------------------------------+
| tashaboot | 3-2 |              | SCR_EL3.NS = 1, eret to EL2     |
+-----------+-----+--------------+----------------------------------+
|           | 2   | virt scrub   | HCR/CNTHCTL/CPTR/HSTR, CNTFRQ,  |
|           |     |              | VBAR_EL2, MMU off, tlbi alle2   |
+-----------+-----+--------------+----------------------------------+
|           | 2   |              | load Image over semihosting,    |
|           |     |              | validate header, place per      |
|           |     |              | booting.rst                     |
+-----------+-----+--------------+----------------------------------+
|           | 2   | caches clean | flush dcache, inval icache,     |
|           |     |              | args ride x20/x21, regs last    |
+-----------+-----+--------------+----------------------------------+
| payload   | 2   | fresh start  | x0 = dtb, x1-x3 = 0, DAIF       |
|           |     |              | masked, br to image entry       |
+-----------+-----+--------------+----------------------------------+

Two handoff bugs the kernel caught, both AAPCS clobbers in the final
jump. Cache maintenance was called after the register setup, x0-x18
are caller saved, so tb_flush_dcache_all() wiped the dtb pointer and
the kernel spun in setup_machine_fdt() with an invalid device tree
blob. The flush helpers also clobbered x1 (u-boot's void call
convention left mov x1, x0 in cache.S) which handed the kernel a wild
x0. The arguments ride in x20/x21 across the cache calls now, callee
saved, and the register setup is the last thing before the branch.

What is missing on purpose: no SMP bringup (secondary cores park),
no PSCI, no initrd or root filesystem handoff, single serial
console. Those come next.

Signed-off-by: Bradley Morgan <brads@mainlining.org>

2 files changed, 118 insertions, 0 deletions

diff --git a/arch/arm64/lib/cache.S b/arch/arm64/lib/cache.S
new file mode 100644
index 0000000..d8ccea2
--- /dev/null
+++ b/arch/arm64/lib/cache.S
@@ -0,0 +1,100 @@
+/* SPDX-License-Identifier: GPL-2.0+ */
+/*
+ * cache.S - set/way cache maintenance, walked off CLIDR_EL1 the same
+ * way u-boot and the kernel's own __flush_dcache_all do it. needed
+ * before jumping to the payload so it starts from memory, not cache.
+ *
+ * Copyright (C) 2026 Bradley Morgan <brads@mainlining.org>
+ */
+
+#include <asm/linkage.h>
+
+.pushsection .text.tb_dcache_level, "ax"
+ENTRY(tb_dcache_level)
+	lsl	x12, x0, #1
+	msr	csselr_el1, x12		/* select cache level */
+	isb				/* sync change of ccsidr_el1 */
+	mrs	x6, ccsidr_el1		/* read the new ccsidr_el1 */
+	ubfx	x2, x6, #0, #3		/* x2 <- log2(cache line size)-4 */
+	ubfx	x3, x6, #3, #10		/* x3 <- number of cache ways - 1 */
+	ubfx	x4, x6, #13, #15	/* x4 <- number of cache sets - 1 */
+	add	x2, x2, #4		/* x2 <- log2(cache line size) */
+	clz	w5, w3			/* x5 <- bit position of #ways */
+	/* x12 <- cache level << 1 */
+	/* x2  <- line length offset */
+	/* x3  <- number of cache ways - 1 */
+	/* x4  <- number of cache sets - 1 */
+	/* x5  <- bit position of #ways */
+
+loop_set:
+	mov	x6, x3			/* x6 <- working copy of #ways */
+loop_way:
+	lsl	x7, x6, x5
+	orr	x9, x12, x7		/* map way and level to cisw value */
+	lsl	x7, x4, x2
+	orr	x9, x9, x7		/* map set number to cisw value */
+	dc	cisw, x9		/* clean & invalidate by set/way */
+	subs	x6, x6, #1		/* decrement the way */
+	b.ge	loop_way
+	subs	x4, x4, #1		/* decrement the set */
+	b.ge	loop_set
+
+	ret
+ENDPROC(tb_dcache_level)
+.popsection
+
+/*
+ * void tb_flush_dcache_all(void)
+ *
+ * clean & invalidate the whole D cache by set/way.
+ */
+.pushsection .text.tb_flush_dcache_all, "ax"
+ENTRY(tb_flush_dcache_all)
+	mov	x1, x0
+	dsb	sy
+	mrs	x10, clidr_el1		/* read clidr_el1 */
+	ubfx	x11, x10, #24, #3	/* x11 <- loc */
+	cbz	x11, finished		/* if loc is 0, exit */
+	mov	x15, lr
+	mov	x0, #0			/* start flush at cache level 0 */
+	/* x0  <- cache level */
+	/* x10 <- clidr_el1 */
+	/* x11 <- loc */
+	/* x15 <- return address */
+
+loop_level:
+	add	x12, x0, x0, lsl #1	/* x12 <- tripled cache level */
+	lsr	x12, x10, x12
+	and	x12, x12, #7		/* x12 <- cache type */
+	cmp	x12, #2
+	b.lt	skip			/* skip if no cache or icache */
+	bl	tb_dcache_level		/* flush this level */
+skip:
+	add	x0, x0, #1		/* increment cache level */
+	cmp	x11, x0
+	b.gt	loop_level
+
+	mov	x0, #0
+	msr	csselr_el1, x0		/* restore csselr_el1 */
+	dsb	sy
+	isb
+	mov	lr, x15
+
+finished:
+	ret
+ENDPROC(tb_flush_dcache_all)
+.popsection
+
+/*
+ * void tb_invalidate_icache_all(void)
+ *
+ * I cache invalidation to PoU, one ic iallu covers the local core.
+ */
+.pushsection .text.tb_invalidate_icache_all, "ax"
+ENTRY(tb_invalidate_icache_all)
+	ic	iallu
+	dsb	sy
+	isb
+	ret
+ENDPROC(tb_invalidate_icache_all)
+.popsection
diff --git a/arch/arm64/lib/semihosting.S b/arch/arm64/lib/semihosting.S
new file mode 100644
index 0000000..6e3fc31
--- /dev/null
+++ b/arch/arm64/lib/semihosting.S
@@ -0,0 +1,18 @@
+/* SPDX-License-Identifier: GPL-2.0+ */
+/*
+ * semihosting.S - the trap instruction itself. qemu answers this when
+ * it is started with -semihosting, and nothing happens without it, so
+ * every caller has to cope with the no-debugger case.
+ *
+ * Copyright (C) 2026 Bradley Morgan <brads@mainlining.org>
+ */
+
+#include <asm/linkage.h>
+
+.pushsection .text.smh_trap, "ax"
+/* long smh_trap(unsigned int sysnum, void *addr); */
+ENTRY(smh_trap)
+	hlt	#0xf000
+	ret
+ENDPROC(smh_trap)
+.popsection