From 9dbdb15abf8ffb9dbdd972d6bbbcea9a3591e2a3 Mon Sep 17 00:00:00 2001 From: Bradley Morgan Date: Sat, 3 Oct 2026 20:02:29 +0000 Subject: 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 --- common/console.c | 74 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 74 insertions(+) create mode 100644 common/console.c (limited to 'common/console.c') diff --git a/common/console.c b/common/console.c new file mode 100644 index 0000000..a7108ba --- /dev/null +++ b/common/console.c @@ -0,0 +1,74 @@ +/* + * console.c - the console dprintf writes to. semihosting SYS_WRITE0, + * the firmware service on the qemu dev path, the arm64 stand-in for + * the bios teletype the osdev loaders use. on real hardware this is + * the one file that changes. + * + * Copyright (c) 2026 Bradley Morgan + * + * Permission is hereby granted, free of charge, to any person obtaining + * a copy of this software and associated documentation files + * (the "Software"), to deal in the Software without restriction, + * including without limitation the rights to use, copy, modify, merge, + * publish, distribute, sublicense, and/or sell copies of the Software, + * and to permit persons to whom the Software is furnished to do so, + * subject to the following conditions: + * + * The above copyright notice and this permission notice shall be + * included in all copies or substantial portions of the Software. + * + * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, + * EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF + * MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. + * IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY + * CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, + * TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE + * SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE. + */ + +#include +#include +#include + +/* + * lk's _dprintf sink. printf buffers a line here then hands it to the + * host, semihosting wants zero terminated strings not counts. + */ +#define TB_CONSOLE_MAX 256 + +static char console_buf[TB_CONSOLE_MAX]; +static size_t console_len; + +static void console_flush(void) +{ + if (console_len == 0) + return; + console_buf[console_len] = '\0'; + smh_write0(console_buf); + console_len = 0; +} + +void _putchar(char c) +{ + if (console_len >= TB_CONSOLE_MAX - 1) + console_flush(); + if (c == '\n') { + /* the host terminal wants cr lf, not lf alone */ + console_buf[console_len++] = '\r'; + } + console_buf[console_len++] = c; + if (c == '\n') + console_flush(); +} + +int tb_console_init(void) +{ + /* + * the probe is one harmless call: SYS_GET_ERRNO with no file + * handle open. a host answers, bare metal ignores the trap. + */ + if (!smh_probe()) + return -1; + console_len = 0; + return 0; +} -- cgit v1.2.3