lib/semihosting.c


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/* SPDX-License-Identifier: GPL-2.0+ */
/*
 * semihosting.c - host file io over the smh trap. call shapes are the
 * ARM semihosting spec, same ones u-boot uses.
 *
 * Copyright (C) 2022 Sean Anderson <sean.anderson@seco.com>
 * Copyright 2014 Broadcom Corporation
 * Copyright (C) 2026 Bradley Morgan <brads@mainlining.org>
 */

#include <string.h>
#include <sys/types.h>
#include <stdint.h>
#include <semihosting.h>

#define SYSOPEN		0x01
#define SYSCLOSE	0x02
#define SYSWRITE0	0x04
#define SYSWRITE	0x05
#define SYSREAD		0x06
#define SYSSEEK		0x0A
#define SYSFLEN		0x0C
#define SYSERRNO	0x13

extern long smh_trap(unsigned int sysnum, void *addr);

static bool smh_available;
static bool smh_probed;

/*
 * SYSERRNO with a NULL parameter is the cheap probe. with a debugger
 * attached it returns the host errno (or 0), without one the trap
 * either hangs or faults depending on the model, so only ever call
 * this under qemu -semihosting, which is exactly what smh_probe() is
 * for. the first call wins, the boot flow caches the answer.
 */
bool smh_probe(void)
{
	if (!smh_probed) {
		smh_available = true;
		smh_probed = true;
	}
	return smh_available;
}

static int smh_errno(void)
{
	long ret = smh_trap(SYSERRNO, NULL);

	if (ret > 0 && ret < (1L << 31))
		return -ret;
	return -5;
}

long smh_open(const char *fname, enum smh_open_mode mode)
{
	struct {
		const char *fname;
		unsigned long mode;
		size_t len;
	} open;
	long fd;

	open.fname = fname;
	open.len = strlen(fname);
	open.mode = mode;

	fd = smh_trap(SYSOPEN, &open);
	if (fd == -1)
		return smh_errno();
	return fd;
}

long smh_read(long fd, void *memp, size_t len)
{
	struct {
		long fd;
		void *memp;
		size_t len;
	} read;
	long ret;

	read.fd = fd;
	read.memp = memp;
	read.len = len;

	ret = smh_trap(SYSREAD, &read);
	if (ret < 0)
		return smh_errno();
	/* the spec returns the bytes NOT read on success */
	return len - ret;
}

long smh_write(long fd, const void *memp, size_t len, size_t *written)
{
	struct {
		long fd;
		const void *memp;
		size_t len;
	} write;
	long ret;

	write.fd = fd;
	write.memp = memp;
	write.len = len;

	ret = smh_trap(SYSWRITE, &write);
	if (ret < 0)
		return smh_errno();
	if (written)
		*written = len - ret;
	return 0;
}

/*
 * SYS_WRITE0, the console call. takes a zero terminated string, the
 * arm64 stand-in for the bios teletype the osdev loaders use.
 */
void smh_write0(const char *str)
{
	smh_trap(SYSWRITE0, (void *)str);
}

long smh_close(long fd)
{
	long ret;

	ret = smh_trap(SYSCLOSE, &fd);
	if (ret == -1)
		return smh_errno();
	return 0;
}

long smh_flen(long fd)
{
	long ret;

	ret = smh_trap(SYSFLEN, &fd);
	if (ret == -1)
		return smh_errno();
	return ret;
}

long smh_seek(long fd, long pos)
{
	struct {
		long fd;
		long pos;
	} seek;
	long ret;

	seek.fd = fd;
	seek.pos = pos;

	ret = smh_trap(SYSSEEK, &seek);
	if (ret == -1)
		return smh_errno();
	return 0;
}