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Initial commit

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Signed-off-by: Robert Allan James <robert.allan.james@gmail.com>
This commit is contained in:
Robert Allan James
2026-09-01 12:07:32 -04:00
parent d2a0305703
commit 58c59e87e5
11 changed files with 218 additions and 16 deletions
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src/starkernel/boot/uefi_loader.c
+49 -1
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@@ -68,6 +68,18 @@ extern void kernel_main(BootInfo *boot_info);
#if defined(ARCH_AMD64)
#define COM1_BASE 0x3F8
/* Set by raw_serial_init() only after a 16550 presence probe succeeds.
* When 0, raw_serial_putc() drops bytes immediately so boot can never
* wedge on a board whose 0x3F8 decode has no UART behind it (real
* mini-PCs like the Beelink SER5 typically expose no legacy COM port,
* whereas QEMU always emulates one — this is why this hang only bites
* on hardware). */
static int raw_serial_ready = 0;
/* Bounded THRE poll bound: provides the same "drop, don't hang" safety
* even if the probe above passes on a phantom decode. */
#define RAW_SERIAL_THRE_MAX_SPIN 100000u
/**
* @brief Write a byte to an x86 I/O port via @c OUT (early-boot raw path).
*
@@ -137,6 +149,29 @@ static void raw_serial_init(void)
raw_outb(COM1_BASE + 2, 0xC7);
/* RTS/DSR set */
raw_outb(COM1_BASE + 4, 0x0B);
/* 16550 presence probe: the scratch register (COM1+7) reads back what
* was written only when a real UART owns this decode. Unclaimed ports
* (or no I/O bridge routing) return garbage/0xFF, so the probe fails
* and serial is disabled — otherwise raw_serial_putc()'s THRE poll
* could spin forever on a board with no COM port. */
raw_outb(COM1_BASE + 7, 0x5A);
raw_serial_ready = (raw_inb(COM1_BASE + 7) == 0x5A) ? 1 : 0;
if (raw_serial_ready)
{
/* Sanity-check that THRE can actually be observed; if not, the
* decode is a phantom — disable serial rather than risk a spin. */
unsigned int spin = 0;
while ((raw_inb(COM1_BASE + 5) & 0x20) == 0 && spin < RAW_SERIAL_THRE_MAX_SPIN)
{
++spin;
}
if (spin >= RAW_SERIAL_THRE_MAX_SPIN)
{
raw_serial_ready = 0;
}
}
}
/**
@@ -154,7 +189,20 @@ static void raw_serial_init(void)
*/
static void raw_serial_putc(char c)
{
while ((raw_inb(COM1_BASE + 5) & 0x20) == 0) { }
/* No UART behind 0x3F8 (or probe failed): drop the byte, never block. */
if (!raw_serial_ready) return;
unsigned int spin = 0;
while ((raw_inb(COM1_BASE + 5) & 0x20) == 0)
{
/* Bounded: on hardware whose probe passed but THRE never asserts
* (phantom decode), drop the character instead of hanging boot. */
if (++spin >= RAW_SERIAL_THRE_MAX_SPIN)
{
raw_serial_ready = 0;
return;
}
}
raw_outb(COM1_BASE + 0, (uint8_t)c);
}