Re: Custom boot loader stub

On Fri, 17 Oct 2025 12:32:36 +0300
Valery Ushakov <[email protected]> wrote:

> Yes, as I said, see how sparc boot code does that: "romp",
> common/prompdev.c, and ofwboot/promlib.c - its ofw subclass.  Start
> here:
> 
>   https://github.com/NetBSD/src/blob/d37c7118/sys/arch/sparc/stand/common/srt0.S#L152

Well, I managed to figure out how to use Open Firmware client interface.
I can print various diagnostic messages on boot:

{0} ok boot /pci@0/pci@0/pci@1/pci@0/pci@1/pci@0/usb@0,2/storage@3/disk@0
Boot device: /pci@0/pci@0/pci@1/pci@0/pci@1/pci@0/usb@0,2/storage@3/disk@0  File and args: 

Running SPARC-V9 bootblk

ihandle_stdin  0xFEB51F90
ihandle_stdout 0xFEB51D88


The only issue I'm having is reading from stdin always returns -2.

I looked at Illumos src/psm/promif/ieee1275/common/prom_getchar.c
and it is very similar to my function:

int64_t ofw_read_stdin(char *strbuf, uint32_t strbuf_len)
{
	ofw_cell_cdt ci_arg[7];

	/* If stdin ihandle is not initialised, return -1 */
	if (ihandle_stdin == (ihandle_cdt)-1) { return -1; }

	ci_arg[0] = (ofw_cell_cdt)"read";         /* Service */
	ci_arg[1] = (ofw_cell_cdt)3;              /* Number of arguments */
	ci_arg[2] = (ofw_cell_cdt)1;              /* Number of return values */
	ci_arg[3] = (ofw_cell_cdt)ihandle_stdin;  /* Arg1: stdin ihandle */
	ci_arg[4] = (ofw_cell_cdt)strbuf;         /* Arg2: strbuf address */
	ci_arg[5] = (ofw_cell_cdt)strbuf_len;     /* Arg3: str length */
	ci_arg[6] = (ofw_cell_cdt)-1;             /* Ret1: return value */

	/* Call into cif handler */
	ofw_cif_handler(ci_arg);

	return (int64_t)ci_arg[6];
}

But when I try to read 1 char from stdin:

i64 = ofw_read_stdin(strbuf, 1);

The return value is -2 and no char is read from stdin. The hex values for
stdin and stdout ihandles seem to be correct and match the ones from:
{0} ok show-props /chosen

Similar code works for stdout, so not sure if I'm missing something.

The bootblk seems to work up to 10 KiB in size, any bigger and I get
ERROR: Last Trap: Fast Data Access MMU Miss

Which I guess is expected. When I have some spare time, I will try
loading the 2nd stage bootloader and may be using "chain" function
to execute that.