Re: [PATCH qemu v20] spapr: Implement Open Firmware client interface

[BALATON Zoltan]

On Sat, 22 May 2021, BALATON Zoltan wrote:
> On Sat, 22 May 2021, Alexey Kardashevskiy wrote:
> > VOF itself does not prints anything in this patch.
>
> However it seems to be needed for linux as the first thing it does seems to 
> be getting /chosen/stdout and calls exit if it returns nothing. So I'll need 
> this at least for linux. (I think MorphOS may also query it to print a banner 
> or some messages but not sure it needs it, at least it does not abort right 
> away if not found.)
>
> > > but to see Linux output do I need a stdout in VOF or it will just open the 
> > > serial with its own driver and use that?
> > > So I'm not sure what's the stdout parts in the current vof patch does and 
> > > if I need that for anything. I'll try to experiment with it some more but 
> > > fixing the ld and Kconfig seems to be enough to get it work for me.
> >
> > So for the client to print something, /chosen/stdout needs to have a valid 
> > ihandle.
> > The only way to get a valid ihandle is having a valid phandle which 
> > vof_client_open() can open.
> > A valid phandle is a phandle of any node in the device tree. On spapr we 
> > pick some spapr-vty, open it and store in /chosen/stdout.
> >
> > From this point output from the client can be seen via a tracepoint.

I've got it now. Looking at the original firmware device tree dump:

https://osdn.net/projects/qmiga/wiki/SubprojectPegasos2/attach/PegasosII_OFW-Dump.txt

I see that /chosen/stdout points to "screen" which is an alias to 
/bootconsole. Just adding an empty /bootconsole node in the device tree 
and vof_client_open_store() that as /chosen/stdout works and I get output 
via vof_write traces so this is enough for now to test Linux. Properly 
connecting a serial backend can thus be postponed.

So with this the Linux kernel does not abort on the first device tree 
access but starts to decompress itself then the embedded initrd and 
crashes at calling setprop:

[...]
vof_client_handle: setprop

Thread 4 "qemu-system-ppc" received signal SIGSEGV, Segmentation fault.
(gdb) bt
#0  0x0000000000000000 in  ()
#1  0x0000555555a5c2bf in vof_setprop
    (vof=0x7ffff48e9420, vallen=4, valaddr=<optimized out>, pname=<optimized 
out>, nodeph=8, fdt=0x7fff8aaff010, ms=0x5555564f8800)
    at ../hw/ppc/vof.c:308
#2  0x0000555555a5c2bf in vof_client_handle
    (nrets=1, rets=0x7ffff48e93f0, nargs=4, args=0x7ffff48e93c0, service=0x7ffff48e9460 
"setprop",
     vof=0x7ffff48e9420, fdt=0x7fff8aaff010, ms=0x5555564f8800) at 
../hw/ppc/vof.c:842
#3  0x0000555555a5c2bf in vof_client_call
    (ms=0x5555564f8800, vof=vof@entry=0x55555662a3d0, 
fdt=fdt@entry=0x7fff8aaff010, args_real=args_real@entry=23580472)
    at ../hw/ppc/vof.c:935

loooks like it's trying to set /chosen/linux,initrd-start:

(gdb) up
#1  0x0000555555a5c2bf in vof_setprop (vof=0x7ffff48e9420, vallen=4, valaddr=<optimized 
out>, pname=<optimized out>, nodeph=8,
    fdt=0x7fff8aaff010, ms=0x5555564f8800) at ../hw/ppc/vof.c:308
308             if (!vmc->setprop(ms, nodepath, propname, val, vallen)) {
(gdb) p nodepath
$1 = "/chosen\000\060/rPC,750CXE/", '\000' <repeats 234 times>
(gdb) p propname
$2 = 
"linux,initrd-start\000linux,initrd-end\000linux,cmdline-timeout\000bootarg"
(gdb) p val
$3 = <optimized out>

I think I need the callback for setprop in TYPE_VOF_MACHINE_IF. I can copy 
spapr_vof_setprop() but some explanation on why that's needed might help. 
Ciould I just do fdt_setprop in my callback as vof_setprop() would do 
without a machine callback or is there some special handling needed for 
these properties?

Regards.
BALATON Zoltan