[PATCH v2 00/12] PolarFire SoC reset controller & clock cleanups

[Conor.Dooley at microchip.com]

On 14/08/2022 12:41, Conor.Dooley at microchip.com wrote:
>>
>> Doesn't really matter since thats long enough to get past the switch
>> out of earlycon which is where the clang built kernel dies.

I had forgotten something obvious: keep_bootcon. With that, on the sbi
console I get a nice:

[    0.581754] Unable to handle kernel NULL pointer dereference at virtual address 00000000000000b1
[    0.591520] Oops [#1]
[    0.594045] Modules linked in:
[    0.597435] CPU: 0 PID: 1 Comm: swapper/0 Not tainted 6.0.0-rc1-00011-g8e1459cf4eca #1
[    0.606188] Hardware name: Microchip PolarFire-SoC Icicle Kit (DT)
[    0.613012] epc : __clk_register+0x4a6/0x85c
[    0.617759]  ra : __clk_register+0x49e/0x85c
[    0.622489] epc : ffffffff803faf7c ra : ffffffff803faf74 sp : ffffffc80400b720
[    0.630466]  gp : ffffffff810e93f8 tp : ffffffe77fe60000 t0 : ffffffe77ffb3800
[    0.638443]  t1 : 000000000000000a t2 : ffffffffffffffff s0 : ffffffc80400b7c0
[    0.646420]  s1 : 0000000000000001 a0 : 0000000000000001 a1 : 0000000000000000
[    0.654396]  a2 : 0000000000000001 a3 : 0000000000000000 a4 : 0000000000000000
[    0.662373]  a5 : ffffffff803a5810 a6 : 0000000200000022 a7 : 0000000000000006
[    0.670350]  s2 : ffffffff81099d48 s3 : ffffffff80d6e28e s4 : 0000000000000028
[    0.678327]  s5 : ffffffff810ed3c8 s6 : ffffffff810ed3d0 s7 : ffffffe77ffbc100
[    0.686304]  s8 : ffffffe77ffb1540 s9 : ffffffe77ffb1540 s10: 0000000000000008
[    0.694281]  s11: 0000000000000000 t3 : 00000000000000c6 t4 : 0000000000000007
[    0.702258]  t5 : ffffffff810c78c0 t6 : ffffffe77ff88cd0
[    0.708125] status: 0000000200000120 badaddr: 00000000000000b1 cause: 000000000000000d
[    0.716869] [<ffffffff803fb892>] devm_clk_hw_register+0x62/0xaa
[    0.723420] [<ffffffff80403412>] mpfs_clk_probe+0x1e0/0x244
[    0.729592] [<ffffffff80457dea>] platform_probe+0x82/0xa6
[    0.735581] [<ffffffff8045593c>] call_driver_probe+0x22/0xa4
[    0.741848] [<ffffffff804557da>] really_probe+0x13a/0x27a
[    0.747819] [<ffffffff804549f8>] __driver_probe_device+0xc4/0xee
[    0.754460] [<ffffffff804554d0>] driver_probe_device+0x3c/0x196
[    0.761013] [<ffffffff804552cc>] __device_attach_driver+0xa2/0x18c
[    0.767853] [<ffffffff8045284a>] bus_for_each_drv+0x76/0xba
[    0.774016] [<ffffffff804547ca>] __device_attach+0xa0/0x15a
[    0.780179] [<ffffffff80454896>] device_initial_probe+0x12/0x1a
[    0.786732] [<ffffffff804529c6>] bus_probe_device+0x2e/0x7c
[    0.792895] [<ffffffff8044f290>] device_add+0x21a/0x3aa
[    0.798693] [<ffffffff805c323a>] of_device_add+0x28/0x32
[    0.804578] [<ffffffff805c3c54>] of_platform_device_create_pdata+0x88/0xb2
[    0.812181] [<ffffffff805c3e70>] of_platform_bus_create+0x14a/0x1a8
[    0.819117] [<ffffffff805c3ea6>] of_platform_bus_create+0x180/0x1a8
[    0.826053] [<ffffffff805c3f2a>] of_platform_populate+0x5c/0x96
[    0.832606] [<ffffffff8082bbaa>] of_platform_default_populate_init+0xc2/0xd4
[    0.840400] [<ffffffff800020e8>] do_one_initcall+0xbc/0x216
[    0.846563] [<ffffffff8080126c>] do_initcall_level+0x82/0x94
[    0.852830] [<ffffffff808011ae>] do_initcalls+0x5c/0x98
[    0.858611] [<ffffffff8080114a>] do_basic_setup+0x20/0x28
[    0.864583] [<ffffffff808010e0>] kernel_init_freeable+0xba/0x104
[    0.871223] [<ffffffff80746856>] kernel_init+0x22/0x1ba
[    0.877013] [<ffffffff80003700>] ret_from_exception+0x0/0xc
[    0.883218] ---[ end trace 0000000000000000 ]---

At least I had somewhere to start. Since QEMU repros, I continued there with:
	$(QEMU)/qemu-system-riscv64 -s -S -M microchip-icicle-kit \
		-m 2G -smp 5 \
		-kernel $(vmlinux_bin) \
		-dtb $(dtb) \
		-initrd $(initramfs) \
		-display none -serial null \
		-serial stdio

Dumping the backtrace at the devm_clk_hw_register() callsites shows that
it passes for:
- the main pll in mpfs_clk_register_mssplls()
- the "cfg" clocks in mpfs_clk_register_cfgs()
- the first 4 periph clocks...

It fails on "clk_periph_timer" - which uses a different parent, that it
tries to find using the macro:
#define PARENT_CLK(PARENT) (&mpfs_cfg_clks[CLK_##PARENT].cfg.hw)

If parent is RTCREF, that becomes: &mpfs_cfg_clks[33].cfg.hw

And yeah, thats well beyond the end of the array!

Booting with gcc, and looking at the clock summary:
  mssrefclk                            1        1        0   125000000          0     0  50000         Y
     clk_rtcref                        0        0        0     1000000          0     0  50000         Y
        clk_periph_timer               0        0        0     1000000          0     0  50000         N

This is reported correctly, so how this works for either gcc or clang now, or
gcc with the patches applied, I am not really too sure..

On v6.0-rc1, gcc and clang show:
  mssrefclk                            1        1        0   125000000          0     0  50000         Y
     clk_rtcref                        0        0        0     1000000          0     0  50000         Y

I guess this may fail gracefully as the pointer is actually null, but
the extra level of indirection added by my patch started tripping up
the error checking. I must just not have noticed because I added RTCREF
in a bit of a hurry trying to fix a bug in time for 5.18:
https://lore.kernel.org/linux-clk/986c73df-9634-d18b-eed3-37584fa2ea89@conchuod.ie/#t

Obviously I've got to fix the bug itself now, but now I am left wondering
about GCC's behaviour rather than clang/llvm's!

Thanks Nathan - if even just for being a rubber duck :)

Conor.

>>
>> Didn't get a chance to look at disassembly etc today, but as I said
>> last night it reproduces with GNU binutils.
>>
>> Thanks,
>> Conor.
>>
>> On another note, brought up our QEMU port's state today so fixing
>> it is now on the good ole, ever expanding todo list :)
>