Re: IP27: R14000: Unexpected General Exception in cpu_set_fpu_fcsr_mask()

[Joshua Kinard <kumba@gentoo.org>]

On 06/02/2015 07:32, Maciej W. Rozycki wrote:
> On Tue, 2 Jun 2015, Joshua Kinard wrote:
> 
>> I commented on it being odd because out of four CPUs, #2 was coming up with a
>> sign-extended value, twice (I tested two reboot cycles, same both times).  I'm
>> not fully knowledgable of IP27 hardware, and am probably one of the few on the
>> planet in possession of R14K node boards, so this might be a quirk of these
>> specific nodes.  Would need others to test to verify, I guess.
> 
>  That's how the CFC1 instruction works, it sign-extends the 32-bit value 
> written to the destination register on 64-bit processors.  So if the chip 
> has come out of reset with FCSR.FCC[7] set, then the bit will be repeated 
> across bits 63:32 when the bit pattern from FCSR has been transferred to a 
> general-purpose register.
> 
>> As for a typo, nope:
>>
>> 	__enable_fpu(FPU_AS_IS);
>>
>> 	fcsr = read_32bit_cp1_register(CP1_STATUS);
>> ->	pr_info("CPU%d: FCSR is: %08lx\n", smp_processor_id(), fcsr);
>> 	fcsr &= ~mask;
> 
>  OK, thanks for confirming.  So it looks like the cause of the exception 
> vanished at the same time.  There's no harm in reinitialising FCSR here 
> though, any vendor-specific bits possibly set will be cleared on process 
> creation anyway.

I had a thought and moved the printk to be near the top of cpu_probe(), since
it was possible the value of fcsr was getting clobbered by earlier code.  sure
enough, I got saner-looking values:

From full power-down:
# dmesg | grep FCSR
[    0.000000] CPU0: FCSR is: 000051a1
[    0.319163] CPU1: FCSR is: d0828324
[    0.364956] CPU2: FCSR is: a8011980
[    0.404819] CPU3: FCSR is: 00000000

Following a warm reboot cycle:
# dmesg | grep FCSR
[    0.000000] CPU0: FCSR is: 00000000
[    0.319248] CPU1: FCSR is: 00000000
[    0.364920] CPU2: FCSR is: 00000000
[    0.404893] CPU3: FCSR is: 00000000

Following a second full power-down:
# dmesg | grep FCSR
[    0.000000] CPU0: FCSR is: c6001100
[    0.318236] CPU1: FCSR is: 908203a4
[    0.364976] CPU2: FCSR is: a9801888
[    0.404828] CPU3: FCSR is: 08000088

With your new patch posted, it boots up just fine.

--J