[Buildroot] [PATCH] ci20_defconfig: disable madd instructions to avoid FPU bug

[Vicente Olivert Riera]

Hello Thomas,

On 18/10/16 13:54, Thomas Petazzoni wrote:
> Hello,
> 
> On Tue, 18 Oct 2016 13:24:21 +0100, Vicente Olivert Riera wrote:
>> MIPS Creator CI20 has an Ingenic JZ4780 SoC which features an XBurst
>> CPU. The FPU on that CPU has a bug that can generate incorrect results
>> in certain cases. The problem shows up when you have several of these
>> instructions in sequence with dependant operands.
>>
>> Using the -mno-fused-madd option prevents gcc from emitting these
>> instructions.
>>
>> More details here:
>>  - https://groups.google.com/forum/#!topic/mips-creator-ci20/spDB2jjbizM
>>  - https://android.googlesource.com/platform/build/+/90ce45347064210585a3a1f59a0514c22c753c8a
>>
>> Signed-off-by: Vicente Olivert Riera <Vincent.Riera at imgtec.com>
>> ---
>>  configs/ci20_defconfig | 2 ++
>>  1 file changed, 2 insertions(+)
>>
>> diff --git a/configs/ci20_defconfig b/configs/ci20_defconfig
>> index 0455170..1894554 100644
>> --- a/configs/ci20_defconfig
>> +++ b/configs/ci20_defconfig
>> @@ -8,6 +8,8 @@ BR2_PACKAGE_HOST_LINUX_HEADERS_CUSTOM_3_18=y
>>  
>>  # system
>>  BR2_TARGET_GENERIC_GETTY_PORT="ttyS4"
>> +# Avoid FPU bug
>> +BR2_TARGET_OPTIMIZATION="-mno-fused-madd"
> 
> Unfortunately, this means that other people using the same MIPS CPU,
> but not using your defconfig will fall into this issue.

True.

> For the Intel X1000, which also had a bug, we've added a separate
> entry in arch/Config.in.x86. Should we be doing the same here for this
> MIPS CPU ?
> 
> Note: this is *really* a question. I am not sure we want to make such a
> change to the MIPS CPU selection.

We could add an Ingenic XBurst entry in the MIPS CPU selection, and then
add that -mno-fused-madd option to the wrapper based on that selection.

If you are OK with that, please let me know and I'll cook the patch.

Vincent.

> Thomas
>