Problems of using APU/FPU under linux

[Shanyuan Gao]

No, actually I am using head_4xx.S and I cannot find FPU_UNAVAILABLE  
in there. Do I need to set it?

And I commented out the _GLOBAL(giveup_fpu) in head_4xx.S because it  
has conflicts with fpu.S



Shan


On Apr 15, 2008, at 2:37 PM, Stephen Neuendorffer wrote:

> Shanyuan,
>
> Did you install the FPU_UNAVAILABLE trap in head_40x.S?
>
>
>> -----Original Message-----
>> From: Shanyuan Gao [mailto:sygao.research at gmail.com]
>> Sent: Tuesday, April 15, 2008 11:34 AM
>> To: Yoshio Kashiwagi; linuxppc-embedded at ozlabs.org
>> Cc: Stephen Neuendorffer; John Bonesio
>> Subject: Re: Problems of using APU/FPU under linux
>>
>> Thank you, Yoshio!!
>>
>> I just applied the change, seems it works! But it doesn't work
>> correctly. I mean it won't give me traps any more, but the answer is
>> not correctly. I just tried to multiply two float numbers. But it
>> gives me 0.
>>
>> The first time I change the reg.h was to enable apu enable, apu
>> exception enable and fpu enable. It gives me answer 0.
>> The second try I did was enabling apu enable and apu exception,
>> because I notice that inside /arch/powerpc/kernel/fpu.S, it will
>> enable FPU in load_up_fpu. So I guess I cannot enable FPU all the
>> time. However, this time it gave me trap again, well, with a
>> different MSR.
>>
>> Now my guess is load_up_fpu is not working correctly. I am working on
>> that.
>>
>>
>> Shan
>>
>>
>>
>> On Apr 15, 2008, at 2:20 PM, Yoshio Kashiwagi wrote:
>>
>>> Hi,
>>>
>>> The following modification is required if you use APU in user space.
>>>
>>> in include/asm-powerpc/reg.h
>>>
>>> -#define MSR_USER   (MSR_KERNEL|MSR_PR|MSR_EE)
>>> +#define MSR_USER   (MSR_KERNEL|MSR_PR|MSR_EE|MSR_VEC)
>>>
>>> Yoshio Kashiwagi - Nissin Systems
>>>
>>>> Thank you very much, Steve and John!
>>>>
>>>> My advisor and I discussed how Linux works with APU/FPU a few days
>>>> ago.
>>>  And he had the same thoughts with John. My naive guess was it would
>>> automatically decode FP operations and mask the trap. Now it
>>> answers my
>>> second question. I will try it later.
>>>>
>>>> But for my first question, I searched all (almost all) the files,
>>>> such
>>> as head.S, entry.S, head_4xx.S, etc. And added following three lines
>>> before mtmsr or MTMSRD �are used
>>>>
>>>> ori � � � �r10, r10, 1<<13 �/* enable fpu */
>>>> oris � � �r10, r10, 1<<9 � �/* enable apu */
>>>> oris � � �r10, r10, 1<<3 � �/* enable apu exception */
>>>>
>>>> However the MSR in trap prompts keeps the same (2d030) before and
>>> after I added those lines.�
>>>>
>>>> [�� 31.819079] Bad trap at PC: 10000458, MSR: 2d030,
>>>> vector=800��� Not
>>> tainted
>>>> [�� 31.887027]�� Signal: 5
>>>> [�� 31.887042]�� Code:�� 0
>>>> [�� 31.887058]�� Addr:�� 0
>>>> Trace/breakpoint trap
>>>>
>>>> I guess there must be some places, like some interrupts that  
>>>> changed
>>> the MSR that I didn't know. �
>>>>
>>>> And for FP exceptions, it has two bits (two modes) in MSR. I think
>>> they are for such exceptions like divided by zero. Do I need to set
>>> them
>>> also?
>>>>
>>>> In my previous build, I also added PPC_FPU under config 40x in  
>>>> arch/
>>> ppc/Kconfig. It compiled arch/powerpc/kernel/fpu.S in, but didn't
>>> help.
>>> I will try CONFIG_PPC_FPU later.
>>>>
>>>>
>>>> Shan
>>>>
>>>> On Apr 14, 2008, at 2:32 PM, John Bonesio wrote:
>>>>
>>>> Hi,
>>>>
>>>> The Linux kernel itself doesn't issue floating point instructions
>>> other than to save and restore the fpu state when necessary.
>>>>
>>>> In Linux, the way it saves and restores the fpu state is to make  
>>>> use
>>> of the trap. When the trap (fpu unavailable) occurs, it loads the  
>>> fpu
>>> state for the current task, sets up the MSR, and returns to re- 
>>> try the
>>> instruction.
>>>>
>>>> So, getting the trap is normal. If the FPU is not being set up
>>> correctly, then there may be a problem with the restoring of the
>>> state.
>>>>
>>>> When you guild the Linux kernel, you need to have CONFIG_PPC_FPU
>>> enabled. Otherwise the kernel does not setup the fpu exception
>>> handling.
>>>>
>>>> - John
>>>>
>>>>
>>>> On Monday 14 April 2008 10:35, Stephen Neuendorffer wrote:
>>>>
>>>> I'm not sure exactly what's going on here.� Generally speaking,
>>>> if you
>>>> have the FPU instantiated in the design and enable the APU in the
>>>> msr,
>>>> then the processor should decode FP instructions and send them
>>> directly
>>>> to the APU with no trap.� I haven't done this myself, or I could
>>>> probably give you some better help...
>>>>
>>>> One thing you should be aware of is that the there are gcc compiler
>>>> patches which are necessary to get the FPU working properly.�
>>>> However,
>>> I
>>>> don't think the failure mode that these patches workaround would
>>>> cause
>>> a
>>>> trap, so my guess is that there is still something else wrong.
>>>>
>>>> Steve
>>>>
>>>> -----Original Message-----
>>>> From: linuxppc-embedded-bounces 
>>>> +stephen=neuendorffer.name at ozlabs.org
>>>> [mailto:linuxppc-embedded-
>>>> bounces+stephen=neuendorffer.name at ozlabs.org] On Behalf Of Shanyuan
>>>> Gao
>>>> Sent: Monday, April 14, 2008 9:18 AM
>>>> To: linuxppc-embedded at ozlabs.org
>>>> Subject: Problems of using APU/FPU under linux
>>>>
>>>> Hi,
>>>>
>>>> Recently I was trying to make APU/FPU working under Linux on Xilinx
>>>> ML410. The standalone programs work perfectly. However under Linux,
>>>> when I try to use a floating point operation, like *fmuls*, it will
>>>> give me a *trap*.
>>>>
>>>> By studying the user guide from Xilinx and dumping the object  
>>>> files,
>>>> I know I need to change the corresponding bits (APU enable, FP
>>>> enable, maybe APU Exception enable) in Machine State Register. I
>>>> guess I need to enable the bits whenever before the kernel uses
>>>> *mtmsr*. However, it doesn't work. I got the same trap with the  
>>>> same
>>>> MSR, as I had no APU/FPU before. I also tried to add the FPU.S  
>>>> to ppc
>>>> tree, but it doesn't work either.
>>>>
>>>> The questions are
>>>> 1. I guess there might be some place that changed MSR after all my
>>>> changes. But I don't know where. And can I write a kernel module to
>>>> change the MSR after booting in Linux? (well, it's hard for me
>>>> though)
>>>>
>>>> 2. Does it have any exception/interrupt mechanism to direct FP
>>>> operation to APU/FPU? Or after enabling APU/FPU it will mask the
>>>> exception/interrupt and decode FP operation by itself?
>>>>
>>>>
>>>> Any ideas are appreciated. Thank you very much!
>>>>
>>>>
>>>> Shan
>>>
>>
>