Problems of using APU/FPU under linux

[Shanyuan Gao]

Sorry I didn't make myself clear. I just want to use FPU as a normal  
co-processor under Linux. I mean different programs and maybe  
multiple users, well, for single precision. So I think the exception  
handler really helps!

Thank you Steve and John, I added the exception handler. It solved  
the trap problem. However, I still got the answer 0 for 1.25f *  
3.75f. I am working on it :)


Shan



On Apr 15, 2008, at 2:59 PM, John Bonesio wrote:

> Hi Shan,
>
> It's not clear what you're trying to do. If you have only one  
> application using the FPU, and you're setting up the APU and FPU  
> from that user application, I'm not sure if you need the FPU  
> unavailable handler.
>
> If you're going to have multiple applications use the FPU, then you  
> should use the FPU unavailable handler. In this case, you'll need  
> to look in head_44x.S to see how the FPU unavailable handler is set  
> up, and do something similar in head_4xx.S
>
> If you're going to do all FPU from one user application, you just  
> need to make sure the kernel doesn't ever clear the FP Available  
> bit in the MSR.
>
> Is this making sense?
>
> model #1: One user app owns/uses FPU: Use Yoshio's suggesting on  
> getting access to the MSR to setup the fpu from the user  
> application. Don't use FPU unavailable handler, Don't use  
> load_up_fpu(). Check kernel code to make sure it doesn't clear FPU  
> available bit on a context switch.
> model #2: Kernel owns FPU, user apps use FPU: Set up the MSR for  
> fpu access in kernel startup code. Set up FPU unavailable handler  
> in kernel. Use load_up_fpu(). Make sure the kernel clears the FPU  
> available bit in the MSR on context switches.
>
> Hope this helps,
>
> - John
>
> On Tuesday 15 April 2008 11:47, Shanyuan Gao wrote:
>> 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
>>>>>
>>>>
>>>
>>
>>
>>
>