How to use SPE on MPC8541

[Andy Fleming]

On Sep 28, 2005, at 03:02, Gérard Guével wrote:

>
> Andy,
>
>
>> Your driver runs in kernel space.  The kernel has the SPE bit off.
>> The MSR state is process-specific.  If the code executes, the
>> MSR bit
>> is set.  Why do you want to see if the bit is set?
>>
>
> OK, this is a bad idea to use a driver to check the msr register.
>
> I don't especially want to see if the bit is set, I just want
> to improve the board performance for a Linux application :-).
>
> To check the performance, I used the Dhrystone 2.1 benchmark with
> the standard glibc (strcpy, strcmp, ...) on one part, and with
> the freescale SPE library on the other part (vstrcpy, vstrcmp, ...).
>
> I already verified in the binary elf file that the right functions are
> called.
> When I run the benchmark, I get the same MIPS with and without SPE  
> code.

Hmm... This is very strange, because Dhrystone is exactly the  
benchmark this was tested on.  How did you determine that the SPE  
functions are called?


> I ran the same benchmark on the same board without OS,
> with a personal pseudo glibc, I have the same MIPS as under Linux,
> with the freescale library, I gain 40% of perf.
>
> That's I want to retreive with the Linux OS.

I'm not sure why you aren't seeing a performance gain, but I assure  
you that, if SPE instructions weren't working, Dhrystone would  
crash.  The only other possibility I can think of is that the SPE  
versions of the functions aren't being called.