Subject: Re: [PATCH v2 2/2] powerpc: oprofile: enable support for ppc750 processors

From: Benjamin Herrenschmidt <[email-masked]>
Date: Fri, 27 Feb 2009 10:13:57 +1100

On Thu, 2009-02-26 at 16:30 -0600, Andy Fleming wrote:
> 
> I know this saves you some code, but it seems hacky.  It would be
> better to modify oprofile to detect the proper cpu type.  Also, this
> will screw things up if you try to use the different event set that
> the 750 has.

Agreed. Note that Jack Miller (CC) has some patches for the oprofile
side.

> Also, one more concern is the long-standing errata which makes this
> quite dangerous.  All of the versions of the 750 I'm aware of have a
> bug where if a Performance Monitor exception occurs within one cycle
> of the Decrementer exception, the cpu will lose the ability to return
> from the interrupt (SRR0/SRR1 become corrupted).  It's possible the
> 750s you have modified to support oprofile don't have this errata.
> Alternatively, we can decide we don't care, as you have to be root to
> use oprofile.  But this is why I didn't add support for anything
> before the 7450.

I think we need to advertise it as broken in some way... I -think- the
latest batch of IBM 750CL have that bug fixed but I'm not 100% certain.

Cheers,
Ben.