Subject: Re: [PATCH 5/9] powerpc: Introduce VSX thread_struct and CONFIG_VSX

From: Michael Neuling <[email-masked]>
Date: Thu, 19 Jun 2008 15:37:51 +1000

In message <[email-masked]> you wrote
:
> 
> On Jun 18, 2008, at 11:35 PM, Michael Neuling wrote:
> 
> > In message <5AEB0769-1394-4924-803D- 
> > [email-masked]> you wrote
> > :
> >>>
> >>>
> >>> Index: linux-2.6-ozlabs/include/asm-powerpc/processor.h
> >>> ===================================================================
> >>> --- linux-2.6-ozlabs.orig/include/asm-powerpc/processor.h
> >>> +++ linux-2.6-ozlabs/include/asm-powerpc/processor.h
> >>> @@ -78,6 +78,7 @@ extern long kernel_thread(int (*fn)(void
> >>> /* Lazy FPU handling on uni-processor */
> >>> extern struct task_struct *last_task_used_math;
> >>> extern struct task_struct *last_task_used_altivec;
> >>> +extern struct task_struct *last_task_used_vsx;
> >>> extern struct task_struct *last_task_used_spe;
> >>>
> >>> #ifdef CONFIG_PPC32
> >>> @@ -136,8 +137,13 @@ typedef struct {
> >>> 	unsigned long seg;
> >>> } mm_segment_t;
> >>>
> >>> +#ifdef CONFIG_VSX
> >>> +#define TS_FPR(i) fpvsr.fp[i].fpr
> >>> +#define TS_FPRSTART fpvsr.fp
> >>> +#else
> >>> #define TS_FPR(i) fpr[i]
> >>> #define TS_FPRSTART fpr
> >>> +#endif
> >>>
> >>> struct thread_struct {
> >>> 	unsigned long	ksp;		/* Kernel stack pointer */
> >>> @@ -155,8 +161,19 @@ struct thread_struct {
> >>> 	unsigned long	dbcr0;		/* debug control register values */
> >>> 	unsigned long	dbcr1;
> >>> #endif
> >>> +#ifdef CONFIG_VSX
> >>> +	/* First 32 VSX registers (overlap with fpr[32]) */
> >>> +	union {
> >>> +		struct {
> >>> +			double fpr;
> >>> +			double vsrlow;
> >>> +		} fp[32];
> >>> +		vector128	vsr[32];
> 
> how about:
> 
> 	union {
> 		struct {
> 			double fp;
> 			double vsrlow;
> 		} fpr;
> 		vector128 v;
> 	} fpvsr[32];

Arrh, yep, makes more sense to put the array definition outside the
union.  I'll change.

> 
> >>>
> >>> +	} fpvsr __attribute__((aligned(16)));
> >>
> >> Do we really need a union here?  what would happen if you just  
> >> changed
> >> the type of fpr[32] from double to vector if #CONFIG_VSX?
> >>
> >> I really dont like the union and think we can just make the storage
> >> look opaque which is the key.  I doubt we every really care about
> >> using fpr[] as a double in the kernel.
> >
> > I did something similar to this for the first cut of this patch, but  
> > it
> > made the code accessing this structure much less readable.
> 
> really, what code is that?

Any code that has to read/write the top or bottom 64 bits _only_ of the
128 bit vector.

The signals code is a good example where, for backwards compatibility,
we need to read/write the old 64 bit FP regs, from the 128 bit value in
the struct.

Similarly, the way we've extended the signals interface for VSX, you
need to read/write out the bottom 64 bits (vsrlow) of a 128 bit value.

eg. the simple:
     current->thread.fpvsr.fp[i].vsrlow = buf[i]
would turn into some abomination/macro.

Mikey