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Subject: Re: Hijacking CPU_FTR_VSX for BGQ QPX
From: Benjamin Herrenschmidt <benh@kernel.crashing.org>
To: Jimi Xenidis <jimix@pobox.com>
Date: Sat, 10 Nov 2012 06:57:09 +1100
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On Fri, 2012-11-09 at 11:43 -0600, Jimi Xenidis wrote:
> The CPU_FTR_* values are pretty tight (a few bits left) yes I need to save and restore the QPX registers.
> There are 32 QPX registers, each 32 bytes in size, it is otherwise managed by the FPSCR and MSR[FP]
> 
> I was thinking that I could hijack the VSX, since there is no plan to add it to embedded yet.
> I could be explicit or create an alieas fo the same bit, but the basic effect (after increasing the save area size) would be something like the diff below.
> Thoughts?

Don't. Use a different bit, we can always split the mask again if
needed, move more bits to mmu_features etc...

> -#ifdef CONFIG_VSX
> +#if defined (CONFIG_VSX) && defined(CONFIG_BGQ)
> +# error "This code depends on CONFIG_VSX and CONFIG_BGQ being exclusive
> +#elif defined (CONFIG_VSX)
> +# define _REST_32VSRS(n,c,base) REST_32VSRS(n,c,base)
> +# define _SAVE_32VSRS(n,c,base) SAVE_32VSRS(n,c,base)
> +#elif defined(CONFIG_BGQ)

Make a CONFIG_PPC_QPX or something like that specifically for the QPX
stuff that you can then "select" from CONFIG_PPC_BGQ (don't do just
CONFIG_BGQ).

And don't just "hijack" stuff like that, it should be a runtime option,
so add a new set etc... it should be possible to build a kernel that
boots on a BGQ or a hypothetical BookE chip with VSX.

> +# define _REST_32VSRS(n,c,base) REST_32QRS(n,c,base)
> +# define _SAVE_32VSRS(n,c,base) SAVE_32QRS(n,c,base)
> +#endif
> +
> +#if defined (CONFIG_VSX) || defined(CONFIG_BGQ)
>  #define REST_32FPVSRS(n,c,base)						\
>  BEGIN_FTR_SECTION							\
>  	b	2f;							\
>  END_FTR_SECTION_IFSET(CPU_FTR_VSX);					\
>  	REST_32FPRS(n,base);						\
>  	b	3f;							\
> -2:	REST_32VSRS(n,c,base);						\
> +2:	_REST_32VSRS(n,c,base);						\
>  3:
>  
>  #define SAVE_32FPVSRS(n,c,base)						\
> @@ -41,7 +51,7 @@ BEGIN_FTR_SECTION							\
>  END_FTR_SECTION_IFSET(CPU_FTR_VSX);					\
>  	SAVE_32FPRS(n,base);						\
>  	b	3f;							\
> -2:	SAVE_32VSRS(n,c,base);						\
> +2:	_SAVE_32VSRS(n,c,base);						\
>  3:
>  #else
>  #define REST_32FPVSRS(n,b,base)	REST_32FPRS(n, base)

Cheers,
Ben.