[Skiboot] [PATCH 07/16] core: hardcode P9 large decrementer width

[Stewart Smith]

Nicholas Piggin <npiggin at gmail.com> writes:
> There is no good reason to "discover" the large decremeter width.
> We are firmware, we know the large decrementer width.
>
> Signed-off-by: Nicholas Piggin <npiggin at gmail.com>

Oliver - do you remember the original motivation around detecting it
rather than just coding how many bits there were? Was it because we were
experimenting with things in sim before the HW design was finalised? Or
are we just being fancy?

> ---
>  core/cpu.c | 43 +++++++------------------------------------
>  1 file changed, 7 insertions(+), 36 deletions(-)
>
> diff --git a/core/cpu.c b/core/cpu.c
> index 5d933bf5c..e7ffe3003 100644
> --- a/core/cpu.c
> +++ b/core/cpu.c
> @@ -1044,48 +1044,19 @@ void init_boot_cpu(void)
>  	init_hid();
>  }
>  
> -static void enable_large_dec(bool on)
> -{
> -	u64 lpcr = mfspr(SPR_LPCR);
> -
> -	if (on)
> -		lpcr |= SPR_LPCR_P9_LD;
> -	else
> -		lpcr &= ~SPR_LPCR_P9_LD;
> -
> -	mtspr(SPR_LPCR, lpcr);
> -	isync();
> -}
> -
> -#define HIGH_BIT (1ull << 63)
> -
>  static int find_dec_bits(void)
>  {
> -	int bits = 65; /* we always decrement once */
> -	u64 mask = ~0ull;
> +	int bits;
>  
>  	if (proc_gen < proc_gen_p9)
> -		return 32;
> -
> -	/* The ISA doesn't specify the width of the decrementer register so we
> -	 * need to discover it. When in large mode (LPCR.LD = 1) reads from the
> -	 * DEC SPR are sign extended to 64 bits and writes are truncated to the
> -	 * physical register width. We can use this behaviour to detect the
> -	 * width by starting from an all 1s value and left shifting until we
> -	 * read a value from the DEC with it's high bit cleared.
> -	 */
> -
> -	enable_large_dec(true);
> -
> -	do {
> -		bits--;
> -		mask = mask >> 1;
> -		mtspr(SPR_DEC, mask);
> -	} while (mfspr(SPR_DEC) & HIGH_BIT);
> -
> -	enable_large_dec(false);
> +		bits = 32;
> +	else if (proc_gen == proc_gen_p9)
> +		bits = 56;
> +	else
> +		assert(0);
>  
>  	prlog(PR_DEBUG, "CPU: decrementer bits %d\n", bits);
> +
>  	return bits;
>  }
>  
> -- 
> 2.18.0
>
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-- 
Stewart Smith
OPAL Architect, IBM.