Re: [PATCH v2,08/10] MIPS: MIPS32R2 optimisations for pipeline stalls and code size.

[David Daney <david.daney@cavium.com>]

On 05/11/2012 01:21 PM, Steven J. Hill wrote:
> From: "Steven J. Hill"<sjhill@mips.com>
>
> If the CPU type is selected as MIPS32R2, then we can surround
> some code with #ifdef's to reduce the binary size. Detect when
> to use 'ehb' instruction to avoid pipeline stalls. Utilise the
> 'ins' and 'ext' MIPS32R2 instructions to reduce the size of
> exception handlers.
>
> Signed-off-by: Steven J. Hill<sjhill@mips.com>
> ---
>   arch/mips/mm/tlbex.c |   48 +++++++++++++++++++++++++++++++++++++++++++-----
>   1 file changed, 43 insertions(+), 5 deletions(-)
>
> diff --git a/arch/mips/mm/tlbex.c b/arch/mips/mm/tlbex.c
> index 897b727..7b84001 100644
> --- a/arch/mips/mm/tlbex.c
> +++ b/arch/mips/mm/tlbex.c
> @@ -74,10 +74,12 @@ static inline int __maybe_unused bcm1250_m3_war(void)
>   	return BCM1250_M3_WAR;
>   }
>
> +#ifndef CONFIG_CPU_MIPS32_R2
>   static inline int __maybe_unused r10000_llsc_war(void)
>   {
>   	return R10000_LLSC_WAR;
>   }
> +#endif

Totally useless addition of #if.  It Does nothing to the generated code, 
only introducing ugliness and fragility to the source.

In general, we are trying to reduce the number of #if in this file (and 
the kernel in general) rather than increase them.


>
>   static int use_bbit_insns(void)
>   {
> @@ -340,6 +342,7 @@ static void __cpuinit build_restore_work_registers(u32 **p)
>    */
>   extern unsigned long pgd_current[];
>
> +# ifndef CONFIG_CPU_MIPS32_R2
>   /*
>    * The R3000 TLB handler is simple.
>    */
> @@ -379,6 +382,7 @@ static void __cpuinit build_r3000_tlb_refill_handler(void)
>
>   	dump_handler((u32 *)ebase, 32);
>   }
> +# endif /* !CONFIG_CPU_MIPS32_R2 */
>   #endif /* CONFIG_MIPS_PGD_C0_CONTEXT */
>
>   /*
> @@ -449,8 +453,22 @@ static void __cpuinit build_tlb_write_entry(u32 **p, struct uasm_label **l,
>   	}
>
>   	if (cpu_has_mips_r2) {
> -		if (cpu_has_mips_r2_exec_hazard)
> -			uasm_i_ehb(p);
> +		/*
> +		 * The architecture spec says an ehb is required here,
> +		 * but a number of cores do not have the hazard and
> +		 * using an ehb causes an expensive pipeline stall.
> +		 */
> +		if (cpu_has_mips_r2_exec_hazard) {
> +			switch (current_cpu_type()) {
> +			case CPU_M14KC:
> +			case CPU_74K:
> +				break;
> +

Can this be rolled into the implementation of 
cpu_has_mips_r2_exec_hazard, thus leaving tlbex.c looking cleaner.

> +			default:
> +				uasm_i_ehb(p);
> +				break;
> +			}
> +		}
>   		tlbw(p);
>   		return;
>   	}
> @@ -910,7 +928,7 @@ build_get_pgde32(u32 **p, unsigned int tmp, unsigned int ptr)
>   #else
>   	/*
>   	 * smp_processor_id()<<  3 is stored in CONTEXT.
> -         */
> +	 */
>   	uasm_i_mfc0(p, ptr, C0_CONTEXT);
>   	UASM_i_LA_mostly(p, tmp, pgdc);
>   	uasm_i_srl(p, ptr, ptr, 23);
> @@ -921,6 +939,13 @@ build_get_pgde32(u32 **p, unsigned int tmp, unsigned int ptr)
>   #endif
>   	uasm_i_mfc0(p, tmp, C0_BADVADDR); /* get faulting address */
>   	uasm_i_lw(p, ptr, uasm_rel_lo(pgdc), ptr);
> +
> +	if (cpu_has_mips32r2) {
> +		uasm_i_ext(p, tmp, tmp, PGDIR_SHIFT, (32 - PGDIR_SHIFT));
> +		uasm_i_ins(p, ptr, tmp, PGD_T_LOG2, (32 - PGDIR_SHIFT));
> +		return;
> +	}
> +
>   	uasm_i_srl(p, tmp, tmp, PGDIR_SHIFT); /* get pgd only bits */
>   	uasm_i_sll(p, tmp, tmp, PGD_T_LOG2);
>   	uasm_i_addu(p, ptr, ptr, tmp); /* add in pgd offset */
> @@ -956,6 +981,15 @@ static void __cpuinit build_adjust_context(u32 **p, unsigned int ctx)
>
>   static void __cpuinit build_get_ptep(u32 **p, unsigned int tmp, unsigned int ptr)
>   {
> +	if (cpu_has_mips32r2) {

Doesn't the optimization also apply to mips64r2?

> +		/* For MIPS32R2, PTE ptr offset is obtained from BadVAddr */
> +		UASM_i_MFC0(p, tmp, C0_BADVADDR);
> +		UASM_i_LW(p, ptr, 0, ptr);
> +		uasm_i_ext(p, tmp, tmp, PAGE_SHIFT+1, PGDIR_SHIFT-PAGE_SHIFT-1);
> +		uasm_i_ins(p, ptr, tmp, PTE_T_LOG2+1, PGDIR_SHIFT-PAGE_SHIFT-1);
> +		return;
> +	}
> +
>   	/*
>   	 * Bug workaround for the Nevada. It seems as if under certain
>   	 * circumstances the move from cp0_context might produce a
> @@ -1496,9 +1530,11 @@ iPTE_SW(u32 **p, struct uasm_reloc **r, unsigned int pte, unsigned int ptr,
>   # endif
>   		UASM_i_SC(p, pte, 0, ptr);
>
> +#ifndef CONFIG_CPU_MIPS32_R2

Ugh.

>   	if (r10000_llsc_war())
>   		uasm_il_beqzl(p, r, pte, label_smp_pgtable_change);
>   	else
> +#endif
>   		uasm_il_beqz(p, r, pte, label_smp_pgtable_change);
>
>   # ifdef CONFIG_64BIT_PHYS_ADDR
> @@ -1632,7 +1668,7 @@ build_pte_modifiable(u32 **p, struct uasm_reloc **r,
>   	}
>   }
>
> -#ifndef CONFIG_MIPS_PGD_C0_CONTEXT
> +#if !defined(CONFIG_MIPS_PGD_C0_CONTEXT)&&  !defined(CONFIG_CPU_MIPS32_R2)
>
>
>   /*
> @@ -1786,7 +1822,7 @@ static void __cpuinit build_r3000_tlb_modify_handler(void)
>
>   	dump_handler(handle_tlbm, ARRAY_SIZE(handle_tlbm));
>   }
> -#endif /* CONFIG_MIPS_PGD_C0_CONTEXT */
> +#endif /* !CONFIG_MIPS_PGD_C0_CONTEXT&&  !CONFIG_CPU_MIPS32_R2 */
>
>   /*
>    * R4000 style TLB load/store/modify handlers.
> @@ -2103,6 +2139,7 @@ void __cpuinit build_tlb_refill_handler(void)
>   #endif
>
>   	switch (current_cpu_type()) {
> +#ifndef CONFIG_CPU_MIPS32_R2
>   	case CPU_R2000:
>   	case CPU_R3000:
>   	case CPU_R3000A:
> @@ -2132,6 +2169,7 @@ void __cpuinit build_tlb_refill_handler(void)
>   		panic("No R8000 TLB refill handler yet");
>   		break;
>
> +#endif /* !CONFIG_CPU_MIPS32_R2 */
>   	default:
>   		if (!run_once) {
>   			scratch_reg = allocate_kscratch();