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Date:   Tue, 12 Sep 2023 06:53:09 -0400
From:   Guo Ren <guoren@kernel.org>
To:     Jisheng Zhang <jszhang@kernel.org>, Arnd Bergmann <arnd@arndb.de>
Cc:     Paul Walmsley <paul.walmsley@sifive.com>,
        Palmer Dabbelt <palmer@dabbelt.com>,
        Albert Ou <aou@eecs.berkeley.edu>,
        Arnd Bergmann <arnd@arndb.de>, linux-riscv@lists.infradead.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH] riscv: errata: thead: use riscv_nonstd_cache_ops for CMO
Message-ID: <ZQBDFa0fGNiaqAgh@gmail.com>
References: <20230912072740.2544-1-jszhang@kernel.org>
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In-Reply-To: <20230912072740.2544-1-jszhang@kernel.org>
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On Tue, Sep 12, 2023 at 03:27:40PM +0800, Jisheng Zhang wrote:
> Previously, we use alternative mechanism to dynamically patch
> the CMO operations for THEAD C906/C910 during boot for performance
> reason. But as pointed out by Arnd, "there is already a significant
> cost in accessing the invalidated cache lines afterwards, which is
> likely going to be much higher than the cost of an indirect branch".
> And indeed, there's no performance difference with GMAC and EMMC per
> my test on Sipeed Lichee Pi 4A board.
> 
> Use riscv_nonstd_cache_ops for THEAD C906/C910 CMO to simplify
> the alternative code, and to acchieve Arnd's goal -- "I think
> moving the THEAD ops at the same level as all nonstandard operations
> makes sense, but I'd still leave CMO as an explicit fast path that
> avoids the indirect branch. This seems like the right thing to do both
> for readability and for platforms on which the indirect branch has a
> noticeable overhead."
> 
> Signed-off-by: Jisheng Zhang <jszhang@kernel.org>
> ---
>  arch/riscv/Kconfig.errata            |  1 +
>  arch/riscv/errata/thead/errata.c     | 76 +++++++++++++++++++++++++++-
>  arch/riscv/include/asm/errata_list.h | 50 +++---------------
>  3 files changed, 81 insertions(+), 46 deletions(-)
> 
> diff --git a/arch/riscv/Kconfig.errata b/arch/riscv/Kconfig.errata
> index 566bcefeab50..d7972914f9de 100644
> --- a/arch/riscv/Kconfig.errata
> +++ b/arch/riscv/Kconfig.errata
> @@ -78,6 +78,7 @@ config ERRATA_THEAD_CMO
>  	bool "Apply T-Head cache management errata"
>  	depends on ERRATA_THEAD && MMU
>  	select RISCV_DMA_NONCOHERENT
> +	select RISCV_NONSTANDARD_CACHE_OPS
>  	default y
>  	help
>  	  This will apply the cache management errata to handle the
> diff --git a/arch/riscv/errata/thead/errata.c b/arch/riscv/errata/thead/errata.c
> index 0554ed4bf087..1c320abfe446 100644
> --- a/arch/riscv/errata/thead/errata.c
> +++ b/arch/riscv/errata/thead/errata.c
> @@ -12,8 +12,10 @@
>  #include <asm/alternative.h>
>  #include <asm/cacheflush.h>
>  #include <asm/cpufeature.h>
> +#include <asm/dma-noncoherent.h>
>  #include <asm/errata_list.h>
>  #include <asm/hwprobe.h>
> +#include <asm/io.h>
>  #include <asm/patch.h>
>  #include <asm/vendorid_list.h>
>  
> @@ -33,6 +35,75 @@ static bool errata_probe_pbmt(unsigned int stage,
>  	return false;
>  }
>  
> +/*
> + * dcache.ipa rs1 (invalidate, physical address)
> + * | 31 - 25 | 24 - 20 | 19 - 15 | 14 - 12 | 11 - 7 | 6 - 0 |
> + *   0000001    01010      rs1       000      00000  0001011
> + * dache.iva rs1 (invalida, virtual address)
> + *   0000001    00110      rs1       000      00000  0001011
Remove dache.iva rs1 ...

> + *
> + * dcache.cpa rs1 (clean, physical address)
> + * | 31 - 25 | 24 - 20 | 19 - 15 | 14 - 12 | 11 - 7 | 6 - 0 |
> + *   0000001    01001      rs1       000      00000  0001011
Remove dcache.cpa rs1 ...

> + * dcache.cva rs1 (clean, virtual address)
> + *   0000001    00101      rs1       000      00000  0001011
Remove dcache.cva rs1 ...

> + *
> + * dcache.cipa rs1 (clean then invalidate, physical address)
> + * | 31 - 25 | 24 - 20 | 19 - 15 | 14 - 12 | 11 - 7 | 6 - 0 |
> + *   0000001    01011      rs1       000      00000  0001011
> + * dcache.civa rs1 (... virtual address)
> + *   0000001    00111      rs1       000      00000  0001011
Remove dcache.civa rs1 ...

> + *
> + * sync.s (make sure all cache operations finished)
> + * | 31 - 25 | 24 - 20 | 19 - 15 | 14 - 12 | 11 - 7 | 6 - 0 |
> + *   0000000    11001     00000      000      00000  0001011
> + */
> +#define THEAD_inval_A0	".long 0x0265000b"
> +#define THEAD_clean_A0	".long 0x0255000b"
> +#define THEAD_flush_A0	".long 0x0275000b"
> +#define THEAD_SYNC_S	".long 0x0190000b"
> +
> +#define THEAD_CMO_OP(_op, _start, _size, _cachesize)			\
> +asm volatile("mv a0, %1\n\t"						\
> +	     "j 2f\n\t"							\
> +	     "3:\n\t"							\
> +	     THEAD_##_op##_A0 "\n\t"					\
> +	     "add a0, a0, %0\n\t"					\
> +	     "2:\n\t"							\
> +	     "bltu a0, %2, 3b\n\t"					\
> +	     THEAD_SYNC_S						\
> +	     : : "r"(_cachesize),					\
> +		 "r"((unsigned long)(_start) & ~((_cachesize) - 1UL)),	\
> +		 "r"((unsigned long)(_start) + (_size))			\
> +	     : "a0")
> +
> +static void thead_errata_cache_inv(phys_addr_t paddr, size_t size)
> +{
> +	void *vaddr = phys_to_virt(paddr);
No need to phys_to_virt, and we could use paddr directly (dcache.ipa
rs1).

> +
> +	THEAD_CMO_OP(inval, vaddr, size, riscv_cbom_block_size);
> +}
> +
> +static void thead_errata_cache_wback(phys_addr_t paddr, size_t size)
> +{
> +	void *vaddr = phys_to_virt(paddr);
> +
> +	THEAD_CMO_OP(clean, vaddr, size, riscv_cbom_block_size);
> +}
Please remove the thead_errata_cache_wback because T-HEAD processors
would prioritize using an invalid cacheline instead of evicting an
existing cacheline. When we do dcache clean, the following operations
are to let other interconnect masters read. So, keeping wback_inv for
T-HEAD processors is the best choice, and maybe some other processors'
vendor has a different idea, but please use the wback_inv instead of
wback_only for the T-HEAD processors.

> +
> +static void thead_errata_cache_wback_inv(phys_addr_t paddr, size_t size)
> +{
> +	void *vaddr = phys_to_virt(paddr);
Ditto.

> +
> +	THEAD_CMO_OP(flush, vaddr, size, riscv_cbom_block_size);
> +}
> +
> +static const struct riscv_nonstd_cache_ops thead_errata_cmo_ops = {
> +	.wback = &thead_errata_cache_wback,
Please  .wback = &thead_errata_cache_wback_inv,


> +	.inv = &thead_errata_cache_inv,
> +	.wback_inv = &thead_errata_cache_wback_inv,
> +};
> +
>  static bool errata_probe_cmo(unsigned int stage,
>  			     unsigned long arch_id, unsigned long impid)
>  {
> @@ -48,6 +119,8 @@ static bool errata_probe_cmo(unsigned int stage,
>  	if (stage == RISCV_ALTERNATIVES_BOOT) {
>  		riscv_cbom_block_size = L1_CACHE_BYTES;
>  		riscv_noncoherent_supported();
> +		if (IS_ENABLED(CONFIG_RISCV_NONSTANDARD_CACHE_OPS))
> +			riscv_noncoherent_register_cache_ops(&thead_errata_cmo_ops);
>  	}
>  
>  	return true;
> @@ -77,8 +150,7 @@ static u32 thead_errata_probe(unsigned int stage,
>  	if (errata_probe_pbmt(stage, archid, impid))
>  		cpu_req_errata |= BIT(ERRATA_THEAD_PBMT);
>  
> -	if (errata_probe_cmo(stage, archid, impid))
> -		cpu_req_errata |= BIT(ERRATA_THEAD_CMO);
> +	errata_probe_cmo(stage, archid, impid);
>  
>  	if (errata_probe_pmu(stage, archid, impid))
>  		cpu_req_errata |= BIT(ERRATA_THEAD_PMU);
> diff --git a/arch/riscv/include/asm/errata_list.h b/arch/riscv/include/asm/errata_list.h
> index b55b434f0059..ea33288f8a25 100644
> --- a/arch/riscv/include/asm/errata_list.h
> +++ b/arch/riscv/include/asm/errata_list.h
> @@ -24,9 +24,8 @@
>  
>  #ifdef CONFIG_ERRATA_THEAD
>  #define	ERRATA_THEAD_PBMT 0
> -#define	ERRATA_THEAD_CMO 1
> -#define	ERRATA_THEAD_PMU 2
> -#define	ERRATA_THEAD_NUMBER 3
> +#define	ERRATA_THEAD_PMU 1
> +#define	ERRATA_THEAD_NUMBER 2
>  #endif
>  
>  #ifdef __ASSEMBLY__
> @@ -94,54 +93,17 @@ asm volatile(ALTERNATIVE(						\
>  #define ALT_THEAD_PMA(_val)
>  #endif
>  
> -/*
> - * dcache.ipa rs1 (invalidate, physical address)
> - * | 31 - 25 | 24 - 20 | 19 - 15 | 14 - 12 | 11 - 7 | 6 - 0 |
> - *   0000001    01010      rs1       000      00000  0001011
> - * dache.iva rs1 (invalida, virtual address)
> - *   0000001    00110      rs1       000      00000  0001011
> - *
> - * dcache.cpa rs1 (clean, physical address)
> - * | 31 - 25 | 24 - 20 | 19 - 15 | 14 - 12 | 11 - 7 | 6 - 0 |
> - *   0000001    01001      rs1       000      00000  0001011
> - * dcache.cva rs1 (clean, virtual address)
> - *   0000001    00101      rs1       000      00000  0001011
> - *
> - * dcache.cipa rs1 (clean then invalidate, physical address)
> - * | 31 - 25 | 24 - 20 | 19 - 15 | 14 - 12 | 11 - 7 | 6 - 0 |
> - *   0000001    01011      rs1       000      00000  0001011
> - * dcache.civa rs1 (... virtual address)
> - *   0000001    00111      rs1       000      00000  0001011
> - *
> - * sync.s (make sure all cache operations finished)
> - * | 31 - 25 | 24 - 20 | 19 - 15 | 14 - 12 | 11 - 7 | 6 - 0 |
> - *   0000000    11001     00000      000      00000  0001011
> - */
> -#define THEAD_inval_A0	".long 0x0265000b"
> -#define THEAD_clean_A0	".long 0x0255000b"
> -#define THEAD_flush_A0	".long 0x0275000b"
> -#define THEAD_SYNC_S	".long 0x0190000b"
> -
>  #define ALT_CMO_OP(_op, _start, _size, _cachesize)			\
> -asm volatile(ALTERNATIVE_2(						\
> -	__nops(6),							\
> +asm volatile(ALTERNATIVE(						\
> +	__nops(5),							\
>  	"mv a0, %1\n\t"							\
>  	"j 2f\n\t"							\
>  	"3:\n\t"							\
>  	CBO_##_op(a0)							\
>  	"add a0, a0, %0\n\t"						\
>  	"2:\n\t"							\
> -	"bltu a0, %2, 3b\n\t"						\
> -	"nop", 0, RISCV_ISA_EXT_ZICBOM, CONFIG_RISCV_ISA_ZICBOM,	\
> -	"mv a0, %1\n\t"							\
> -	"j 2f\n\t"							\
> -	"3:\n\t"							\
> -	THEAD_##_op##_A0 "\n\t"						\
> -	"add a0, a0, %0\n\t"						\
> -	"2:\n\t"							\
> -	"bltu a0, %2, 3b\n\t"						\
> -	THEAD_SYNC_S, THEAD_VENDOR_ID,					\
> -			ERRATA_THEAD_CMO, CONFIG_ERRATA_THEAD_CMO)	\
> +	"bltu a0, %2, 3b\n\t",						\
> +	0, RISCV_ISA_EXT_ZICBOM, CONFIG_RISCV_ISA_ZICBOM)		\
>  	: : "r"(_cachesize),						\
>  	    "r"((unsigned long)(_start) & ~((_cachesize) - 1UL)),	\
>  	    "r"((unsigned long)(_start) + (_size))			\
> -- 
> 2.40.1
> 
> 
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