Re: [RFC PATCH v2 07/10] perf: RISC-V: Move T-Head PMU to CPU feature alternative framework

[Conor Dooley]

Hey,

On Thu, Oct 19, 2023 at 10:01:19PM +0800, Yu Chien Peter Lin wrote:

$subject: perf: RISC-V: Move T-Head PMU to CPU feature alternative framework

IMO, this should be "RISC-V, perf:" or just "RISC-V" as the changes
being made to the arch code are far more meaningful than those
elsewhere.

> The custom PMU extension was developed to support perf event sampling
> prior to the ratification of Sscofpmf. Instead of utilizing the standard
> bits and CSR of Sscofpmf, a set of custom CSRs is added. So we may
> consider it as a CPU feature rather than an erratum.
> 
> T-Head cores need to append "xtheadpmu" to the riscv,isa-extensions
> for each cpu node in device tree, and enable CONFIG_THEAD_CUSTOM_PMU
> for proper functioning as of this commit.

And in doing so, you regress break perf for existing DTs :(
You didn't add the property to existing DTS in-kernel either, so if this
series was applied, perf would just entirely stop working, no?

> Signed-off-by: Yu Chien Peter Lin <peterlin@xxxxxxxxxxxxx>
> ---
> Hi All,
> 
> This is in preparation for introducing other PMU alternative.
> We follow Conor's suggestion [1] to use cpu feature alternative
> framework rather than errta, if you want to stick with errata
> alternative or have other issues, please let me know. Thanks.

Personally, I like this conversion, but it is going to regress support
for perf on any T-Head cores which may be a bitter pill to get people to
actually accept...
Perhaps we could add this "improved" detection in parallel, and
eventually remove the m*id based stuff in the future.

> [1] https://patchwork.kernel.org/project/linux-riscv/patch/20230907021635.1002738-4-peterlin@xxxxxxxxxxxxx/#25503860
> 
> Changes v1 -> v2:
>   - New patch
> ---
>  arch/riscv/Kconfig.errata            | 13 -------------
>  arch/riscv/errata/thead/errata.c     | 19 -------------------
>  arch/riscv/include/asm/errata_list.h | 15 +--------------
>  arch/riscv/include/asm/hwcap.h       |  1 +
>  arch/riscv/kernel/cpufeature.c       |  1 +
>  drivers/perf/Kconfig                 | 13 +++++++++++++
>  drivers/perf/riscv_pmu_sbi.c         | 16 ++++++++++++++--
>  7 files changed, 30 insertions(+), 48 deletions(-)
> 
> diff --git a/arch/riscv/Kconfig.errata b/arch/riscv/Kconfig.errata
> index 566bcefeab50..35dfb19d6a29 100644
> --- a/arch/riscv/Kconfig.errata
> +++ b/arch/riscv/Kconfig.errata
> @@ -85,17 +85,4 @@ config ERRATA_THEAD_CMO
>  
>  	  If you don't know what to do here, say "Y".
>  
> -config ERRATA_THEAD_PMU
> -	bool "Apply T-Head PMU errata"
> -	depends on ERRATA_THEAD && RISCV_PMU_SBI
> -	default y
> -	help
> -	  The T-Head C9xx cores implement a PMU overflow extension very
> -	  similar to the core SSCOFPMF extension.
> -
> -	  This will apply the overflow errata to handle the non-standard
> -	  behaviour via the regular SBI PMU driver and interface.
> -
> -	  If you don't know what to do here, say "Y".
> -
>  endmenu # "CPU errata selection"
> diff --git a/arch/riscv/errata/thead/errata.c b/arch/riscv/errata/thead/errata.c
> index 0554ed4bf087..5de5f7209132 100644
> --- a/arch/riscv/errata/thead/errata.c
> +++ b/arch/riscv/errata/thead/errata.c
> @@ -53,22 +53,6 @@ static bool errata_probe_cmo(unsigned int stage,
>  	return true;
>  }
>  
> -static bool errata_probe_pmu(unsigned int stage,
> -			     unsigned long arch_id, unsigned long impid)
> -{
> -	if (!IS_ENABLED(CONFIG_ERRATA_THEAD_PMU))
> -		return false;
> -
> -	/* target-c9xx cores report arch_id and impid as 0 */
> -	if (arch_id != 0 || impid != 0)
> -		return false;
> -
> -	if (stage == RISCV_ALTERNATIVES_EARLY_BOOT)
> -		return false;
> -
> -	return true;
> -}
> -
>  static u32 thead_errata_probe(unsigned int stage,
>  			      unsigned long archid, unsigned long impid)
>  {
> @@ -80,9 +64,6 @@ static u32 thead_errata_probe(unsigned int stage,
>  	if (errata_probe_cmo(stage, archid, impid))
>  		cpu_req_errata |= BIT(ERRATA_THEAD_CMO);
>  
> -	if (errata_probe_pmu(stage, archid, impid))
> -		cpu_req_errata |= BIT(ERRATA_THEAD_PMU);
> -
>  	return cpu_req_errata;
>  }
>  
> diff --git a/arch/riscv/include/asm/errata_list.h b/arch/riscv/include/asm/errata_list.h
> index c190393aa9db..1b5354a50d55 100644
> --- a/arch/riscv/include/asm/errata_list.h
> +++ b/arch/riscv/include/asm/errata_list.h
> @@ -25,8 +25,7 @@
>  #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_NUMBER 2
>  #endif
>  
>  #ifdef __ASSEMBLY__
> @@ -147,18 +146,6 @@ asm volatile(ALTERNATIVE_2(						\
>  	    "r"((unsigned long)(_start) + (_size))			\
>  	: "a0")
>  
> -#define THEAD_C9XX_RV_IRQ_PMU			17
> -#define THEAD_C9XX_CSR_SCOUNTEROF		0x5c5
> -
> -#define ALT_SBI_PMU_OVERFLOW(__ovl)					\
> -asm volatile(ALTERNATIVE(						\
> -	"csrr %0, " __stringify(CSR_SSCOUNTOVF),			\
> -	"csrr %0, " __stringify(THEAD_C9XX_CSR_SCOUNTEROF),		\
> -		THEAD_VENDOR_ID, ERRATA_THEAD_PMU,			\
> -		CONFIG_ERRATA_THEAD_PMU)				\
> -	: "=r" (__ovl) :						\
> -	: "memory")
> -
>  #endif /* __ASSEMBLY__ */
>  
>  #endif
> diff --git a/arch/riscv/include/asm/hwcap.h b/arch/riscv/include/asm/hwcap.h
> index b7b58258f6c7..d3082391c901 100644
> --- a/arch/riscv/include/asm/hwcap.h
> +++ b/arch/riscv/include/asm/hwcap.h
> @@ -58,6 +58,7 @@
>  #define RISCV_ISA_EXT_ZICSR		40
>  #define RISCV_ISA_EXT_ZIFENCEI		41
>  #define RISCV_ISA_EXT_ZIHPM		42
> +#define RISCV_ISA_EXT_XTHEADPMU		43
>  
>  #define RISCV_ISA_EXT_MAX		64
>  
> diff --git a/arch/riscv/kernel/cpufeature.c b/arch/riscv/kernel/cpufeature.c
> index 1cfbba65d11a..4a3fb017026c 100644
> --- a/arch/riscv/kernel/cpufeature.c
> +++ b/arch/riscv/kernel/cpufeature.c
> @@ -181,6 +181,7 @@ const struct riscv_isa_ext_data riscv_isa_ext[] = {
>  	__RISCV_ISA_EXT_DATA(svinval, RISCV_ISA_EXT_SVINVAL),
>  	__RISCV_ISA_EXT_DATA(svnapot, RISCV_ISA_EXT_SVNAPOT),
>  	__RISCV_ISA_EXT_DATA(svpbmt, RISCV_ISA_EXT_SVPBMT),
> +	__RISCV_ISA_EXT_DATA(xtheadpmu, RISCV_ISA_EXT_XTHEADPMU),

Again, this would need to be documented in the dt-binding to be
acceptable.

>  };
>  
>  const size_t riscv_isa_ext_count = ARRAY_SIZE(riscv_isa_ext);
> diff --git a/drivers/perf/Kconfig b/drivers/perf/Kconfig
> index 273d67ecf6d2..c71b6f16bdfa 100644
> --- a/drivers/perf/Kconfig
> +++ b/drivers/perf/Kconfig
> @@ -86,6 +86,19 @@ config RISCV_PMU_SBI
>  	  full perf feature support i.e. counter overflow, privilege mode
>  	  filtering, counter configuration.
>  
> +config THEAD_CUSTOM_PMU
> +	bool "T-Head custom PMU support"
> +	depends on RISCV_ALTERNATIVE && RISCV_PMU_SBI
> +	default y
> +	help
> +	  The T-Head C9xx cores implement a PMU overflow extension very
> +	  similar to the core SSCOFPMF extension.
> +
> +	  This will patch the overflow CSR and handle the non-standard
> +	  behaviour via the regular SBI PMU driver and interface.
> +
> +	  If you don't know what to do here, say "Y".
> +
>  config ARM_PMU_ACPI
>  	depends on ARM_PMU && ACPI
>  	def_bool y
> diff --git a/drivers/perf/riscv_pmu_sbi.c b/drivers/perf/riscv_pmu_sbi.c
> index f340db9ce1e2..790fc20fe094 100644
> --- a/drivers/perf/riscv_pmu_sbi.c
> +++ b/drivers/perf/riscv_pmu_sbi.c
> @@ -20,10 +20,21 @@
>  #include <linux/cpu_pm.h>
>  #include <linux/sched/clock.h>
>  
> -#include <asm/errata_list.h>
>  #include <asm/sbi.h>
>  #include <asm/hwcap.h>
>  
> +#define THEAD_C9XX_RV_IRQ_PMU		17
> +#define THEAD_C9XX_CSR_SCOUNTEROF	0x5c5
> +
> +#define ALT_SBI_PMU_OVERFLOW(__ovl)					\
> +asm volatile(ALTERNATIVE(						\
> +	"csrr %0, " __stringify(CSR_SSCOUNTOVF),			\
> +	"csrr %0, " __stringify(THEAD_C9XX_CSR_SCOUNTEROF),		\
> +		0, RISCV_ISA_EXT_XTHEADPMU,				\
> +		CONFIG_THEAD_CUSTOM_PMU)				\
> +	: "=r" (__ovl) :						\
> +	: "memory")
> +
>  #define SYSCTL_NO_USER_ACCESS	0
>  #define SYSCTL_USER_ACCESS	1
>  #define SYSCTL_LEGACY		2
> @@ -805,7 +816,8 @@ static int pmu_sbi_setup_irqs(struct riscv_pmu *pmu, struct platform_device *pde
>  	if (riscv_isa_extension_available(NULL, SSCOFPMF)) {
>  		riscv_pmu_irq_num = RV_IRQ_PMU;
>  		riscv_pmu_use_irq = true;
> -	} else if (IS_ENABLED(CONFIG_ERRATA_THEAD_PMU) &&
> +	} else if (riscv_isa_extension_available(NULL, XTHEADPMU) &&
> +		   IS_ENABLED(CONFIG_THEAD_CUSTOM_PMU) &&
>  		   riscv_cached_mvendorid(0) == THEAD_VENDOR_ID &&
>  		   riscv_cached_marchid(0) == 0 &&
>  		   riscv_cached_mimpid(0) == 0) {

Can all of the m*id checks be removed, since the firmware is now
explicitly telling us that the T-Head PMU is supported?

Cheers,
Conor.

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