Subject: Re: [RFC 1/3] powernv/cpuidle : Support for pre-entry and post exit of stop state in firmware

From: Gautham R Shenoy <[email-masked]>
Date: Mon, 27 Apr 2020 15:56:26 +0530

Hi Abhishek,

On Sun, Apr 26, 2020 at 09:10:25PM -0500, Abhishek Goel wrote:
> This patch provides kernel framework fro opal support of save restore
> of sprs in idle stop loop. Opal support for stop states is needed to
> selectively enable stop states or to introduce a quirk quickly in case
> a buggy stop state is present.
> 
> We make a opal call from kernel if firmware-stop-support for stop
> states is present and enabled. All the quirks for pre-entry of stop
> state is handled inside opal. A call from opal is made into kernel
> where we execute stop afer saving of NVGPRs.
> After waking up from 0x100 vector in kernel, we enter back into opal.
> All the quirks in post exit path, if any, are then handled in opal,
> from where we return successfully back to kernel.
> For deep stop states in which additional SPRs are lost, saving and
> restoration will be done in OPAL.
> 
> This idea was first proposed by Nick here:
> https://patchwork.ozlabs.org/patch/1208159/
> 
> The corresponding skiboot patch for this kernel patch is here:
> https://patchwork.ozlabs.org/project/skiboot/list/?series=172831
> 
> When we callback from OPAL into kernel, r13 is clobbered. So, to
> access PACA we need to restore it from HSPRGO. In future we can
> handle this into OPAL as in here:
> https://patchwork.ozlabs.org/patch/1245275/
> 
> Signed-off-by: Abhishek Goel <[email-masked]>
> Signed-off-by: Nicholas Piggin <[email-masked]>
> ---
> 
>  v1->v2 : No change in this patch.
> 
>  arch/powerpc/include/asm/opal-api.h        |  8 ++++-
>  arch/powerpc/include/asm/opal.h            |  3 ++
>  arch/powerpc/kernel/idle_book3s.S          |  5 +++
>  arch/powerpc/platforms/powernv/idle.c      | 37 ++++++++++++++++++++++
>  arch/powerpc/platforms/powernv/opal-call.c |  2 ++
>  5 files changed, 54 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/powerpc/include/asm/opal-api.h b/arch/powerpc/include/asm/opal-api.h
> index c1f25a760eb1..a2c782c99c9e 100644
> --- a/arch/powerpc/include/asm/opal-api.h
> +++ b/arch/powerpc/include/asm/opal-api.h
> @@ -214,7 +214,9 @@
>  #define OPAL_SECVAR_GET				176
>  #define OPAL_SECVAR_GET_NEXT			177
>  #define OPAL_SECVAR_ENQUEUE_UPDATE		178
> -#define OPAL_LAST				178
> +#define OPAL_REGISTER_OS_OPS			181
> +#define OPAL_CPU_IDLE				182
> +#define OPAL_LAST				182
> 
>  #define QUIESCE_HOLD			1 /* Spin all calls at entry */
>  #define QUIESCE_REJECT			2 /* Fail all calls with OPAL_BUSY */
> @@ -1181,6 +1183,10 @@ struct opal_mpipl_fadump {
>  	struct	opal_mpipl_region region[];
>  } __packed;
> 
> +struct opal_os_ops {
> +	__be64 os_idle_stop;
> +};
> +
>  #endif /* __ASSEMBLY__ */
> 
>  #endif /* __OPAL_API_H */
> diff --git a/arch/powerpc/include/asm/opal.h b/arch/powerpc/include/asm/opal.h
> index 9986ac34b8e2..3c340bc4df8e 100644
> --- a/arch/powerpc/include/asm/opal.h
> +++ b/arch/powerpc/include/asm/opal.h
> @@ -400,6 +400,9 @@ void opal_powercap_init(void);
>  void opal_psr_init(void);
>  void opal_sensor_groups_init(void);
> 
> +extern int64_t opal_register_os_ops(struct opal_os_ops *os_ops);
> +extern int64_t opal_cpu_idle(__be64 srr1_addr, uint64_t psscr);
> +
>  #endif /* __ASSEMBLY__ */
> 
>  #endif /* _ASM_POWERPC_OPAL_H */
> diff --git a/arch/powerpc/kernel/idle_book3s.S b/arch/powerpc/kernel/idle_book3s.S
> index 22f249b6f58d..8d287d1d06c0 100644
> --- a/arch/powerpc/kernel/idle_book3s.S
> +++ b/arch/powerpc/kernel/idle_book3s.S
> @@ -49,6 +49,8 @@ _GLOBAL(isa300_idle_stop_noloss)
>   */
>  _GLOBAL(isa300_idle_stop_mayloss)
>  	mtspr 	SPRN_PSSCR,r3
> +	mr	r6, r13
> +	mfspr	r13, SPRN_HSPRG0
>  	std	r1,PACAR1(r13)
>  	mflr	r4
>  	mfcr	r5
> @@ -74,6 +76,7 @@ _GLOBAL(isa300_idle_stop_mayloss)
>  	std	r31,-8*18(r1)
>  	std	r4,-8*19(r1)
>  	std	r5,-8*20(r1)
> +	std	r6,-8*21(r1)
>  	/* 168 bytes */
>  	PPC_STOP
>  	b	.	/* catch bugs */
> @@ -91,8 +94,10 @@ _GLOBAL(idle_return_gpr_loss)
>  	ld	r1,PACAR1(r13)
>  	ld	r4,-8*19(r1)
>  	ld	r5,-8*20(r1)
> +	ld	r6,-8*21(r1)
>  	mtlr	r4
>  	mtcr	r5
> +	mr	r13,r6
>  	/*
>  	 * KVM nap requires r2 to be saved, rather than just restoring it
>  	 * from PACATOC. This could be avoided for that less common case
> diff --git a/arch/powerpc/platforms/powernv/idle.c b/arch/powerpc/platforms/powernv/idle.c
> index 78599bca66c2..1841027b25c5 100644
> --- a/arch/powerpc/platforms/powernv/idle.c
> +++ b/arch/powerpc/platforms/powernv/idle.c
> @@ -35,6 +35,7 @@
>  static u32 supported_cpuidle_states;
>  struct pnv_idle_states_t *pnv_idle_states;
>  int nr_pnv_idle_states;
> +static bool firmware_stop_supported;
> 
>  /*
>   * The default stop state that will be used by ppc_md.power_save
> @@ -602,6 +603,25 @@ struct p9_sprs {
>  	u64 uamor;
>  };
> 
> +/*
> + * This function is called from OPAL if firmware support for stop
> + * states is present and enabled. It provides a fallback for idle
> + * stop states via OPAL.
> + */
> +static uint64_t os_idle_stop(uint64_t psscr, bool save_gprs)
> +{
> +	/*
> +	 * For lite state which does not lose even GPRS we call
> +	 * idle_stop_noloss while for all other states we call
> +	 * idle_stop_mayloss. Saving and restoration of other additional
> +	 * SPRs if required is handled in OPAL. All the quirks are also
> +	 * handled in OPAL.
> +	 */
> +	if (!save_gprs)
> +		return isa300_idle_stop_noloss(psscr);

I think PSSCR[ESL|EC] = 0 case is an overkill to go into OPAL and come
back via a callback. That can be handled in the kernel itself.

> +	return isa300_idle_stop_mayloss(psscr);
> +}
> +
>  static unsigned long power9_idle_stop(unsigned long psscr, bool mmu_on)
>  {
>  	int cpu = raw_smp_processor_id();
> @@ -613,6 +633,16 @@ static unsigned long power9_idle_stop(unsigned long psscr, bool mmu_on)
>  	unsigned long mmcr0 = 0;
>  	struct p9_sprs sprs = {}; /* avoid false used-uninitialised */
>  	bool sprs_saved = false;
> +	int rc = 0;
> +
> +	/*
> +	 * Kernel takes decision whether to make OPAL call or not. This logic
> +	 * will be combined with the logic for BE opal to take decision.
> +	 */
> +	if (firmware_stop_supported) {
> +		rc = opal_cpu_idle(cpu_to_be64(__pa(&srr1)), (uint64_t) psscr);

Couple of comments here. 

   1) If PSSCR[ESL|EC] = 0, the current code expects mmu_on=true. When
      we make an OPAL call and come back into the kernel via the
      callback today, we will be in real-mode, with mmu turned off.

   2) You seem to be choosing the opal cpuidle support as the default
      case, and not as a fallback. Thus, with this patch you will miss
      out on the deep stop-state support.

> +		goto out;
> +	}
> 
>  	if (!(psscr & (PSSCR_EC|PSSCR_ESL))) {
>  		/* EC=ESL=0 case */
> @@ -1232,6 +1262,10 @@ static int pnv_parse_cpuidle_dt(void)
>  		pr_warn("opal: PowerMgmt Node not found\n");
>  		return -ENODEV;
>  	}
> +
> +	if (of_device_is_compatible(np, "firmware-stop-supported"))
> +		firmware_stop_supported = true;

IMO, at least for POWER9 generation, you need to do this only when
"idle-stop" device-tree cpu-feature is unavailable.

> +
>  	nr_idle_states = of_property_count_u32_elems(np,
>  						"ibm,cpu-idle-state-flags");
> 
> @@ -1326,6 +1360,7 @@ static int pnv_parse_cpuidle_dt(void)
> 
>  static int __init pnv_init_idle_states(void)
>  {
> +	struct opal_os_ops os_ops;
>  	int cpu;
>  	int rc = 0;
> 
> @@ -1349,6 +1384,8 @@ static int __init pnv_init_idle_states(void)
>  		}
>  	}
> 
> +	os_ops.os_idle_stop = be64_to_cpu(os_idle_stop);
> +	rc = opal_register_os_ops((struct opal_os_ops *)(&os_ops));
>  	/* In case we error out nr_pnv_idle_states will be zero */
>  	nr_pnv_idle_states = 0;
>  	supported_cpuidle_states = 0;
> diff --git a/arch/powerpc/platforms/powernv/opal-call.c b/arch/powerpc/platforms/powernv/opal-call.c
> index 5cd0f52d258f..c885e607ba62 100644
> --- a/arch/powerpc/platforms/powernv/opal-call.c
> +++ b/arch/powerpc/platforms/powernv/opal-call.c
> @@ -293,3 +293,5 @@ OPAL_CALL(opal_mpipl_query_tag,			OPAL_MPIPL_QUERY_TAG);
>  OPAL_CALL(opal_secvar_get,			OPAL_SECVAR_GET);
>  OPAL_CALL(opal_secvar_get_next,			OPAL_SECVAR_GET_NEXT);
>  OPAL_CALL(opal_secvar_enqueue_update,		OPAL_SECVAR_ENQUEUE_UPDATE);
> +OPAL_CALL(opal_register_os_ops,			OPAL_REGISTER_OS_OPS);
> +OPAL_CALL(opal_cpu_idle,			OPAL_CPU_IDLE);
> -- 
> 2.17.1
>