linux-kernel - Re: [PATCH] Revert "powerpc/powernv/idle: Replace CPU feature check with PVR check"

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Message-ID: <20200826094009.GC48285@drishya.in.ibm.com>
Date:   Wed, 26 Aug 2020 15:10:09 +0530
From:   Vaidyanathan Srinivasan <svaidy@...ux.ibm.com>
To:     Pratik Rajesh Sampat <psampat@...ux.ibm.com>
Cc:     linuxppc-dev@...ts.ozlabs.org, mpe@...erman.id.au,
        npiggin@...il.com, mikey@...ling.org, ego@...ux.vnet.ibm.com,
        linux-kernel@...r.kernel.org, pratik.r.sampat@...il.com
Subject: Re: [PATCH] Revert "powerpc/powernv/idle: Replace CPU feature check
 with PVR check"

* Pratik Rajesh Sampat <psampat@...ux.ibm.com> [2020-08-26 13:59:18]:

> Cpuidle stop state implementation has minor optimizations for P10
> where hardware preserves more SPR registers compared to P9.
> The current P9 driver works for P10, although does few extra
> save-restores. P9 driver can provide the required power management
> features like SMT thread folding and core level power savings
> on a P10 platform.
> 
> Until the P10 stop driver is available, revert the commit which
> allows for only P9 systems to utilize cpuidle and blocks all
> idle stop states for P10.
> Cpu idle states are enabled and tested on the P10 platform
> with this fix.
> 
> This reverts commit 8747bf36f312356f8a295a0c39ff092d65ce75ae.
> 
> Fixes: 8747bf36f312 ("powerpc/powernv/idle: Replace CPU feature check with PVR check")
> Signed-off-by: Pratik Rajesh Sampat <psampat@...ux.ibm.com>

Reviewed-by: Vaidyanathan Srinivasan <svaidy@...ux.ibm.com>


> ---
>  @mpe: This revert would resolve a staging issue wherein the P10 stop
>  driver is not yet ready while cpuidle stop states need not be blocked
>  on 5.9 for Power10 systems which could cause SMT folding related
>  performance issues.
> 
>  The P10 stop driver is in the works here:
>  https://lists.ozlabs.org/pipermail/linuxppc-dev/2020-August/216773.html
> 
>  arch/powerpc/platforms/powernv/idle.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/powerpc/platforms/powernv/idle.c b/arch/powerpc/platforms/powernv/idle.c
> index 77513a80cef9..345ab062b21a 100644
> --- a/arch/powerpc/platforms/powernv/idle.c
> +++ b/arch/powerpc/platforms/powernv/idle.c
> @@ -1223,7 +1223,7 @@ static void __init pnv_probe_idle_states(void)
>  		return;
>  	}
>  
> -	if (pvr_version_is(PVR_POWER9))
> +	if (cpu_has_feature(CPU_FTR_ARCH_300))
>  		pnv_power9_idle_init();
>  
>  	for (i = 0; i < nr_pnv_idle_states; i++)


This revert solves the stated problem and makes kernel v5.9 work
reasonable well on P10 with stop states which are required for SMT
mode changes.

Complete P10 driver has been in the works and will build on this fix
and complete the required platform support and optimizations.

--Vaidy