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From: Dave Martin <Dave.Martin@arm.com>
To: Suzuki K Poulose <suzuki.poulose@arm.com>
Cc: mark.rutland@arm.com, catalin.marinas@arm.com, will@kernel.org,
	linux-kernel@vger.kernel.org,
	linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH 1/3] arm64: cpufeature: Fix the type of no FP/SIMD capability
Date: Fri, 11 Oct 2019 12:36:20 +0100	[thread overview]
Message-ID: <20191011113620.GG27757@arm.com> (raw)
In-Reply-To: <20191010171517.28782-2-suzuki.poulose@arm.com>

On Thu, Oct 10, 2019 at 06:15:15PM +0100, Suzuki K Poulose wrote:
> The NO_FPSIMD capability is defined with scope SYSTEM, which implies
> that the "absence" of FP/SIMD on at least one CPU is detected only
> after all the SMP CPUs are brought up. However, we use the status
> of this capability for every context switch. So, let us change
> the scop to LOCAL_CPU to allow the detection of this capability
> as and when the first CPU without FP is brought up.
> 
> Also, the current type allows hotplugged CPU to be brought up without
> FP/SIMD when all the current CPUs have FP/SIMD and we have the userspace
> up. Fix both of these issues by changing the capability to
> BOOT_RESTRICTED_LOCAL_CPU_FEATURE.
> 
> Fixes: 82e0191a1aa11abf ("arm64: Support systems without FP/ASIMD")
> Cc: Will Deacon <will@kernel.org>
> Cc: Mark Rutland <mark.rutland@arm.com>
> Cc: Catalin Marinas <catalin.marinas@arm.com>
> Signed-off-by: Suzuki K Poulose <suzuki.poulose@arm.com>
> ---
>  arch/arm64/kernel/cpufeature.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
> index 9323bcc40a58..0f9eace6c64b 100644
> --- a/arch/arm64/kernel/cpufeature.c
> +++ b/arch/arm64/kernel/cpufeature.c
> @@ -1361,7 +1361,7 @@ static const struct arm64_cpu_capabilities arm64_features[] = {
>  	{
>  		/* FP/SIMD is not implemented */
>  		.capability = ARM64_HAS_NO_FPSIMD,
> -		.type = ARM64_CPUCAP_SYSTEM_FEATURE,
> +		.type = ARM64_CPUCAP_BOOT_RESTRICTED_CPU_LOCAL_FEATURE,

ARM64_HAS_NO_FPSIMD is really a disability, not a capability.

Although we have other things that smell like this (CPU errata for
example), I wonder whether inverting the meaning in the case would
make the situation easier to understand.

So, we'd have ARM64_HAS_FPSIMD, with a minimum (signed) feature field
value of 0.  Then this just looks like an ARM64_CPUCAP_SYSTEM_FEATURE
IIUC.  We'd just need to invert the sense of the check in
system_supports_fpsimd().

>  		.min_field_value = 0,

(Does .min_field_value == 0 make sense, or is it even used?  I thought
only the default has_cpuid_feature() match logic uses that.)

>  		.matches = has_no_fpsimd,
>  	},

Cheers
---Dave

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WARNING: multiple messages have this Message-ID (diff)
From: Dave Martin <Dave.Martin@arm.com>
To: Suzuki K Poulose <suzuki.poulose@arm.com>
Cc: linux-arm-kernel@lists.infradead.org, mark.rutland@arm.com,
	catalin.marinas@arm.com, linux-kernel@vger.kernel.org,
	will@kernel.org
Subject: Re: [PATCH 1/3] arm64: cpufeature: Fix the type of no FP/SIMD capability
Date: Fri, 11 Oct 2019 12:36:20 +0100	[thread overview]
Message-ID: <20191011113620.GG27757@arm.com> (raw)
In-Reply-To: <20191010171517.28782-2-suzuki.poulose@arm.com>

On Thu, Oct 10, 2019 at 06:15:15PM +0100, Suzuki K Poulose wrote:
> The NO_FPSIMD capability is defined with scope SYSTEM, which implies
> that the "absence" of FP/SIMD on at least one CPU is detected only
> after all the SMP CPUs are brought up. However, we use the status
> of this capability for every context switch. So, let us change
> the scop to LOCAL_CPU to allow the detection of this capability
> as and when the first CPU without FP is brought up.
> 
> Also, the current type allows hotplugged CPU to be brought up without
> FP/SIMD when all the current CPUs have FP/SIMD and we have the userspace
> up. Fix both of these issues by changing the capability to
> BOOT_RESTRICTED_LOCAL_CPU_FEATURE.
> 
> Fixes: 82e0191a1aa11abf ("arm64: Support systems without FP/ASIMD")
> Cc: Will Deacon <will@kernel.org>
> Cc: Mark Rutland <mark.rutland@arm.com>
> Cc: Catalin Marinas <catalin.marinas@arm.com>
> Signed-off-by: Suzuki K Poulose <suzuki.poulose@arm.com>
> ---
>  arch/arm64/kernel/cpufeature.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
> index 9323bcc40a58..0f9eace6c64b 100644
> --- a/arch/arm64/kernel/cpufeature.c
> +++ b/arch/arm64/kernel/cpufeature.c
> @@ -1361,7 +1361,7 @@ static const struct arm64_cpu_capabilities arm64_features[] = {
>  	{
>  		/* FP/SIMD is not implemented */
>  		.capability = ARM64_HAS_NO_FPSIMD,
> -		.type = ARM64_CPUCAP_SYSTEM_FEATURE,
> +		.type = ARM64_CPUCAP_BOOT_RESTRICTED_CPU_LOCAL_FEATURE,

ARM64_HAS_NO_FPSIMD is really a disability, not a capability.

Although we have other things that smell like this (CPU errata for
example), I wonder whether inverting the meaning in the case would
make the situation easier to understand.

So, we'd have ARM64_HAS_FPSIMD, with a minimum (signed) feature field
value of 0.  Then this just looks like an ARM64_CPUCAP_SYSTEM_FEATURE
IIUC.  We'd just need to invert the sense of the check in
system_supports_fpsimd().

>  		.min_field_value = 0,

(Does .min_field_value == 0 make sense, or is it even used?  I thought
only the default has_cpuid_feature() match logic uses that.)

>  		.matches = has_no_fpsimd,
>  	},

Cheers
---Dave

  reply	other threads:[~2019-10-11 11:36 UTC|newest]

Thread overview: 46+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-10-10 17:15 [PATCH 0/3] arm64: Fix support for systems without FP/SIMD Suzuki K Poulose
2019-10-10 17:15 ` Suzuki K Poulose
2019-10-10 17:15 ` [PATCH 1/3] arm64: cpufeature: Fix the type of no FP/SIMD capability Suzuki K Poulose
2019-10-10 17:15   ` Suzuki K Poulose
2019-10-11 11:36   ` Dave Martin [this message]
2019-10-11 11:36     ` Dave Martin
2019-10-11 12:13     ` Suzuki K Poulose
2019-10-11 12:13       ` Suzuki K Poulose
2019-10-11 14:21       ` Dave Martin
2019-10-11 14:21         ` Dave Martin
2019-10-11 17:28         ` Suzuki K Poulose
2019-10-11 17:28           ` Suzuki K Poulose
2019-10-14 14:52           ` Dave Martin
2019-10-14 14:52             ` Dave Martin
2019-10-14 15:45             ` Suzuki K Poulose
2019-10-14 15:45               ` Suzuki K Poulose
2019-10-14 15:50               ` Dave P Martin
2019-10-14 15:50                 ` Dave P Martin
2019-10-14 16:57                 ` Ard Biesheuvel
2019-10-14 16:57                   ` Ard Biesheuvel
2019-10-15  9:44                   ` Suzuki K Poulose
2019-10-15  9:44                     ` Suzuki K Poulose
2019-10-15  9:52                     ` Ard Biesheuvel
2019-10-15  9:52                       ` Ard Biesheuvel
2019-10-15 10:24                   ` Dave Martin
2019-10-15 10:24                     ` Dave Martin
2019-10-15 10:30                     ` Ard Biesheuvel
2019-10-15 10:30                       ` Ard Biesheuvel
2019-10-15 13:03                       ` Suzuki K Poulose
2019-10-15 13:03                         ` Suzuki K Poulose
2019-10-15 13:11                         ` Ard Biesheuvel
2019-10-15 13:11                           ` Ard Biesheuvel
2019-10-15 14:05                       ` Dave Martin
2019-10-15 14:05                         ` Dave Martin
2019-10-10 17:15 ` [PATCH 2/3] arm64: nofpsmid: Clear TIF_FOREIGN_FPSTATE flag for early tasks Suzuki K Poulose
2019-10-10 17:15   ` Suzuki K Poulose
2019-10-11 11:26   ` Dave Martin
2019-10-11 11:26     ` Dave Martin
2019-10-17 12:42     ` Suzuki K Poulose
2019-10-17 12:42       ` Suzuki K Poulose
2019-10-17 16:09       ` Dave Martin
2019-10-17 16:09         ` Dave Martin
2019-10-10 17:15 ` [PATCH 3/3] arm64: cpufeature: Set the FP/SIMD compat HWCAP bits properly Suzuki K Poulose
2019-10-10 17:15   ` Suzuki K Poulose
2019-10-17  0:06 ` [PATCH 0/3] arm64: Fix support for systems without FP/SIMD Will Deacon
2019-10-17  0:06   ` Will Deacon

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