linux-kernel - Re: [PATCH] coresight: trbe: Directly use ID_AA64DFR0_EL1_TraceBuffer

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Message-ID: <558cfef3-8ebf-e59f-af20-7a262b2d2de7@arm.com>
Date:   Wed, 2 Aug 2023 09:55:10 +0100
From:   James Clark <james.clark@....com>
To:     Anshuman Khandual <anshuman.khandual@....com>,
        linux-arm-kernel@...ts.infradead.org, suzuki.poulose@....com
Cc:     Mike Leach <mike.leach@...aro.org>, Leo Yan <leo.yan@...aro.org>,
        coresight@...ts.linaro.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] coresight: trbe: Directly use
 ID_AA64DFR0_EL1_TraceBuffer_IMP



On 02/08/2023 07:36, Anshuman Khandual wrote:
> is_trbe_available() checks for the TRBE support via extracting TraceBuffer
> field value from ID_AA64DFR0_EL1, and ensures that it is implemented. This
> replaces the open encoding '0b0001' with 'ID_AA64DFR0_EL1_TraceBuffer_IMP'
> which is now available via sysreg tools. Functional change is not intended.
> 
> Cc: Suzuki K Poulose <suzuki.poulose@....com>
> Cc: Mike Leach <mike.leach@...aro.org>
> Cc: James Clark <james.clark@....com>
> Cc: Leo Yan <leo.yan@...aro.org>
> Cc: coresight@...ts.linaro.org
> Cc: linux-arm-kernel@...ts.infradead.org
> Cc: linux-kernel@...r.kernel.org
> Signed-off-by: Anshuman Khandual <anshuman.khandual@....com>
> ---
>  drivers/hwtracing/coresight/coresight-trbe.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/hwtracing/coresight/coresight-trbe.h b/drivers/hwtracing/coresight/coresight-trbe.h
> index 94e67009848a..ebb9108d8e24 100644
> --- a/drivers/hwtracing/coresight/coresight-trbe.h
> +++ b/drivers/hwtracing/coresight/coresight-trbe.h
> @@ -24,7 +24,7 @@ static inline bool is_trbe_available(void)
>  	unsigned int trbe = cpuid_feature_extract_unsigned_field(aa64dfr0,
>  								 ID_AA64DFR0_EL1_TraceBuffer_SHIFT);
>  
> -	return trbe >= 0b0001;
> +	return trbe >= ID_AA64DFR0_EL1_TraceBuffer_IMP;
>  }
>  
>  static inline bool is_trbe_enabled(void)

Reviewed-by: James Clark <james.clark@....com>

Kind of unrelated to this change, but it seems like a lot of feature
checks like this avoid using any value altogether when it's just 0 or 1.
For example the same check for TRBE in kvm:

  /* Check if we have TRBE implemented and available at the host */
  if (cpuid_feature_extract_unsigned_field(dfr0,
ID_AA64DFR0_EL1_TraceBuffer_SHIFT) &&
  ...