linux-kernel - Re: [PATCH] coresight: etm4x: Do not access TRCIDR1 for identification

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Message-ID: <74d57e87-a32a-1a55-6ff6-6ad7f5cbec49@arm.com>
Date:   Tue, 21 Mar 2023 10:19:01 +0000
From:   Suzuki K Poulose <suzuki.poulose@....com>
To:     Anshuman Khandual <anshuman.khandual@....com>,
        coresight@...ts.linaro.org
Cc:     mike.leach@...aro.org, james.clark@....com,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        Steve Clevenger <scclevenger@...amperecomputing.com>
Subject: Re: [PATCH] coresight: etm4x: Do not access TRCIDR1 for
 identification

On 21/03/2023 10:17, Anshuman Khandual wrote:
> 
> 
> On 3/17/23 17:27, Suzuki K Poulose wrote:
>> CoreSight ETM4x architecture clearly provides ways to identify a device
>> via registers in the "Management" class, TRCDEVARCH and TRCDEVTYPE. These
>> registers can be accessed without the Trace domain being powered on.
>> We additionally added TRCIDR1 as fallback in order to cover for any
>> ETMs that may not have implemented TRCDEVARCH. So far, nobody has
>> reported hitting a WARNING we placed to catch such systems.
>>
>> Also, more importantly it is problematic to access TRCIDR1, which is a "Trace"
>> register via MMIO access, without clearing the OSLK. But we cannot
>> mess with the OSLK until we know for sure that this is an ETMv4 device.
>> Thus, this kind of creates a chicken and egg problem unnecessarily for systems
>> "which are compliant" to the ETMv4 architecture.
>>
>> Let us remove the TRCIDR1 fall back check and rely only on TRCDEVARCH.
>>
>> Reported-by: Steve Clevenger <scclevenger@...amperecomputing.com>
>> Link: https://lore.kernel.org/all/143540e5623d4c7393d24833f2b80600d8d745d2.1677881753.git.scclevenger@os.amperecomputing.com/
>> Cc: Mike Leach <mike.leach@...aro.org>
>> Cc: James Clark <james.clark@....com>
>> Fixes: 8b94db1edaee ("coresight: etm4x: Use TRCDEVARCH for component discovery")
>> Signed-off-by: Suzuki K Poulose <suzuki.poulose@....com>
>> ---
>>   .../hwtracing/coresight/coresight-etm4x-core.c | 18 +++++-------------
>>   1 file changed, 5 insertions(+), 13 deletions(-)
>>
>> diff --git a/drivers/hwtracing/coresight/coresight-etm4x-core.c b/drivers/hwtracing/coresight/coresight-etm4x-core.c
>> index 104333c2c8a3..c1b72d892d7d 100644
>> --- a/drivers/hwtracing/coresight/coresight-etm4x-core.c
>> +++ b/drivers/hwtracing/coresight/coresight-etm4x-core.c
>> @@ -1070,25 +1070,17 @@ static bool etm4_init_iomem_access(struct etmv4_drvdata *drvdata,
>>   				   struct csdev_access *csa)
>>   {
>>   	u32 devarch = readl_relaxed(drvdata->base + TRCDEVARCH);
>> -	u32 idr1 = readl_relaxed(drvdata->base + TRCIDR1);
>>   
>>   	/*
>>   	 * All ETMs must implement TRCDEVARCH to indicate that
>> -	 * the component is an ETMv4. To support any broken
>> -	 * implementations we fall back to TRCIDR1 check, which
>> -	 * is not really reliable.
>> +	 * the component is an ETMv4
>>   	 */
>> -	if ((devarch & ETM_DEVARCH_ID_MASK) == ETM_DEVARCH_ETMv4x_ARCH) {
>> -		drvdata->arch = etm_devarch_to_arch(devarch);
>> -	} else {
>> -		pr_warn("CPU%d: ETM4x incompatible TRCDEVARCH: %x, falling back to TRCIDR1\n",
>> -			smp_processor_id(), devarch);
>> -
>> -		if (ETM_TRCIDR1_ARCH_MAJOR(idr1) != ETM_TRCIDR1_ARCH_ETMv4)
>> -			return false;
>> -		drvdata->arch = etm_trcidr_to_arch(idr1);
> 
> etm_trcidr_to_arch() does not seem to be used else where, should be dropped ?

Good point, will drop it. Thanks for the review.

Suzuki