linux-kernel - Re: [PATCH v3 22/26] coresight: etm4x: Add necessary synchronization for sysreg access

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Message-ID: <7370dc60-ca9c-7b58-0b56-6fcef628304f@arm.com>
Date:   Tue, 10 Nov 2020 10:11:23 +0000
From:   Suzuki K Poulose <suzuki.poulose@....com>
To:     Mathieu Poirier <mathieu.poirier@...aro.org>
Cc:     linux-arm-kernel@...ts.infradead.org, mike.leach@...aro.org,
        coresight@...ts.linaro.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 22/26] coresight: etm4x: Add necessary synchronization
 for sysreg access

On 11/9/20 6:32 PM, Mathieu Poirier wrote:
> On Wed, Oct 28, 2020 at 10:09:41PM +0000, Suzuki K Poulose wrote:
>> As per the specification any update to the TRCPRGCTLR must be synchronized
>> by a context synchronization event (in our case an explicist ISB) before
>> the TRCSTATR is checked.
>>
>> Cc: Mike Leach <mike.leach@...aro.org>
>> Cc: Mathieu Poirier <mathieu.poirier@...aro.org>
>> Signed-off-by: Suzuki K Poulose <suzuki.poulose@....com>
>> ---
>>   drivers/hwtracing/coresight/coresight-etm4x-core.c | 13 +++++++++++++
>>   1 file changed, 13 insertions(+)
>>
>> diff --git a/drivers/hwtracing/coresight/coresight-etm4x-core.c b/drivers/hwtracing/coresight/coresight-etm4x-core.c
>> index e36bc1c722c7..4bc2f15b6332 100644
>> --- a/drivers/hwtracing/coresight/coresight-etm4x-core.c
>> +++ b/drivers/hwtracing/coresight/coresight-etm4x-core.c
>> @@ -178,6 +178,15 @@ static int etm4_enable_hw(struct etmv4_drvdata *drvdata)
>>   	/* Disable the trace unit before programming trace registers */
>>   	etm4x_relaxed_write32(csa, 0, TRCPRGCTLR);
>>   
>> +	/*
>> +	 * If we use system instructions, we need to synchronize the
>> +	 * write to the TRCPRGCTLR, before accessing the TRCSTATR.
>> +	 * See ARM IHI0064F, section
>> +	 * "4.3.7 Synchronization of register updates"
>> +	 */
>> +	if (!csa->io_mem)
>> +		isb();
>> +
> 
> When I first read the documentation on system instruction section 4.3.7 really
> got me thinking...
> 
> At the very top, right after the title "Synchronization of register updates" one
> can read "Software running on the PE...".  Later in the text, when specifying
> the synchronisation rules, the term "trace analyzer" is used.  _Typically_ a trace
> analyzer is an external box.
> 

Very good point. The trace analyzer could still use the system register
to program the ETM and causing a context synchronization event is tricky
from within the trace analyzer. And I agree that there is a bit of
confusion around the synchronization from a self-hosted point of view.
I believe this is true for the self-hosted case too and should be
clarified in the TRM.

> Arm documentation is precise and usually doesn't overlook that kind of detail.
> The question is to understand if a context synchronisation event is also needed
> when programming is done on the PE.  If so I think the documentation should be
> amended.
> 
> In that case:
> 
> Reviewed-by: Mathieu Poirier <mathieu.poirier@...aro.org>
> 

Thanks
Suzuki