RE: [PATCH v3 22/26] coresight: etm4x: Add necessary synchronization for sysreg access

[John Horley]

On 11/10/20 10:11 AM, Suzuki K Poulose wrote:
> 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@xxxxxxxxxx>
>>> Cc: Mathieu Poirier <mathieu.poirier@xxxxxxxxxx>
>>> Signed-off-by: Suzuki K Poulose <suzuki.poulose@xxxxxxx>
>>> ---
>>>   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.
>

The ETM architecture uses "trace analyzer" to mean self-hosted software and an external debugger. It's a useful term that generically covers "the thing that's in charge of tracing" and "the thing that's capturing and/or decoding the trace", regardless of whether either of these are external or self-hosted (or even a mixture!).

So in 4.3.7, yes this does mean that context synchronization events are needed to synchronize register updates when using system instructions to program the trace unit.

I'll take a look at what we can improve here :-)

Cheers, John.

>> 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@xxxxxxxxxx>
>>
>
>Thanks
>Suzuki
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