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

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Date:   Tue, 10 Nov 2020 10:35:42 -0700
From:   Mathieu Poirier <mathieu.poirier@...aro.org>
To:     Suzuki K Poulose <suzuki.poulose@....com>
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 Tue, Nov 10, 2020 at 10:11:23AM +0000, 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@...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.

I think John has given us the final answer here - the current implementation
stands.

> 
> > 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