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

[Suzuki K Poulose]

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.

> 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