Re: [PATCH v4 5/6] coresight: etm4x: save/restore state across CPU low power states

[Andrew Murray <andrew.murray@arm.com>]

On Fri, Aug 02, 2019 at 11:54:12AM +0100, Suzuki K Poulose wrote:
> 
> 
> On 30/07/2019 13:51, Andrew Murray wrote:
> > Some hardware will ignore bit TRCPDCR.PU which is used to signal
> > to hardware that power should not be removed from the trace unit.
> > Let's mitigate against this by conditionally saving and restoring
> > the trace unit state when the CPU enters low power states.
> > 
> > This patchset introduces a firmware property named
> > 'arm,coresight-needs-save-restore' - when this is present the
> > hardware state will be conditionally saved and restored.
> > 
> > A module parameter 'pm_save_enable' is also introduced which can
> > be configured to override the firmware property. This can be set
> > to never allow save/restore, to conditionally allow it, or to
> > do as the firmware indicates (default).
> > 
> > We avoid saving the hardware state when coresight isn't in use
> > to reduce PM latency - we can't determine this by reading the
> > claim tags (TRCCLAIMCLR) as these are 'trace' registers which need
> > power and clocking, something we can't easily provide in the PM
> > context. Therefore we rely on the existing drvdata->mode internal
> > state that is set when self-hosted coresight is used (and powered).
> > 
> > As we do not have a simple way of determining if an external agent
> > is using coresight, we don't save/restore for this use case.
> > 
> > Signed-off-by: Andrew Murray <andrew.murray@arm.com>
> > ---
> >   drivers/hwtracing/coresight/coresight-etm4x.c | 322 ++++++++++++++++++
> >   drivers/hwtracing/coresight/coresight-etm4x.h |  64 ++++
> >   2 files changed, 386 insertions(+)
> > 
> > diff --git a/drivers/hwtracing/coresight/coresight-etm4x.c b/drivers/hwtracing/coresight/coresight-etm4x.c
> > index a128b5063f46..30f118792289 100644
> > --- a/drivers/hwtracing/coresight/coresight-etm4x.c
> > +++ b/drivers/hwtracing/coresight/coresight-etm4x.c
> 
> 
> > +static int etm4_cpu_pm_notify(struct notifier_block *nb, unsigned long cmd,
> > +			      void *v)
> > +{
> > +	struct etmv4_drvdata *drvdata;
> > +	unsigned int cpu = smp_processor_id();
> > +
> > +	if (!etmdrvdata[cpu])
> > +		return 0;
> > +
> > +	drvdata = etmdrvdata[cpu];
> > +
> > +	if (!drvdata->save_state)
> > +		return NOTIFY_OK;
> > +
> > +	if (WARN_ON_ONCE(drvdata->cpu != smp_processor_id()))
> > +		return NOTIFY_BAD;
> 
> minor nit: you may skip the second call to smp_processor_id() as this is called
> on from non-preemptible context.
> 
> > +
> > +	switch (cmd) {
> > +	case CPU_PM_ENTER:
> > +		/* save the state if self-hosted coresight is in use */
> > +		if (local_read(&drvdata->mode))
> > +			if (etm4_cpu_save(drvdata))
> > +				return NOTIFY_BAD;
> > +		break;
> > +	case CPU_PM_EXIT:
> > +	case CPU_PM_ENTER_FAILED:
> > +		/* trcclaimset is set when there is state to restore */
> > +		if (drvdata->state_needs_restore)
> > +			etm4_cpu_restore(drvdata);
> > +		break;
> > +	default:
> > +		return NOTIFY_DONE;
> > +	}
> > +
> > +	return NOTIFY_OK;
> > +}
> > +
> 
> 
> > +static void etm4_cpu_pm_unregister(void)
> > +{
> > +	cpu_pm_unregister_notifier(&etm4_cpu_pm_nb);
> > +}
> > +#else
> > +static int etm4_cpu_pm_register(void) { return 0; }
> > +static void etm4_cpu_pm_unregister(void) { }
> > +#endif
> > +
> > +static inline bool etm4_needs_save_restore(struct device *dev)
> > +{
> > +	return fwnode_property_present(dev->fwnode,
> 
> nit: It may be safe to use dev_fwnode(dev), instead of dev->fwnode. But I
> see a lot of existing users of dev->fwnode. Not sure it does have an impact.
> 
> Looks fine to me ,
> 
> Reviewed-by: Suzuki K Poulose <suzuki.poulose@arm.com>

Thanks - I'll make these changes.

Andrew Murray
> 
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