Re: [PATCH v2 5/5] coresight: etm4x: save/restore state across CPU low power states

From: Leo Yan <leo.yan@linaro.org>
To: Andrew Murray <andrew.murray@arm.com>
Cc: Mathieu Poirier <mathieu.poirier@linaro.org>,
	Suzuki K Poulose <suzuki.poulose@arm.com>,
	Alexander Shishkin <alexander.shishkin@linux.intel.com>,
	coresight@lists.linaro.org, Sudeep Holla <sudeep.holla@arm.com>,
	linux-arm-kernel@lists.infradead.org,
	Mike Leach <mike.leach@linaro.org>
Subject: Re: [PATCH v2 5/5] coresight: etm4x: save/restore state across CPU low power states
Date: Fri, 28 Jun 2019 17:12:46 +0800	[thread overview]
Message-ID: <20190628091246.GE32370@leoy-ThinkPad-X240s> (raw)
In-Reply-To: <20190628085357.GH34530@e119886-lin.cambridge.arm.com>

On Fri, Jun 28, 2019 at 09:53:58AM +0100, Andrew Murray wrote:

[...]

> > > diff --git a/drivers/hwtracing/coresight/coresight-etm4x.c b/drivers/hwtracing/coresight/coresight-etm4x.c

[...]

> > > +
> > > +#define PARAM_PM_SAVE_DISABLE	0
> > > +#define PARAM_PM_SAVE_ENABLE	1
> > > +#define PARAM_PM_SAVE_FIRMWARE	2
> > > +
> > > +static int pm_save_enable = PARAM_PM_SAVE_FIRMWARE;
> > > +module_param(pm_save_enable, int, 0644);
> > > +MODULE_PARM_DESC(pm_save_enable, "Save/restore state on power down: "
> > > +				  "0 = disabled, 1 = enabled, 2 = firmware");
> > > +
> > 
> > I understand if set pm_save_enable = 2 (firmware), then driver will
> > depend on drvdata->pm_save_enable to make decision for context saving
> > and restoring.
> > 
> > Maybe we can simplize to set pm_save_enable for binary value:
> > 0 (disabled) or 1 (enabled).  The reason is if we set the module
> > parameter 'pm_save_enable = 1', then we can set every ETM device's
> > drvdata->pm_save_enable in initialization phase.  So in the probe
> > function, we can use below code:
> > 
> >   drvdata->pm_save_enable = pm_save_enable ? : etm4_needs_save_restore(dev);
> 
> This means that when the module parameter is set to 1, then we only save/restore
> if the firmware suggests it is needed.

If the module parameter is set to 1, then we will always set every
device 'drvdata->pm_save_enable' to 1.  So in this case, the module
parameter will override the firmware property and always save/restore
contexts for ETM.

> However - what happens on hardware that ignores the PU bit (and thus requires
> save/restore), yet it's firmware doesn't have the
> 'arm,coresight-needs-save-restore' property? There is no way to override the
> firmware and always save/restore.

Actually I suggested to give the module parameter with high priority
and when the module parameter has set to 1, then it can override
firmware 'arm,coresight-needs-save-restore' property.

If we set the module parameter in kernel command line or when load the
module, its value can be used in the function etm4_probe().  So in the
driver probing, it detects the module parameter is 1, then it can
directly set every device 'drvdata->pm_save_enable' to 1.  Thus we can
always save/restore context for ignoring the PU bit case.

> (It's also quite helpful for debugging to be able to change the module parameter
> at run time.)

[...]

> > Do we need to disable trace unit at the end of saving flow?
> 
> At the start of this function we take the OS lock, this has the effect of
> also disabling the trace. Therefore I don't think it's necessary to do more.

Okay, I read the comment and thanks for reminding.

Thanks,
Leo Yan

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