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: Mon, 1 Jul 2019 10:07:10 +0800	[thread overview]
Message-ID: <20190701020710.GC28475@leoy-ThinkPad-X240s> (raw)
In-Reply-To: <20190628092227.GJ34530@e119886-lin.cambridge.arm.com>

Hi Andrew,

On Fri, Jun 28, 2019 at 10:22:28AM +0100, Andrew Murray wrote:
> On Fri, Jun 28, 2019 at 05:12:46PM +0800, Leo Yan wrote:
> > 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.
> 
> Sorry, I seemingly can't read code today.
> 
> > 
> > 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.
> 
> In any case, not only do we want to override the firmware to always
> save/restore. Sometimes we may also want to override the firmware to never
> save/restore (despite the firmware having the
> 'arm,coresight-needs-save-restore' flag present). For example to debug power
> management.
> 
> Thus with this current approach you can override the firmware to either enable
> or disable save/restore.

Thanks for explanation and agree with this.  Just a suggestion, maybe we
can initialize 'drvdata->pm_save_enable' in probe as below:

        if (pm_save_enable == PARAM_PM_SAVE_FIRMWARE)
                drvdata->pm_save_enable = etm4_needs_save_restore(dev);
        else
                drvdata->pm_save_enable = pm_save_enable;

From then on, we can only use 'drvdata->pm_save_enable' and don't need
to check the module parameter anymore.

Thanks,
Leo Yan

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