linux-kernel - Re: [PATCH 5/5] usb: dwc3: qcom: Allow runtime PM

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Message-ID: <20230328125705.GG5695@thinkpad>
Date:   Tue, 28 Mar 2023 18:27:05 +0530
From:   Manivannan Sadhasivam <manivannan.sadhasivam@...aro.org>
To:     Johan Hovold <johan@...nel.org>
Cc:     andersson@...nel.org, Thinh.Nguyen@...opsys.com,
        gregkh@...uxfoundation.org, mathias.nyman@...el.com,
        konrad.dybcio@...aro.org, robh+dt@...nel.org,
        krzysztof.kozlowski+dt@...aro.org, linux-arm-msm@...r.kernel.org,
        linux-usb@...r.kernel.org, linux-kernel@...r.kernel.org,
        devicetree@...r.kernel.org
Subject: Re: [PATCH 5/5] usb: dwc3: qcom: Allow runtime PM

On Tue, Mar 28, 2023 at 02:18:16PM +0200, Johan Hovold wrote:
> On Tue, Mar 28, 2023 at 03:35:01PM +0530, Manivannan Sadhasivam wrote:
> > On Tue, Mar 28, 2023 at 11:46:36AM +0200, Johan Hovold wrote:
> > > On Sat, Mar 25, 2023 at 10:22:17PM +0530, Manivannan Sadhasivam wrote:
> > > > dwc3-qcom driver is capable of doing runtime PM on its own, but currently
> > > > it requires userspace intervention to enable it. But there is no harm in
> > > > letting the driver to enable runtime PM on its own. So let's get rid of the
> > > > "pm_runtime_forbid()" and make sure that the dependency is maintained with
> > > > child devices using "pm_suspend_ignore_children(dev, false)".
> > > 
> > > Well, the potential harm is that these paths have hardly been tested so
> > > enabling it by default is a risk (e.g. as you noticed when trying to
> > > enable this by default). And especially if we don't address the layering
> > > violations first.
> > > 
> > 
> > I certainly tested this on a couple of boards with host and gadget mode and
> > noticed no issue (except one issue noticed by Steev on a docking station with
> > display but that should be related to orientation switch).
> > 
> > Even if we allow runtime PM on this driver, still userspace needs to enable it
> > for dwc3 and xhci drivers. So this essentially reduces one step in that process
> > if someone tries to enable runtime PM for usb intentionally. So I don't forsee a
> > potential harm here.
> 
> Well this whole driver is a mess so I don't have any problem imagining
> ways in which things can break. ;)
> 
> > > > Also during remove(), the device needs to be waken up first if it was
> > > > runtime suspended. Finally, pm_runtime_allow() can be removed.
> > > > 
> > > > Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@...aro.org>
> > > > ---
> > > >  drivers/usb/dwc3/dwc3-qcom.c | 5 +++--
> > > >  1 file changed, 3 insertions(+), 2 deletions(-)
> > > > 
> > > > diff --git a/drivers/usb/dwc3/dwc3-qcom.c b/drivers/usb/dwc3/dwc3-qcom.c
> > > > index f1059dfcb0e8..5f26bb66274f 100644
> > > > --- a/drivers/usb/dwc3/dwc3-qcom.c
> > > > +++ b/drivers/usb/dwc3/dwc3-qcom.c
> > > > @@ -920,7 +920,7 @@ static int dwc3_qcom_probe(struct platform_device *pdev)
> > > >  	qcom->is_suspended = false;
> > > >  	pm_runtime_set_active(dev);
> > > >  	pm_runtime_enable(dev);
> > > > -	pm_runtime_forbid(dev);
> > > > +	pm_suspend_ignore_children(dev, false);
> > > 
> > > There's no need to explicitly disable ignore-children as that is the
> > > default.
> > > 
> > 
> > Other dwc drivers were doing it, so I thought someone (maintainer) wanted to
> > explicitly disable ignore_children. But if that's not the case, I can remove it.
> 
> Yeah, please remove it. I doubt these runtime pm implementations have
> gotten much review.
> 
> Note how several dwc3 glue drivers just do an unconditional get in
> probe(), which means that these paths are probably never exercised at
> all and effectively amounts to that pm_runtime_forbid() you are removing
> here.
> 
> Probably there to tick off "runtime pm" on some internal project
> manager's list of "features".
> 

Agree.

> > > >  	return 0;
> > > >  
> > > > @@ -948,6 +948,8 @@ static int dwc3_qcom_remove(struct platform_device *pdev)
> > > >  	struct device *dev = &pdev->dev;
> > > >  	int i;
> > > >  
> > > > +	pm_runtime_get_sync(dev);
> > > 
> > > This call needs to be balanced. But this is a fix for a bug in the
> > > current implementation that should go in a separate patch.
> > > 
> > 
> > Ok. For balancing I could add pm_runtime_put_noidle() before pm_runtime_disable.
> 
> You should do it after disabling runtime pm.
> 

May I know why?

Thanks,
Mani

> > > > +
> > > >  	device_remove_software_node(&qcom->dwc3->dev);
> > > >  	of_platform_depopulate(dev);
> > > >  
> > > > @@ -960,7 +962,6 @@ static int dwc3_qcom_remove(struct platform_device *pdev)
> > > >  	dwc3_qcom_interconnect_exit(qcom);
> > > >  	reset_control_assert(qcom->resets);
> > > >  
> > > > -	pm_runtime_allow(dev);
> > > >  	pm_runtime_disable(dev);
> > > >  
> > > >  	return 0;
> 
> Johan

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