linux-kernel - Re: [PATCH 05/12] PCI: aardvark: add suspend to RAM support

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Date:   Tue, 11 Dec 2018 14:16:27 +0000
From:   Lorenzo Pieralisi <lorenzo.pieralisi@....com>
To:     "Rafael J. Wysocki" <rjw@...ysocki.net>, sboyd@...nel.org
Cc:     Miquel Raynal <miquel.raynal@...tlin.com>, sudeep.holla@....com,
        Gregory Clement <gregory.clement@...tlin.com>,
        Jason Cooper <jason@...edaemon.net>,
        Andrew Lunn <andrew@...n.ch>,
        Sebastian Hesselbarth <sebastian.hesselbarth@...il.com>,
        Thomas Petazzoni <thomas.petazzoni@...tlin.com>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        devicetree@...r.kernel.org, Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>, linux-pci@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        Antoine Tenart <antoine.tenart@...tlin.com>,
        Maxime Chevallier <maxime.chevallier@...tlin.com>,
        Nadav Haklai <nadavh@...vell.com>
Subject: Re: [PATCH 05/12] PCI: aardvark: add suspend to RAM support

On Tue, Dec 04, 2018 at 10:42:19PM +0100, Rafael J. Wysocki wrote:
> On Tuesday, December 4, 2018 10:45:58 AM CET Lorenzo Pieralisi wrote:
> > On Mon, Dec 03, 2018 at 11:00:20PM +0100, Rafael J. Wysocki wrote:
> > > On Monday, December 3, 2018 4:38:46 PM CET Miquel Raynal wrote:
> > > > Hi Lorenzo,
> > > > 
> > > > Lorenzo Pieralisi <lorenzo.pieralisi@....com> wrote on Mon, 3 Dec 2018
> > > > 10:27:08 +0000:
> > > > 
> > > > > [+Rafael, Sudeep]
> > > > > 
> > > > > On Fri, Nov 23, 2018 at 03:18:24PM +0100, Miquel Raynal wrote:
> > > > > > Add suspend and resume callbacks. The priority of these are
> > > > > > "_noirq()", to workaround early access to the registers done by the
> > > > > > PCI core through the ->read()/->write() callbacks at resume time.
> > > > > > 
> > > > > > Signed-off-by: Miquel Raynal <miquel.raynal@...tlin.com>
> > > > > > ---
> > > > > >  drivers/pci/controller/pci-aardvark.c | 52 +++++++++++++++++++++++++++
> > > > > >  1 file changed, 52 insertions(+)
> > > > > > 
> > > > > > diff --git a/drivers/pci/controller/pci-aardvark.c b/drivers/pci/controller/pci-aardvark.c
> > > > > > index 108b3f15c410..7ecf1ac4036b 100644
> > > > > > --- a/drivers/pci/controller/pci-aardvark.c
> > > > > > +++ b/drivers/pci/controller/pci-aardvark.c
> > > > > > @@ -1108,6 +1108,55 @@ static int advk_pcie_setup_clk(struct advk_pcie *pci
> > > > > >  	return ret;
> > > > > >  }
> > > > > >  
> > > > > > +static int __maybe_unused advk_pcie_suspend(struct device *dev)
> > > > > > +{
> > > > > > +	struct advk_pcie *pcie = dev_get_drvdata(dev);
> > > > > > +
> > > > > > +	advk_pcie_disable_phy(pcie);
> > > > > > +
> > > > > > +	clk_disable_unprepare(pcie->clk);  
> > > > > 
> > > > > I have noticed it is common practice, still, I would like to check whether
> > > > > it is allowed to call functions that may sleep in a NOIRQ suspend/resume
> > > > > callback ?
> > > > 
> > > > You are right this is weird. I double checked and for instance,
> > > > pcie-mediatek.c, pci-tegra.c and pci-imx6.c do the exact same thing. There are
> > > > probably other cases where drivers call functions that may sleep from a NOIRQ
> > > > context. I am interested to know if this is valid and if not, what is the
> > > > alternative?
> > > > 
> > > 
> > > Yes, it is valid.  _noirq means that the high-level action handlers
> > > will not be invoked for interrupts occurring during that period, but
> > > that doesn't apply to timer interrupts.
> > > 
> > > IOW, don't expect *your* IRQ handler to be invoked then (if this is
> > > not a timer IRQ), but you can sleep.
> > 
> > Hi Rafael, all,
> > 
> > I did not ask my question (that may be silly) properly apologies. I know
> > that the S2R context allows sleeping the question is, in case
> > clk_disable_unprepare() (and resume counterparts) sleeps,
> 
> If it just sleeps, then this is not a problem, but if it actually *waits*
> for something meaningful to happen (which I guess is what you really mean),
> then things may go awry.
> 
> > what is going to wake it up, given that we are in the S2R NOIRQ phase and as
> > you said the action handlers (that are possibly required to wake up the eg
> > clk_disable_unprepare() caller) are disabled (unless, AFAIK,
> > IRQF_NO_SUSPEND is passed at IRQ request time in the respective driver).
> 
> So if it waits for an action handler to do something and wake it up, it may
> very well deadlock.  I have no idea if that really happens, though.
> 
> > The clk API implementations back-ends are beyond my depth, I just wanted
> > to make sure I understand how the S2R flow is expected to work in this
> > specific case.
> 
> Action handlers won't run unless the IRQs are marked as IRQF_NO_SUSPEND
> (well, there are a few more complications I don't recall exactly, but
> that's the basic rule).  If anything depends on them to run, it will block.

Stephen, any comments on this ? I would like to understand if it is safe
to call a clk_*unprepare/prepare_* function (that may have a blocking
back-end waiting on a wake-up event triggered by an IRQ action) in the
suspend/resume NOIRQ phase.

It is not clear how the unprepare/prepare() callers can possibly know
whether it is safe to block at that stage given that IRQ actions are
suspended and the wake-up may never trigger.

Thanks,
Lorenzo