linux-kernel - Re: [PATCH v3] PCI: Xilinx NWL: Modifying irq chip for legacy interrupts

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Message-ID: <86r33j7cjy.fsf@arm.com>
Date:   Tue, 31 Jan 2017 10:23:45 +0000
From:   Marc Zyngier <marc.zyngier@....com>
To:     Bharat Kumar Gogada <bharat.kumar.gogada@...inx.com>
Cc:     "bhelgaas\@google.com" <bhelgaas@...gle.com>,
        "paul.gortmaker\@windriver.com" <paul.gortmaker@...driver.com>,
        "robh\@kernel.org" <robh@...nel.org>,
        "colin.king\@canonical.com" <colin.king@...onical.com>,
        "linux-pci\@vger.kernel.org" <linux-pci@...r.kernel.org>,
        "michal.simek\@xilinx.com" <michal.simek@...inx.com>,
        "linux-arm-kernel\@lists.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "linux-kernel\@vger.kernel.org" <linux-kernel@...r.kernel.org>,
        Ravikiran Gummaluri <rgummal@...inx.com>,
        "arnd\@arndb.de" <arnd@...db.de>
Subject: Re: [PATCH v3] PCI: Xilinx NWL: Modifying irq chip for legacy interrupts

On Tue, Jan 31 2017 at 09:34:43 AM, Bharat Kumar Gogada <bharat.kumar.gogada@...inx.com> wrote:
>  > On Tue, Jan 31 2017 at 08:59:12 AM, Bharat Kumar Gogada
>> <bharat.kumar.gogada@...inx.com> wrote:
>> > - Adding mutex lock for protecting legacy mask register
>> > - Few wifi end points which only support legacy interrupts, performs
>> > hardware reset functionalities after disabling interrupts by invoking
>> > disable_irq and then re-enable using enable_irq, they enable hardware
>> > interrupts first and then virtual irq line later.
>> > - The legacy irq line goes low only after DEASSERT_INTx is received.As
>> > the legacy irq line is high immediately after hardware interrupts are
>> > enabled but virq of EP is still in disabled state and EP handler is
>> > never executed resulting no DEASSERT_INTx.If dummy irq chip is used,
>> > interrutps are not masked and system is hanging with CPU stall.
>> > - Adding irq chip functions instead of dummy irq chip for legacy
>> > interrupts.
>> > - Legacy interrupts are level sensitive, so using handle_level_irq is
>> > more appropriate as it is masks interrupts until End point handles
>> > interrupts and unmasks interrutps after End point handler is executed.
>> > - Legacy interrupts are level triggered, virtual irq line of End Point
>> > shows as edge in /proc/interrupts.
>> > - Setting irq flags of virtual irq line of EP to level triggered at
>> > the time of mapping.
>> >
>> > Signed-off-by: Bharat Kumar Gogada <bharatku@...inx.com>
>> > ---
>> >  drivers/pci/host/pcie-xilinx-nwl.c |   43
>> +++++++++++++++++++++++++++++++++++-
>> >  1 files changed, 42 insertions(+), 1 deletions(-)
>> >
>> > diff --git a/drivers/pci/host/pcie-xilinx-nwl.c
>> > b/drivers/pci/host/pcie-xilinx-nwl.c
>> > index 43eaa4a..76dd094 100644
>> > --- a/drivers/pci/host/pcie-xilinx-nwl.c
>> > +++ b/drivers/pci/host/pcie-xilinx-nwl.c
>> > @@ -184,6 +184,7 @@ struct nwl_pcie {
>> >  	u8 root_busno;
>> >  	struct nwl_msi msi;
>> >  	struct irq_domain *legacy_irq_domain;
>> > +	struct mutex leg_mask_lock;
>> >  };
>> >
>> >  static inline u32 nwl_bridge_readl(struct nwl_pcie *pcie, u32 off) @@
>> > -395,11 +396,50 @@ static void nwl_pcie_msi_handler_low(struct irq_desc
>> *desc)
>> >  	chained_irq_exit(chip, desc);
>> >  }
>> >
>> > +static void nwl_mask_leg_irq(struct irq_data *data) {
>> > +	struct irq_desc *desc = irq_to_desc(data->irq);
>> > +	struct nwl_pcie *pcie;
>> > +	u32 mask;
>> > +	u32 val;
>> > +
>> > +	pcie = irq_desc_get_chip_data(desc);
>> > +	mask = 1 << (data->hwirq - 1);
>> > +	mutex_lock(&pcie->leg_mask_lock);
>> > +	val = nwl_bridge_readl(pcie, MSGF_LEG_MASK);
>> > +	nwl_bridge_writel(pcie, (val & (~mask)), MSGF_LEG_MASK);
>> > +	mutex_unlock(&pcie->leg_mask_lock);
>> 
>> Have you looked at which context this is called in? In a number of cases, the
>> mask/unmask methods are called whilst you're in an interrupt context. If you
>> sleep there (which is what happens with a contended mutex), you die horribly.
>> 
>> Given these constraints, you should use raw_spin_lock_irqsave and
>> co, since this
>> can be called from both interrupt and non-interrupt contexts.
>> 
> I have seen very few wifi drivers calling these in MAC flow,

Very few is already too many. But I'm afraid you're missing the point
entirely: This patch is about using handle_level_irq as the flow
handler. The first thing handle_level_irq does is to mask the
interrupt. If you have a competing mask or unmask operation in progress
on another CPU (or in the middle of one on the same CPU when the
interrupt fired), your system explodes.

Please have a look at Documentation/DocBook/kernel-locking.tmpl.

> raw_spin_lock_irqsave
> looks more safe, will do it.

Thanks,

	M.
-- 
Jazz is not dead. It just smells funny.