RE: [PATCH v5] PCI: Xilinx NWL: Modifying irq chip for legacy interrupts

[Bharat Kumar Gogada <bharat.kumar.gogada@xilinx.com>]

> -----Original Message-----
> From: Marc Zyngier [mailto:marc.zyngier@arm.com]
> Sent: Thursday, February 09, 2017 9:33 PM
> To: Bharat Kumar Gogada <bharatku@xilinx.com>; bhelgaas@google.com;
> robh@kernel.org; paul.gortmaker@windriver.com; colin.king@canonical.com;
> linux-pci@vger.kernel.org
> Cc: linux-arm-kernel@lists.infradead.org; linux-kernel@vger.kernel.org;
> michal.simek@xilinx.com; arnd@arndb.de; Ravikiran Gummaluri
> <rgummal@xilinx.com>
> Subject: Re: [PATCH v5] PCI: Xilinx NWL: Modifying irq chip for legacy interrupts
> 
> On 09/02/17 15:16, Bharat Kumar Gogada wrote:
> >>
> >> On 09/02/17 12:01, Bharat Kumar Gogada wrote:
> >>>> On 06/02/17 07:03, Bharat Kumar Gogada wrote:
> >>>>> +static struct irq_chip nwl_leg_irq_chip = {
> >>>>> +	.name = "nwl_pcie:legacy",
> >>>>> +	.irq_enable = nwl_unmask_leg_irq,
> >>>>> +	.irq_disable = nwl_mask_leg_irq,
> >>>>
> >>>> You don't need these two if they are implemented in terms of
> mask/unmask.
> >>>
> >>> These are being invoked by some drivers other than interrupt flow.
> >>> Ex: drivers/net/wireless/ath/ath9k/main.c
> >>> static int ath_reset_internal(struct ath_softc *sc, struct
> >>> ath9k_channel *hchan) {
> >>>          ....
> >>>          disable_irq(sc->irq);
> >>>          tasklet_disable(&sc->intr_tq);
> >>>         ...
> >>>         ...
> >>>         enable_irq(sc->irq);
> >>>         spin_unlock_bh(&sc->sc_pcu_lock); } For us masking/unmasking
> >>> is the way to enable/disable interrupts.
> >>
> >> And if you looked at the way disable_irq is implemented, you would
> >> have found out that it falls back to masking if there is no disable
> >> method, preserving the semantic you expect.
> >>
> > Yes I did see, but this fall back requires extra "IRQ_DISABLE_UNLAZY" flag to
> be set to each virq.
> 
> No it doesn't. If you do a disable_irq(), the interrupt is flagged as disabled, but
> nothing gets done. If an interrupt actually fires, then the interrupts gets masked,
> and the handler is not called.
Yes agreed, this is where the problem comes for us. Here is the scenario
Ex:drivers/net/wireless/ath/ath9k/main.c
static int ath_reset_internal(struct ath_softc *sc, struct ath9k_channel *hchan)
{
  ....
   ath9k_hw_set_interrupts(ah);
   ath9k_hw_enable_interrupts(ah);
   ...
  enable_irq(sc->irq);
  ...
}
If you observe this they enable hardware interrupts first and then call enable_irq, at this point of time
virq is in disabled state. So, if interrupt is raised in this period of time the handler is never invoked 
and DEASEERT_INTx will not be seen. As I mentioned in my subject the irq line between bridge and 
GIC goes low only after it sees DEASSERT_INTx. But since DEASSERT_INTx is never seen line is always high
causing cpu stall.
So for this kind of EP's we need those two methods.

Bharat

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