Re: [dpdk-dev] [EXT] Re: [PATCH v3 2/3] eal: add ack interrupt API

[Jerin Jacob Kollanukkaran <jerinj@marvell.com>]

> -----Original Message-----
> From: Burakov, Anatoly <anatoly.burakov@intel.com>
> Sent: Thursday, July 18, 2019 6:43 PM
> To: Nithin Kumar Dabilpuram <ndabilpuram@marvell.com>; Hyong Youb Kim
> <hyonkim@cisco.com>; David Marchand <david.marchand@redhat.com>;
> Thomas Monjalon <thomas@monjalon.net>; Ferruh Yigit
> <ferruh.yigit@intel.com>; Bruce Richardson <bruce.richardson@intel.com>
> Cc: Jerin Jacob Kollanukkaran <jerinj@marvell.com>; John Daley
> <johndale@cisco.com>; Shahed Shaikh <shshaikh@marvell.com>;
> dev@dpdk.org
> Subject: [EXT] Re: [dpdk-dev] [PATCH v3 2/3] eal: add ack interrupt API
> On 18-Jul-19 9:46 AM, Nithin Dabilpuram wrote:
> > Add new ack interrupt API to avoid using
> > VFIO_IRQ_SET_ACTION_TRIGGER(rte_intr_enable()) for acking interrupt
> > purpose for VFIO based interrupt handlers.
> > This implementation is specific to Linux.
> >
> > Using rte_intr_enable() for acking interrupt has below issues
> >
> >   * Time consuming to do for every interrupt received as it will
> >     free_irq() followed by request_irq() and all other initializations
> >   * A race condition because of a window between free_irq() and
> >     request_irq() with packet reception still on and device still
> >     enabled and would throw warning messages like below.
> >     [158764.159833] do_IRQ: 9.34 No irq handler for vector
> >
> > In this patch, rte_intr_ack() is a no-op for VFIO_MSIX/VFIO_MSI
> > interrupts as they are edge triggered and kernel would not mask the
> > interrupt before delivering the event to userspace and we don't need to
> ack.
> >
> > Signed-off-by: Nithin Dabilpuram <ndabilpuram@marvell.com>
> > Signed-off-by: Jerin Jacob <jerinj@marvell.com>
> > Tested-by: Shahed Shaikh <shshaikh@marvell.com>
> > ---
> 
> <snip>
> 
> >    */
> >   int rte_intr_disable(const struct rte_intr_handle *intr_handle);
> >
> > +/**
> > + * It acks an interrupt raised for the specified handle.
> > + *
> > + * Call this function to ack an interrupt from interrupt
> > + * handler either from application or driver, so that
> > + * new interrupts are raised.
> > + *
> > + * @note PMD generally calls this function at the end of its IRQ callback.
> > + *    Internally, it unmasks the interrupt if possible. For INTx, unmasking
> > + *    is required as the interrupt is auto-masked prior to invoking
> > + *    callback. For MSI/MSI-X, unmasking is typically not needed as the
> > + *    interrupt is not auto-masked. In fact, for interrupt handle types
> > + *    VFIO_MSIX and VFIO_MSI, this function is no-op.
> 
> I've added this comment in previous revision's discussion, but i'll copy it here
> as well.
> 
> Does the user of this API even cares about these details? I would think that it
> would be easier to just mandate calling this function at the end of each
> interrupt callback, regardless of which interrupt mode is used.
> 
> IMO internal details (about no-ops and such) are better explained in the
> implementation, not in the public-facing API header.

+1 to move the comment under @note to implementation.