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

[Nithin Kumar Dabilpuram <ndabilpuram@marvell.com>]

On 7/18/2019 6:57 PM, Jerin Jacob Kollanukkaran wrote:
>> -----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.
Ok. I'll just move this note alone to implementation of function in next 
revision.
>