linux-kernel - RE: INTMS/INTMC not being used in NVME interrupt handling

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Date:   Thu, 17 May 2018 11:15:59 +0000
From:   Bharat Kumar Gogada <bharatku@...inx.com>
To:     Keith Busch <keith.busch@...ux.intel.com>
CC:     "linux-nvme@...ts.infradead.org" <linux-nvme@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "keith.busch@...el.com" <keith.busch@...el.com>,
        "axboe@...com" <axboe@...com>, "hch@....de" <hch@....de>,
        "sagi@...mberg.me" <sagi@...mberg.me>
Subject: RE: INTMS/INTMC not being used in NVME interrupt handling

> > Hi,
> >
> > As per NVME specification:
> > 7.5.1.1 Host Software Interrupt Handling It is recommended that host
> > software utilize the Interrupt Mask Set and Interrupt Mask Clear
> > (INTMS/INTMC) registers to efficiently handle interrupts when configured
> to use pin based or MSI messages.
> >
> > In kernel 4.14, drivers/nvme/host/pci.c function nvme_isr doesn't  use
> > these registers.
> >
> > Any reason why these registers are not used in nvme interrupt handler ?
> 
> I think you've answered your own question: we process completions in the
> interrupt context. The interrupt is already masked at the CPU level in this
> context, so there should be no reason to mask them at the device level.
> 
> > Why NVMe driver is not using any bottom half and processing all
> > completion queues in interrupt handler ?
> 
> Performance.
Thanks keith. 
Currently driver isn't setting any Coalesce count.  
So the NVMe card will raise interrupt for every single completion queue ?

For legacy interrupt for each CQ 
CQ-> ASSERT_INTA-> DOORBELL-> DEASSERT_INTA is this flow correct ?

Is the following flow valid
CQ1->ASSERT_INTA->CQ2/CQ3->Doorbell->DEASSERT_INTA ?

When using legacy interrupts, if CQ1 is sent followed by ASSERT_INTA, can the EP send 
another CQ2,CQ3.. before DEASSERT_INTA of CQ1 is generated?

Regards,
Bharat