netdev - RE: [PATCH 5/5] PCI: remove pci_enable

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Message-ID: <063D6719AE5E284EB5DD2968C1650D6DCFFBE7F3@AcuExch.aculab.com>
Date:   Tue, 28 Mar 2017 09:07:28 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'David Daney' <ddaney.cavm@...il.com>,
        Christoph Hellwig <hch@....de>
CC:     "bhelgaas@...gle.com" <bhelgaas@...gle.com>,
        "davem@...emloft.net" <davem@...emloft.net>,
        "netanel@...apurnalabs.com" <netanel@...apurnalabs.com>,
        "jcliburn@...il.com" <jcliburn@...il.com>,
        "chris.snook@...il.com" <chris.snook@...il.com>,
        "sgoutham@...ium.com" <sgoutham@...ium.com>,
        "rric@...nel.org" <rric@...nel.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>
Subject: RE: [PATCH 5/5] PCI: remove pci_enable_msix

From: David Daney
> Sent: 27 March 2017 18:31
> On 03/27/2017 10:11 AM, Christoph Hellwig wrote:
> > On Mon, Mar 27, 2017 at 09:59:35AM -0700, David Daney wrote:
> >> On 03/27/2017 01:29 AM, Christoph Hellwig wrote:
> >>> Unused now that all callers switched to pci_alloc_irq_vectors.
> >>>
> >>
> >> And you are aware that the ThunderX GPIO driver that I am attempting to
> >> merge uses this interface.
> >>
> >> If this patch gets merged, should I ask to revert it when the GPIO driver
> >> goes in?
> >
> > No.  You should not use pci_enable_msix in your new driver as I told
> > you before.
> >
> >> You offer no solution for drivers that would benefit from using a sparse
> >> sub set of the available MSI-X vectors.
> >
> > Use pci_enable_msix_{exact,range} for now, as I told you before.
> >
> 
> That still results in twice as many MSI-X being provisioned than are needed.
> 
> For drivers that use a contiguous range of MSI-X, your suggestion is
> usable, but for others you are forcing resources to be wasted.  For what
> end?

There are also drivers that only need some interrupts after a specific
device is opened, and could free them when closed.

This could even be network devices with lots of queues, or a driver
realising that the workload is high and per-cpu interrupts make sense.

So any real change to the interface should allow drivers to allocate
and free individual MSI-X vectors.

I remember a lot of discussions when pci_enable_msix_range() was added,
but almost nothing about this change.

	David