linux-kernel - Re: [PATCH] PCI MSI: allow alignment restrictions on vector allocation

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Message-ID: <20171003213434.GI25517@bhelgaas-glaptop.roam.corp.google.com>
Date:   Tue, 3 Oct 2017 16:34:34 -0500
From:   Bjorn Helgaas <helgaas@...nel.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Daniel Drake <drake@...lessm.com>, linux-kernel@...r.kernel.org,
        linux-pci@...r.kernel.org, x86@...nel.org,
        linux-wireless@...r.kernel.org, ath9k-devel@....qualcomm.com,
        linux@...lessm.com
Subject: Re: [PATCH] PCI MSI: allow alignment restrictions on vector
 allocation

On Tue, Oct 03, 2017 at 11:07:58PM +0200, Thomas Gleixner wrote:
> On Mon, 2 Oct 2017, Thomas Gleixner wrote:
> > On Mon, 2 Oct 2017, Thomas Gleixner wrote:
> > > On Mon, 2 Oct 2017, Daniel Drake wrote:
> > >   2) The affinity setting of straight MSI interrupts (w/o remapping) on x86
> > >      requires to make the affinity change from the interrupt context of the
> > >      current active vector in order not to lose interrupts or worst case
> > >      getting into a stale state.
> > > 
> > >      That works for single vectors, but trying to move all vectors in one
> > >      go is more or less impossible, as there is no reliable way to
> > >      determine that none of the other vectors is on flight.
> > > 
> > >      There might be some 'workarounds' for that, but I rather avoid that
> > >      unless we get an official documented one from Intel/AMD.
> > 
> > Thinking more about it. That might be actually a non issue for MSI, but we
> > have that modus operandi in the current code and we need to address that
> > first before even thinking about multi MSI support.
> 
> But even if its possible, it's very debatable whether it's worth the effort
> when this driver just can use the legacy INTx.and be done with it.

Daniel said "Legacy interrupts do not work on that module, so MSI
support is required," so I assume he means INTx doesn't work.  Maybe
the driver could poll?  I don't know how much slower that would be,
but at least it would penalize the broken device, not everybody.

Bjorn