linux-kernel - Re: [PATCH RFC 54/77] ntb: Ensure number of MSIs on SNB is enough for the link interrupt

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Message-ID: <20131007165056.GA24536@jonmason-lab>
Date:	Mon, 7 Oct 2013 09:50:57 -0700
From:	Jon Mason <jon.mason@...el.com>
To:	Alexander Gordeev <agordeev@...hat.com>
Cc:	linux-kernel@...r.kernel.org, Bjorn Helgaas <bhelgaas@...gle.com>,
	Ralf Baechle <ralf@...ux-mips.org>,
	Michael Ellerman <michael@...erman.id.au>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Martin Schwidefsky <schwidefsky@...ibm.com>,
	Ingo Molnar <mingo@...hat.com>, Tejun Heo <tj@...nel.org>,
	Dan Williams <dan.j.williams@...el.com>,
	Andy King <acking@...are.com>,
	Matt Porter <mporter@...nel.crashing.org>,
	stable@...r.kernel.org, linux-pci@...r.kernel.org,
	linux-mips@...ux-mips.org, linuxppc-dev@...ts.ozlabs.org,
	linux390@...ibm.com, linux-s390@...r.kernel.org, x86@...nel.org,
	linux-ide@...r.kernel.org, iss_storagedev@...com,
	linux-nvme@...ts.infradead.org, linux-rdma@...r.kernel.org,
	netdev@...r.kernel.org, e1000-devel@...ts.sourceforge.net,
	linux-driver@...gic.com,
	Solarflare linux maintainers <linux-net-drivers@...arflare.com>,
	"VMware, Inc." <pv-drivers@...are.com>, linux-scsi@...r.kernel.org
Subject: Re: [PATCH RFC 54/77] ntb: Ensure number of MSIs on SNB is enough
 for the link interrupt

On Sat, Oct 05, 2013 at 11:43:04PM +0200, Alexander Gordeev wrote:
> On Wed, Oct 02, 2013 at 05:48:05PM -0700, Jon Mason wrote:
> > On Wed, Oct 02, 2013 at 12:49:10PM +0200, Alexander Gordeev wrote:
> > > Signed-off-by: Alexander Gordeev <agordeev@...hat.com>
> > > ---
> > >  drivers/ntb/ntb_hw.c |    2 +-
> > >  1 files changed, 1 insertions(+), 1 deletions(-)
> > > 
> > > diff --git a/drivers/ntb/ntb_hw.c b/drivers/ntb/ntb_hw.c
> > > index de2062c..eccd5e5 100644
> > > --- a/drivers/ntb/ntb_hw.c
> > > +++ b/drivers/ntb/ntb_hw.c
> > > @@ -1066,7 +1066,7 @@ static int ntb_setup_msix(struct ntb_device *ndev)
> > >  		/* On SNB, the link interrupt is always tied to 4th vector.  If
> > >  		 * we can't get all 4, then we can't use MSI-X.
> > >  		 */
> > > -		if (ndev->hw_type != BWD_HW) {
> > > +		if ((rc < SNB_MSIX_CNT) && (ndev->hw_type != BWD_HW)) {
> > 
> > Nack, this check is unnecessary.
> 
> If SNB can do more than SNB_MSIX_CNT MSI-Xs then this check is needed
> to enable less than maximum MSI-Xs in case the maximum was not allocated.
> Otherwise SNB will fallback to single MSI instead of multiple MSI-Xs.

Per the comment in the code snippet above, "If we can't get all 4,
then we can't use MSI-X".  There is already a check to see if more
than 4 were acquired.  So it's not possible to hit this.  Even if it
was, don't use SNB_MSIX_CNT here (limits.msix_cnt is the preferred
variable).  Also, the "()" are unnecessary.

Thanks,
Jon

> -- 
> Regards,
> Alexander Gordeev
> agordeev@...hat.com
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