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

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Message-ID: <20131007203114.GD24536@jonmason-lab>
Date:	Mon, 7 Oct 2013 13:31:18 -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 Mon, Oct 07, 2013 at 08:38:45PM +0200, Alexander Gordeev wrote:
> On Mon, Oct 07, 2013 at 09:50:57AM -0700, Jon Mason wrote:
> > 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.
> 
> The changelog is definitely bogus. I meant here an improvement to the
> existing scheme, not a conversion to the new one:
> 
> 	msix_entries = msix_table_size(val);
> 
> Getting i.e. 16 vectors here.
> 
> 	if (msix_entries > ndev->limits.msix_cnt) {

On SNB HW, limits.msix_cnt is set to SNB_MSIX_CNT (4)
http://lxr.free-electrons.com/source/drivers/ntb/ntb_hw.c#L558

> 		rc = -EINVAL;
> 		goto err;
> 	}
> 
> Upper limit check i.e. succeeds.
> 
> 	[...]
> 
> 	rc = pci_enable_msix(pdev, ndev->msix_entries, msix_entries);
> 
> pci_enable_msix() does not success and returns i.e. 8 here, should retry.

Per the above, since our upper bound is 4.  We will either have this
return 0 for all 4 or a number between 1 and 3 (or an error, but
that's not relevant to this discussion).

> 	if (rc < 0)
> 		goto err1;
> 	if (rc > 0) {
> 		/* 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) {
> 
> On SNB bail out here, although could have continue with 8 vectors.
> Can only use SNB_MSIX_CNT here, since limits.msix_cnt is the upper limit.

Since we can guarantee that rc is between 1 and 3 at this point (on
SNB HW), we should error out.

Thanks,
Jon


> 
> 			rc = -EIO;
> 			goto err1;
> 		}
> 
> 		[...]
> 	}
> 
> -- 
> Regards,
> Alexander Gordeev
> agordeev@...hat.com
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