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Date:   Sun, 15 Oct 2017 05:14:50 +0000
From:   Yuval Mintz <yuvalm@...lanox.com>
To:     Yunsheng Lin <linyunsheng@...wei.com>,
        "davem@...emloft.net" <davem@...emloft.net>
CC:     "huangdaode@...ilicon.com" <huangdaode@...ilicon.com>,
        "xuwei5@...ilicon.com" <xuwei5@...ilicon.com>,
        "liguozhu@...ilicon.com" <liguozhu@...ilicon.com>,
        "Yisen.Zhuang@...wei.com" <Yisen.Zhuang@...wei.com>,
        "gabriele.paoloni@...wei.com" <gabriele.paoloni@...wei.com>,
        "john.garry@...wei.com" <john.garry@...wei.com>,
        "linuxarm@...wei.com" <linuxarm@...wei.com>,
        "salil.mehta@...wei.com" <salil.mehta@...wei.com>,
        "lipeng321@...wei.com" <lipeng321@...wei.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH net-next 0/2] Add mqprio hardware offload support in hns3
 driver

> Hi, Yuval
> 
> On 2017/10/13 4:21, Yuval Mintz wrote:
> >> This patchset adds a new hardware offload type in mqprio before adding
> >> mqprio hardware offload support in hns3 driver.
> >
> > I think one of the biggest issues in tying this to DCB configuration is the
> > non-immediate [and possibly non persistent] configuration.
> >
> > Scenario #1:
> > User is configuring mqprio offloaded with 3 TCs while device is in willing
> mode.
> > Would you expect the driver to immediately respond with a success or
> instead
> > delay the return until the DCBx negotiation is complete and the operational
> > num of TCs is actually 3?
> 
> Well, when user requsts the mqprio offloaded by a hardware shared by DCB,
> I expect
> the user is not using the dcb tool.
> If user is still using dcb tool, then result is undefined.
> 
> The scenario you mention maybe can be enforced by setting willing to zero
> when user
> is requesting the mqprio offload, and restore the willing bit when unloaded
> the mqprio
> offload.

Sounds a bit harsh but would probably work.

> But I think the real issue is that dcb and mqprio shares the tc system in the
> stack,
> the problem may be better to be fixed in the stack rather than in the driver,
> as you
> suggested in the DCB patchset. What do you think?

What did you have in mind?

> 
> >
> > Scenario #2:
> > Assume user explicitly offloaded mqprio with 3 TCs, but now DCB
> configuration
> > has changed on the peer side and 4 TCs is the new negotiated operational
> value.
> > Your current driver logic would change the number of TCs underneath the
> user
> > configuration [and it would actually probably work due to mqprio being a
> crappy
> > qdisc]. But was that the user actual intention?
> > [I think the likely answer in this scenario is 'yes' since the alternative is no
> better.
> > But I still thought it was worth mentioning]
> 
> You are right, the problem also have something to do with mqprio and dcb
> sharing
> the tc in the stack.
> 
> Druing testing, when user explicitly offloaded mqprio with 3 TCs, all
> queue has a default pfifo mqprio attached, after DCB changes the tc num to
> 4,
> using tc qdisc shows some queue does not have a default pfifo mqprio
> attached.

Really? Then what did it show? 
[I assume it has some pfifo attached, and it's an mqprio dump kind of an issue]

> 
> Maybe we can add a callback to notify mqprio the configuration has changed.
> 

Which would do what?
You already have the notifications available for monitoring using dcbnl logic if the
configuration change [for user]; So user can re-configure whatever it wants.
But other than dropping all the qdisc configurations and going back to the default
qdiscs, what default action would mqprio be able to do when configuration changes
that actually makes sense?

> Thanks
> Yunsheng Lin
> 
> >
> > Cheers,
> > Yuval
> >
> >>
> >> Yunsheng Lin (2):
> >>   mqprio: Add a new hardware offload type in mqprio
> >>   net: hns3: Add mqprio hardware offload support in hns3 driver
> >>
> >>  drivers/net/ethernet/hisilicon/hns3/hnae3.h        |  1 +
> >>  .../net/ethernet/hisilicon/hns3/hns3pf/hclge_dcb.c | 23 +++++++++++
> >>  .../net/ethernet/hisilicon/hns3/hns3pf/hns3_enet.c | 46
> ++++++++++++++-
> >> -------
> >>  include/uapi/linux/pkt_sched.h                     |  1 +
> >>  4 files changed, 55 insertions(+), 16 deletions(-)
> >>
> >> --
> >> 1.9.1
> >
> >
> >

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