netdev - RE: [PATCH net-next] net: hns3: Add support to change MTU in hardware & netdev

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Message-ID: <F4CC6FACFEB3C54C9141D49AD221F7F93B86FB7E@FRAEML521-MBX.china.huawei.com>
Date:   Fri, 18 Aug 2017 16:10:07 +0000
From:   Salil Mehta <salil.mehta@...wei.com>
To:     Andrew Lunn <andrew@...n.ch>
CC:     "davem@...emloft.net" <davem@...emloft.net>,
        "Zhuangyuzeng (Yisen)" <yisen.zhuang@...wei.com>,
        "lipeng (Y)" <lipeng321@...wei.com>,
        "mehta.salil.lnk@...il.com" <mehta.salil.lnk@...il.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-rdma@...r.kernel.org" <linux-rdma@...r.kernel.org>,
        Linuxarm <linuxarm@...wei.com>
Subject: RE: [PATCH net-next] net: hns3: Add support to change MTU in
 hardware & netdev

Hi Andrew

> -----Original Message-----
> From: Andrew Lunn [mailto:andrew@...n.ch]
> Sent: Friday, August 18, 2017 5:02 PM
> To: Salil Mehta
> Cc: davem@...emloft.net; Zhuangyuzeng (Yisen); lipeng (Y);
> mehta.salil.lnk@...il.com; netdev@...r.kernel.org; linux-
> kernel@...r.kernel.org; linux-rdma@...r.kernel.org; Linuxarm
> Subject: Re: [PATCH net-next] net: hns3: Add support to change MTU in
> hardware & netdev
> 
> > for example,
> > file: drivers/net/ethernet/chelsio/cxgb4/cxgb4_main.c
> > static int init_one(struct pci_dev *pdev, const struct pci_device_id
> *ent)
> >
> >  		netdev->priv_flags |= IFF_UNICAST_FLT;
> >
> > +		/* MTU range: 81 - 9600 */
> > +		netdev->min_mtu = 81;
> > +		netdev->max_mtu = MAX_MTU;
> 
> In this cause, the driver is not using the default values. So it sets
> them.
> 
> Anyway, try it. After your alloc_etherdev_mqs(), print the value of
> min_mtu. It should already be set to MIN_ETH_MTU.
I understand your point. In this case, I would like to keep the
range being set by the driver just to be more explicit. 
So for now keep this initialization in the driver?

Thanks
Salil
> 
> > I see. IMHO HNS3 is currently limited by maximum buffer per
> descriptor
> > which is 64k. I am sure such frames would get dropped in the hardware
> > itself and which I guess should be more preferable than dropping in
> > driver since it saves you some precious cpu cycles?
> 
> If you hardware handles this, then you don't need to do anything.
Fine. Thanks!

Salil
> 
>    Andrew