netdev - Re: [net-next PATCH] octeontx2-pf: Change receive buffer size using ethtool

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Date:   Fri, 14 Jan 2022 11:28:35 +0530
From:   sundeep subbaraya <sundeep.lkml@...il.com>
To:     Jakub Kicinski <kuba@...nel.org>
Cc:     Subbaraya Sundeep <sbhatta@...vell.com>,
        David Miller <davem@...emloft.net>, netdev@...r.kernel.org,
        hariprasad <hkelam@...vell.com>,
        Geetha sowjanya <gakula@...vell.com>,
        Sunil Kovvuri Goutham <sgoutham@...vell.com>
Subject: Re: [net-next PATCH] octeontx2-pf: Change receive buffer size using ethtool

Hi Jakub,

On Fri, Jan 14, 2022 at 12:01 AM Jakub Kicinski <kuba@...nel.org> wrote:
>
> On Thu, 13 Jan 2022 12:07:42 +0530 sundeep subbaraya wrote:
> > > Should we use rx_buf_len = 0 as a way for users to reset the rxbuf len
> > > to the default value? I think that would be handy.
> > >
> > Before this patch we calculate each receive buffer based on mtu set by user.
> > Now user can use rx-buf-len but the old mtu based calculation is also there.
> > Here I am using rx_buf_len == 0 as a switch to calculate buffer length
> > using mtu or
> > just use length set by user. So here I am not setting rx_buf_len to some
> > default value.
> >
> > > > --- a/drivers/net/ethernet/marvell/octeontx2/nic/otx2_pf.c
> > > > +++ b/drivers/net/ethernet/marvell/octeontx2/nic/otx2_pf.c
> > > > @@ -66,6 +66,8 @@ static int otx2_change_mtu(struct net_device *netdev, int new_mtu)
> > > >                   netdev->mtu, new_mtu);
> > > >       netdev->mtu = new_mtu;
> > > >
> > > > +     pf->hw.rbuf_len = 0;
> > >
> > > Why reset the buf len on mtu change?
> > >
> > As explained above buffer size will be calculated using mtu
> > now instead of rx-buf-len from ethtool.
>
> IIUC you're saying that the way to get back to the default buffer size
> is to change the MTU?
>
> It was discussed on the list in the past in the context of RSS
> indirection tables and the conclusion was that we should not reset
> explicit user configuration (in case of RSS indir table this means
> user-set table survives change in the number of queues).
>
> Having one config reset another is pretty painful to deal with for
> the users. I had to fix many cases of this sort of problem in the
> production env I'm working with. Turning one knob resets other knobs
> so it takes multiple runs of the config daemon (chef or alike) to
> get to the target configuration.
>
> In my mind if user has set the rx-buf-len it should survive all other
> config changes. User can reset to default by setting rx-buf-len to 0.
> It should be possible to set rx-buf-len and mtu in any order and have
> the same result.

Thanks for the clear explanation. I will change as per your comments
and send the next spin.

Sundeep