netdev - Re: [PATCH net-next 01/15] sfc: support setting MTU even if not privileged to configure MAC fully

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Message-ID: <20200701161636.3399d902@kicinski-fedora-pc1c0hjn.dhcp.thefacebook.com>
Date:   Wed, 1 Jul 2020 16:16:36 -0700
From:   Jakub Kicinski <kuba@...nel.org>
To:     Edward Cree <ecree@...arflare.com>
Cc:     <linux-net-drivers@...arflare.com>, <davem@...emloft.net>,
        <netdev@...r.kernel.org>
Subject: Re: [PATCH net-next 01/15] sfc: support setting MTU even if not
 privileged to configure MAC fully

On Wed, 1 Jul 2020 23:13:13 +0100 Edward Cree wrote:
> On 01/07/2020 20:03, Jakub Kicinski wrote:
> > On Wed, 1 Jul 2020 15:51:25 +0100 Edward Cree wrote:  
> >> Unprivileged functions (such as VFs) may set their MTU by use of the
> >>  'control' field of MC_CMD_SET_MAC_EXT, as used in efx_mcdi_set_mtu().
> >> If calling efx_ef10_mac_reconfigure() from efx_change_mtu(), the NIC
> >>  supports the above (SET_MAC_ENHANCED capability), and regular
> >>  efx_mcdi_set_mac() fails EPERM, then fall back to efx_mcdi_set_mtu().  
> > Is there no way of checking the permission the function has before
> > issuing the firmware call?  
> We could condition on the LINKCTRL flag from the MC_CMD_DRV_ATTACH
>  response we get at start of day; but usually in this driver we've
>  tried to follow the EAFP principle rather than embedding knowledge
>  of the firmware's permissions model into the driver.

I see. I'm actually asking because of efx_ef10_set_udp_tnl_ports().
I'd like to rewrite the UDP tunnel code so that 
NETIF_F_RX_UDP_TUNNEL_PORT only appears on the interface if it 
_really_ can do the offload. ef10 is the only driver I've seen where 
I can't be sure what FW will say.. (other than liquidio, but that's 
more of a kernel<->FW proxy than a driver, sigh).

Is there anything I can condition on there?

> I suppose it might make sense to go straight to efx_mcdi_set_mtu()
>  in the mtu_only && SET_MAC_ENHANCED case, use efx_mcdi_set_mac()
>  otherwise, and thus never have a fallback from one to the other.
> WDYT?

For the change of MTU that indeed seems much cleaner.