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

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Date:   Wed, 1 Jul 2020 23:13:13 +0100
From:   Edward Cree <ecree@...arflare.com>
To:     Jakub Kicinski <kuba@...nel.org>
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 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 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?

-ed