[Intel-wired-lan] [net-next] igb: add function to set I210 transmit mode

[Brown, Aaron F]

> From: Intel-wired-lan [mailto:intel-wired-lan-bounces at lists.osuosl.org] On
> Behalf Of Gangfeng
> Sent: Tuesday, August 9, 2016 11:48 PM
> To: intel-wired-lan at lists.osuosl.org
> Cc: Gangfeng Huang <gangfeng.huang at ni.com>
> Subject: [Intel-wired-lan] [net-next] igb: add function to set I210 transmit
> mode
> 
> From: Gangfeng Huang <gangfeng.huang at ni.com>
> 
> I210 supports two transmit modes, legacy and Qav. The transmit mode is
> configured in TQAVCTRL.QavMode register. Before this patch igb driver
> only support legacy mode. This patch makes it possible to configure the
> transmit mode.
> 
> Example:
> Get the transmit mode:
> $ echo /sys/class/net/eth0/qav_mode
> 0

I think you mean "cat /sys/class/net/eth0/qav_mode" to see the contents, echo just displays the file name.

Having said that, I really appreciate you putting detailed instructions in the description.  It's rather surprising how often basic instructions do not get included. 

> Set transmit mode to qav mode
> $ echo 1 > /sys/class/net/eth0/qav_mode
>  

<snip>

>  #define MAXIMUM_ETHERNET_VLAN_SIZE 1522
> 
> +/* In qav mode, the maximum frame size is 1536 */
> +#define IGB_MAX_QAV_FRAME_SIZE 1536
> +

When I am in qav_mode I am not able to set the MTU beyond 1514, which I believe adds up to 1536 when I add the Ethernet header and a VLAN header, so this part seems good.  However, I am able to toggle into qav_mode with the MTU set to a larger value, the MTU stays at the larger value and I do not get any indication on the console, dmesg or logs that the MTU should be reduced.  I think failing to change to qav_mode with a message stating to reduce the MTU would be the desired behavior here.