Re: [net PATCH 2/2] net: dsa: qca8k: fix MTU calculation

[Vladimir Oltean]

On Fri, Nov 19, 2021 at 03:03:50AM +0100, Ansuel Smith wrote:
> From: Robert Marko <robert.marko@xxxxxxxxxx>
> 
> qca8k has a global MTU, so its tracking the MTU per port to make sure
> that the largest MTU gets applied.
> Since it uses the frame size instead of MTU the driver MTU change function
> will then add the size of Ethernet header and checksum on top of MTU.
> 
> The driver currently populates the per port MTU size as Ethernet frame
> length + checksum which equals 1518.
> 
> The issue is that then MTU change function will go through all of the
> ports, find the largest MTU and apply the Ethernet header + checksum on
> top of it again, so for a desired MTU of 1500 you will end up with 1536.
> 
> This is obviously incorrect, so to correct it populate the per port struct
> MTU with just the MTU and not include the Ethernet header + checksum size
> as those will be added by the MTU change function.
> 
> Fixes: f58d2598cf70 ("net: dsa: qca8k: implement the port MTU callbacks")
> Signed-off-by: Robert Marko <robert.marko@xxxxxxxxxx>
> Signed-off-by: Ansuel Smith <ansuelsmth@xxxxxxxxx>
> ---

Reviewed-by: Vladimir Oltean <olteanv@xxxxxxxxx>