Subject: Re: [PATCH net v5] net: stmmac: Prevent DSA tags from breaking COE

From: Jakub Kicinski <[email-masked]>
Date: Tue, 16 Jan 2024 10:36:37 -0800

On Tue, 16 Jan 2024 17:18:30 +0100 (CET) Romain Gantois wrote:
> > which makes it sound like bit 5 will not be set for a Ethernet II frame
> > with unsupported IP payload, or not an IP frame. Does the bit mean other
> > things in different descriptor formats?  
> 
> The description of this bit in my datasheet is:
> 
> ```
> b5 FT Frame Type
> When set, this bit indicates that the Receive Frame is an Ethernet-type frame 
> (the Length/Type field is greater than or equal to 1,536). When this bit is 
> reset, it indicates that the received frame is an IEEE 802.3 frame. This bit is 
> not valid for Runt frames less than 14 bytes
> ```
> 
> There is no mention of a more subtle check to detect non-IP Ethernet II frames. 
> I ran some tests on my hardware and EDSA-tagged packets consistently come in 
> with status 0b100, so the MAC sets the frame type bit even for frames that don't 
> have an IP ethertype.

Boo, who designed this thing :(

v6 is good to go in then, thank you for investigating and testing!