On 2023-02-07 02:34:41 +0100, Andrew Lunn wrote: > > > > I've ignored "max-frame-size" since the description in > > > > ethernet-controller.yaml claims there is a contradiction in the > > > > Devicetree specification. I suppose it is describing the property > > > > "max-frame-size" with "Specifies maximum packet length ...". > > > > > > Please include it and we'll fix the spec. It is clearly wrong. 2 nios > > > boards use 1518 and the consumer for them says it is MTU. Everything > > > else clearly uses mtu with 1500 or 9000. > > > > Ok, the example in the pdf is 'max-frame-size = <1518>;'. I'll include > > it with the description of ethernet-controller.yaml which specifies it > > as MTU. > > You need to be careful here. Frame and MTU are different things. yes, we are aware. The description in of the property in Documentation/devicetree/bindings/net/ethernet-controller.yaml is: | Maximum transfer unit (IEEE defined MTU), rather than the | maximum frame size (there\'s contradiction in the Devicetree | Specification). The description for the property in the Devicetree is: | Specifies maximum packet length in bytes that the physical interface | can send and receive. While the "packet length" in the description is a little confusing this seems to refer to the ethernet frame size. > The IEEE 802.3 standard says nothing about MTU. I believe MTU is an IP > concept. It is the size of the SDU an Ethernet PDU can carry. This is > typically 1500. > > Historically, the max Ethernet frame size was 1518. But with 802.1Q > which added the VLAN header, all modern hardware actual uses 1522 to > accommodate the extra 4 bytes VLAN header. So i would not actually put > max-frame-size = <1518> anywhere, because it will get copy/pasted and > break VLAN setups. > > It looks like the ibm,emac.txt makes this error, max-frame-size = > <5dc>; 0x5dc is 1500. And there are a few powerpc .dtc using > 1500/0x5dc, which are probably broken. I would not say it is an error. The specification/name and use of "max-frame-size" has clearly diverged. All 4 in-tree users of this property interpret it as MTU. With the exception of the 2 nios2 boards Rob found all device trees use either 1500, 3800 or 9000 as 'max-frame-size'. I think Rob's plan to deal with this conflict between specification and actual use is to accept the use and update the description in the specification. This results in a "max-frame-size" property which describes the maximal payload / MTU. The upside of this is that we can leave all devicetrees and drivers unchanged and avoid breaking out-of-tree users. I'll fix the 2 nios2 boards since those currently end up with a MTU of 1518 in altera_tse_main.c. Janne
