My question for Greg was: Is there an assumption that CONFIG_NET_ETH_PKTSIZE
has to be 1514? So that ultimately a frame must be received completely into
one buffer?
Search for packet and frame in the Ethernet section of the reference
manual. The hardware will DMA up to (I think it was) 2048 bytes without
Jumbo mode. These are the 2K packets. This is for the H7 (and probably
the F7).
However, that should not happen because nothing should ever send packets
that large to the station (and PROMISCUOUS mode should be disabled).
From what you are saying, the packet buffer should also be aligned to
the cache line and be a multiple of the cache line size. So for an MTU
of X, I think that should be
mask = Y - 1
size = ( (X + 18) + mask) & ~mask
buffer = memalign(Y, size);
where
* X is the MTU (like 1500). The MTU does not include the Ethernet
overhead.
* Y is the cache line size, usually 32.
* 18 is the Ethernet overhead (14 byte Ethernet header + 2 byte FCS at
the end). The FCS is usually accounted for the the
CONFIG_GUARD_SIZE which defaults to 2. But some hardware transfers
additional info after the FCS.
The 1514 packet size you mention may not be meaningful without some
clarification. The packet buffer size of 1518 is a 1500 byte MTU plus
the 18 byte Ethernet overhead. Some hardware verifies the FCS and does
not transfer it to memory ... that would make 1514. But that is not a
typical case.
The selection of the MTU should not matter on most networks or with most
network hardware. TCP should follow the negotiated MSS and everything
else should be small. The MTU should be selected based on the
configured properties of the network, resources available in the target,
and the needs of the application
IP size = Ethernet header + MTU + FCS
= Ethernet header + IP header + protocol header + MSS + FCS
IPv4 hosts are required to be able to handle an MSS of at least 536
bytes; IPv6 hosts are required to be able to handle an MSS of 1220
bytes. This results in the minimum sizes and is correct.
