gitproxy/RTLPlayground
1
0
Fork 0
mirror of https://github.com/logicog/RTLPlayground.git synced 2026-09-02 15:02:51 +08:00
Code Issues Packages Projects Releases Wiki Activity

doc: move the L2 multicast and tag word details out of the code

Browse Source 
Review asked for this directly: the hardware layout above port_l2mc_set()
would be better as documentation than as a comment, keeping only the two
lines that say what the function does.

doc/l2.md gains a section on static multicast entries, why delivery uses
the forward action rather than the trap, and the SMI layout of the entry.
doc/CpuPort.md gains the layout of the tag's flags and pmask words, with
the byte order trap that cost an afternoon: writing the flags constant raw
instead of through HTONS puts 0x0020 on the wire as 0x2000, which is EFID
rather than LEARN_DIS, and the ASIC then leaves the 0x8899 header on the
frame.

The comments those paragraphs came from are replaced by a pointer to the
file that now holds them.
This commit is contained in:
d00f
2026-08-18 23:29:10 +02:00
committed by d00f
parent 0160b430f4
commit 4a78b2dc8c
4 changed files with 66 additions and 38 deletions
Download Patch File Download Diff File Expand all files Collapse all files
doc/l2.md
+33
View File
@@ -65,3 +65,36 @@ ASIC and flushing the table in order to quickly forget the learned entries.
3c:18:a0:7e:11:00 0x0001 learned 5
1c:2a:a3:23:00:02 0x0001 learned 7
```
## Static multicast entries
Slow-protocol frames such as LACPDUs and STP BPDUs have to reach the CPU
without being flooded to the other ports. No bridge relays these frames:
their addresses are in the set that 802.1D-2004 clause 7.12.6 forbids a
bridge to forward, and what travels the network is the information, with
every bridge regenerating BPDUs of its own on its designated ports. The reserved-multicast *trap* action
cannot do that on this hardware, because its destination is an external CPU
attached to a physical port, which these boards do not populate. The protocol
modules therefore leave the reserved-multicast action at *forward* and constrain
the egress with a static L2 multicast entry instead: the lookup hits the entry's
own port mask rather than the VLAN flood mask. Verified on a SWTGW218AS both
ways, with the CPU bit cleared, where delivery stops, and with the CPU bit alone,
where nothing egresses.
`port_l2mc_set()` writes one such entry. The SMI layout is the L2 multicast
variant of the table entry:
```
DATA_IN_A = MAC bytes 5..2 -> c2 00 00 <mac_last>
DATA_IN_B = MAC[1..0] | vid<<16 | IVL<<29 | pmask[1:0]<<30
DATA_IN_C = pmask[9:2]
```
Lookups are IVL, so an entry made for VID 0 is never matched and a caller adds
one entry per PVID in use. The write goes through the table access register
with the table selector set to the L2 lookup table, `TBL_L2_UNICAST` in the
code, a name that despite appearances covers the multicast entries as well.
The hardware hashes MAC and VID to pick the bucket slot by itself.
Writing the same MAC and VID again replaces the entry rather than adding a
second one, so a caller can retarget the mask at will, for instance back to all
ports to restore flooding.