storm: meters from 24 up, plainer parser, documentation from the review

Storm control used meters 0 to 39, the same meters the ACL hands out,
so a storm limit and an acl meter could overwrite each other. It now
uses 24 + port * 4 + type, meters 24 to 63.

parse_storm() is no longer reentrant and uses plain locals, looks the
type up in a while loop and returns straight from the one-line cases.
storm_show() uses plain locals too.

The documentation says how IGMP decides between mcast and umcast, how
to get from a meter index to its limit, where the IFG bit of a meter
is, and what an ingress bandwidth limit on the same port does to the
storm limit.

doc/storm_control.md

@@ -12,7 +12,10 @@ storm <port> bcast|mcast|ucast|umcast off
 storm
 ```
 `ucast` is unicast to a MAC address the switch has not learned yet, and
-`umcast` is multicast without an entry in the address table. A limit in
+`umcast` is multicast without an entry in the address table. Multicast groups
+get their entries from IGMP snooping, so without `igmp on` all multicast is
+unknown and only the `umcast` limit applies to it; `mcast` covers the groups
+IGMP has learned. A limit in
 packets per second goes from 1 to 1048575, a limit in kbit/s from 1 to
 10000000. `storm` without arguments prints the limits of every port. The
 limits are kept in the configuration like any other setting, and the Bandwidth
@@ -21,8 +24,11 @@ page of the web interface has a Storm control card to set them.
 ## Registers
 Each port and type has an enable bit and a meter index, and each limit uses
 one of the 64 shared meters of the switch. RTLPlayground gives every port and
-type its own meter, index `port * 4 + type`, so one port's storm does not eat
-into the limit of another.
+type its own meter, index `24 + port * 4 + type`, so one port's storm does not
+eat into the limit of another. Storm control uses meters 24 to 63 and leaves
+0 to 23 to the ACL. The limit of a meter is in `RTL837X_METER_RATE + 4 *
+meter`, and bit `meter % 32` of `RTL837X_METER_MODE + 4 * (meter / 32)`
+selects pps instead of kbit/s.
 ```
 #define RTL837X_STORM_CTRL		0x54e4	/* + 4 * type, bit = port */
 #define RTL837X_STORM_MIDX		0x54f4	/* + 8 * type + 4 * (port / 5), 6 bits per port */
@@ -31,7 +37,15 @@ into the limit of another.
 ```
 In kbit/s mode one step of the rate is 1 kbit/s. Measured with 100 and 1000 byte
 frames, the meter counts a frame with its FCS, but without preamble and
-interframe gap.
+interframe gap. Every meter has a bit that adds them, in `0x5f08 + 4 * (meter
+/ 32)`, which storm control leaves cleared. `IGBW_INC_IFG` and `EGBW_INC_IFG`
+are the matching bits of the bandwidth control.
+
+An ingress bandwidth limit on the same port works on the same arriving frames
+as the storm limit rather than after it, and the two together pass far less
+than either one: a 1 Mbit/s `umcast` limit with a 10 Mbit/s ingress limit let
+0.12 to 0.16 Mbit/s of a 1.4 Gbit/s stream through, where each alone passed
+1.17 and 10.2 Mbit/s.
 
 The type is 0 for broadcast, 1 for multicast, 2 for unknown unicast and 3 for
 unknown multicast. The burst size of a meter (0x5df0 + 4 * meter) counts packets