Re: [openrisc] Understanding traffic cop

["Damjan Lampret" <lampret@opencores.org>]

> Some of the things I am having problems with are:
> 1. What are the signals with names like xi0_wb_dat_o or yi0_wb_dat_o for
> example.
> These ultimately are getting or'ed together to produce the output
> i0_wb_dat_o, but I do not understand
> the separation.

There are two "channels". One channel going to target 0 and the other
channel going to targets 1-8. Number of channels means how many initiators
can independently access targets.

Targets is basically slave peripherals. Initiators are masters.
Initiator/targets are used in case of traffic cop not to confuse with
masters/slaves on traffic cop and in the system (from traffic point of view
master can be something else then from master point of view, rememberwhen
you attach master to a traffic cop, traffic cop interface can be considered
a slave, or do you name the interface as master because you connect master -
anyway it is a matter of convention, so I used initiators and targets,
similar to PCI convention).

So in this case you have 8 initators going to target 0 and the same 8
initiators (as in the second channel) going to targets 1-8 (so targets 1-8
can only be access one at a time, while target 0 is independent). Signals
coming from target 0 are prefixed with x and those in the second channel are
prefixed with y. These of course need to be ORed together because at the end
there is only one set of initiators.

> 2. What is target 0 and initiator 0. Are these real devices on the bus
like
> (cpu and memory) or is this
> a reference to traffic cop itself.

Khm, not sure what you mean. Initiator 0 is not reference to real devices,
you can connect whatever you want. But at the end a master device will be
connected to initiator 0, for example the cpu. Target 0 will usually be
connected to memory, to allow independent access to memory while for example
UARTs etc will be connected to target 1-8 as they are slow and don't have to
be accessible independently.

> 3. What does the comment // From initiators to targets 1-8 (lower part)
and
> // From initiators to targets 1-8 (upper part)
> mean?

The traffic cop is basically split into two parts, lower and upper. There
are two upper parts, each for a channel. For example all initiators are
"merged and arbitrated" together into first channel and result of merging
goes to target 0. This is the case with t0_ch instance of module tc_mi_to_st
(module name means "traffic cop multiple initiators to single target"). So
for first channel going to target 0 there is no lower part.

There is also t18_ch_upper instances, this one also "merges / arbitrates"
the same initiators into second channel going to targets 1-8. (here
tc_mi_to_st would better read as in "traffic cop multiple initiator to
single channel" instead of "single target", but the name single target is
because the module itself is indeed goign to a single target, however
because of the lower part that is demuxing it goes to multiple targets).

So as indicated above, instance t18_ch_lower of module tc_si_to_mt (traffic
cop single initiaror to multiple targets) is used to demux second channel
and connect multiple targets. So together upper and lower part merge/mux
targets 1-8 to initiators.

> 4. What are the z signals like z_wb_cyc_i for?

z is basically second channel. So between muxed initiators (upper part) and
demux targets (lower part).

>
> Ultimately I know this is a multiplexer, but the devil is in the details.
> If there are some other docs I should be
> looking at, let me know.

If you can make more comments in the traffic cop based on what I have
written here, the next guy will have less questions to ask. ;-)

regards,
Damjan

>
> Thanks,
> Damon
>
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