This implements an AXI stream buffer which allows replaying of the first
portion of each packet. The intent is to simplify the implementation of
CSMA/CD. This requires keeping 56 bytes of data to "replay" (slot time
minus the preamble). After these bytes are transmitted, we can only get
late collisions.
We always read from the buffer, as this simplifies the implementation
compared to some kind of hybrid fifo/skid buffer approach. The primary
design problem faced is in determining when it's OK to overwrite the
first byte in the packet. A naïve approach might be to allow overwriting
whenever the slave reads the last byte. However, in the case of a
54-byte packet, we will still need to allow replaying at this point (in
case there is a collision on the last byte). We can't just wait for
m_axis_ready to go high, because that would violate the AXI stream
protocol. To solve this, the slave must assert the done signal when it
is finished with the packet.
Signed-off-by: Sean Anderson <seanga2@gmail.com>
This module integrates the PCS with the descrambler, implements the PMA
(which is just the link monitor), and implements loopback and coltest
functions. This is more of the PCS/PMA, but the descrambler is
technically part of the PMD, so it's the "core" instead.
We deviate from the standard in one important way: the link doesn't come
up until the descambler is locked. I think this makes sense, since if
the descrambler isn't locked, then the incoming data will be gibberish.
I suspect this isn't part of the standard because the descrambler
doesn't have a locked output in X3.263, so IEEE would have had to
specify it.
Loopback is actually implemented in the PMD, but it modifies the
behavior in several places. It disables collisions (unless
coltest is enabled). Additionally, we need to force the link up (to
avoid the lengthy stabilization timer), but ensure it is down for at
least once cycle (to ensure the descrambler desynchronizes).
On the test side, we just go through the "happy path," as many of the
edge conditions are tested for in the submodule tests. Several of those
tests are modified so that their helper functions can be reused in this
test. In particular, the rx path is now async so that we can feed it
rx_data.
Signed-off-by: Sean Anderson <seanga2@gmail.com>
Several interfaces have ce signals. Create a common function for driving
these signals, similar to the Clock function.
Signed-off-by: Sean Anderson <seanga2@gmail.com>
Parametrizing a test over different methods of generating valid data
will be useful for other tests as well. Refactor it out. We have to bind
valids early in with_valids.test, otherwise we will end up binding
with_valids.valid by reference (causing all tests to use saw_valid).
Signed-off-by: Sean Anderson <seanga2@gmail.com>
At the end of the bitstream, we might not have enough bits for valid=2.
If we don't change it to valid=1, instead of marking an X as valid.
Fixes: d351291 ("Initial commit")
Signed-off-by: Sean Anderson <seanga2@gmail.com>
The PMA also has to deal with "recoverd bitstreams" (that is, inputs which
can have 0, 1, or 2 valid bits). Export the core of pcs_send_codes into its
own function, as it is useful for generating these signals.
Signed-off-by: Sean Anderson <seanga2@gmail.com>
In the recieve tests, the harness often has a choice of how fast to feed
data to the module. Up to this point, we have always used the same
strategy (typically random), even when multiple strategies were used
when writing the test. Add parametrization to test different strategies
in each test run. The timing decorator is taken from the cocotb source,
since we can't pass parameters to cocotb.test directly any more.
Signed-off-by: Sean Anderson <seanga2@gmail.com>
Sean Anderson