rfnoc: Ignore errors in ctrlport response packets if ACKs not wanted

In 0caed5529, a change was made to ctrlpoint_endpoint's behavior such
that if a client does not care about checking for ACKs on poke or poll
operations, the code calls `wait_for_ack()` with a flag indicating that
it should not wait for the ACK, but find and remove the corresponding
response from the response queue. This prevents the queue from
potentially growing endlessly with response packets that the client
doesn't even care about.

However, this introduced a subtle, undesired behavioral change. When
`wait_for_ack()` finds the corresponding response for a request, it also
checks the status field of the response to report any errors flagged by
the hardware such as invalid command, routing error, etc. Prior to the
change mentioned above, since `wait_for_ack()` was never called when the
client doesn't want ACKs, the client would never be never notified of
any errors associated with the request. However, with the aforementioned
change in placd, when `wait_for_ack()` is called to find and remove the
unwanted response packet corresponding to the request, errors **are**
checked and reported up the user.

The behavior change was unearthed by the X410 ZBX CPLD initialization
code, which writes an initial value of 0 to all ZBX CPLD registers--even
read-only registers. A control request to write a read-only register
results in a response with CMDERR in its status field, as it should.
However, since the ZBX CPLD register initialization is performed with a
`poke32()` operation which by default doesn't wait for ACKs, this was
never a problem until the change to drain the response queue
inadvertently caused the error to surface. The result is that creating a
USRP session or RFNoC graph session to an X410 device is seen to
occasionally fail with a 'RuntimeError: RuntimeError: Failure to create
rfnoc_graph' message printed to the console.

This commit preserves the desired queue-draining behavior, but ignores
any error status on the response when it is found and removed from the
queue, thus restoring the behavior pre-0caed5529.

1 files changed, 8 insertions, 0 deletions

diff --git a/host/lib/rfnoc/ctrlport_endpoint.cpp b/host/lib/rfnoc/ctrlport_endpoint.cpp
index 2078ecce1..3a569fae1 100644
--- a/host/lib/rfnoc/ctrlport_endpoint.cpp
+++ b/host/lib/rfnoc/ctrlport_endpoint.cpp
@@ -453,6 +453,14 @@ private:
             // Filter by op_code, address and seq_num
             if (rx_ctrl.seq_num == request.seq_num && rx_ctrl.op_code == request.op_code
                 && rx_ctrl.address == request.address) {
+                // If we find the response for our request, and the client
+                // has not requested acknowledgements, then simply return,
+                // ignoring any error status in the response. This restores
+                // the pre-0caed5529 behavior.
+                if (!require_ack) {
+                    return {};
+                }
+
                 // Validate transaction status
                 if (rx_ctrl.status == CMD_CMDERR) {
                     throw uhd::op_failed("Control operation returned a failing status");