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Many budget GSM/LTE modems expose only a single serial port for both commands and notifications, unlike Huawei devices that provide separate command and notification ports. When NotifyPort is set to the same value as CommandPort, Open() now assigns notifyPort = cmdPort so that sanityCheck passes. Callers should skip Watch() in this configuration to avoid read conflicts on the shared file descriptor.
JimScope
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Mar 23, 2026
Mark both xlab/at vendor patches as temporary with links to the upstream PRs that will make them unnecessary: - commands.go: Export Dev field (xlab/at#43) - at.go: Single-port support (xlab/at#42)
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Summary
NotifyPort == CommandPort, aliasnotifyPorttocmdPortinOpen()so thatsanityCheckpassesMotivation
Many budget GSM/LTE modems (e.g. SIM800, Quectel EC25 in single-port mode) expose only one serial port. Currently, if
NotifyPortis set equal toCommandPort,Open()skips opening the notification port, leavingnotifyPortasnil. This causessanityCheckto returnErrClosed, makingSend()andInit()fail.Change
In
Open(), after the dual-port branch, add:```go
} else if d.NotifyPort != "" {
d.notifyPort = d.cmdPort
}
```
This aliases the notification port to the command port fd, allowing
sanityCheckto pass. Callers should not callWatch()in this configuration, since reading from the same fd in two goroutines would cause race conditions.Backward Compatibility
No behavioral change for existing dual-port configurations. Only affects the case where
NotifyPort == CommandPort, which previously failed withErrClosed.