[Copied on requst from https://bugzilla.kernel.org/show_bug.cgi?id=205033] It seems that a lot of Linux kernel USB serial device drivers are ignoring the CREAD setting of termios.c_cflag. The man page is quite clear: CREAD Enable receiver. The glibc man page at https://ftp.gnu.org/old-gnu/Manuals/glibc-2.2.3/html_chapter/libc_17.html states: "Macro: tcflag_t CREAD If this bit is set, input can be read from the terminal. Otherwise, input is discarded when it arrives." When I mask this bit and then call tcsetattr(), I'm still receiving input characters, at least on a pl2303 USB UART. Looking at the source code of drivers/usb/serial/, the *only* driver appearing to respect CREAD is digi_acceleport.c. All others seem to ignore it. To the contrary, most classic serial drivers in drivers/tty/serial seem to implement it. In absence of low-level support in individual drivers to actually disable the receiver in hardware, I would have at least expected the core tty/serial layer to drop/discard any characters received by the hardware while CREAD is not set. But that also doesn't appear to be the case. What's even more worrying is that the tcsetattr() call succeeds, i.e. it is a silent error. I would expect the kernel to either implement the functionalty in one way or another, or simply return tcsetattr() with an error if an unsupported combination (i.e. CFLAG not set) is configured. This is not a theoretical issue. Anyone implementing a half-duplex protocol with shared Rx and Tx line will face the same issue. Am I missing something here? Please don't tell me that I just discovered something that's broken for some 20-odd years, or at the very least as far as normal linux.git history reaches back :/ Please keep me in Cc of any responses, I'm not subscribed to linux-usb. Thanks! Regards, Harald -- - Harald Welte <laforge@xxxxxxxxxxxx> http://laforge.gnumonks.org/ ============================================================================ "Privacy in residential applications is a desirable marketing option." (ETSI EN 300 175-7 Ch. A6)
