Hi, I'm reposting here (originally emailed GregKH and Jiri Slaby directly as they are listed as tty maintainers) as well as responding to a reply from Greg. Original mail: Some people have been reporting bugs using synchronous PPP with the rp-pppoe user-space program. See for example https://github.com/dfskoll/rp-pppoe/issues/32 I've narrowed this down to the N_HDLC line discipline sometimes concatenating two packets on the write side into a single read on the other side. I have attached a proof-of-concept program illustrating the problem. If you run: ./test_n_hdlc with no arguments, then the program makes two writes in quick succession to the tty followed by a read. It generally only takes one or two tries on my computer before I see both writes being combined in a single read. On the other hand, if you run: ./test_n_hdlc foo with a single argument, then the program sleeps for 0.1s between writes, and I never see them being combined in a single read, even after 20 tries. I'm running mainline kernel 6.8.7 on amd64; the processor model name per cpuinfo is: AMD Ryzen Threadripper 3970X 32-Core Processor Regards, Dianne. Greg's reply with my responses inline: > What is wrong with that? Does the N_HDLC line discipline somewhere > state that this is not possible to happen? Normal write combining > happens for other tty ldiscs. Userspace should be able to handle > this, unless again, the N_HDLC ldisc somehow says that this should > never happen. > Ok, I looked at the comments at the top of the the ldisc, and it says > this should not happen, so something is odd, I agree. Right. It's done that way so a PPP 'pty' helper always gets exactly one frame when it reads from the tty file dscriptor. > Did this change recently? Or has this always been the case? Meaning > did something in the kernel change to cause this to break? I haven't used this mode of PPP in over a decade, so I know it used to work, but no longer does. I'm sorry I can't narrow down more precisely as to when it stopped working. > Also, please cc: the linux-serial list for tty issues, doing stuff in > private is generally not a good idea. OK, sorry about that... I misread the bug reporting instructions and forgot to Cc the lists. Regards, Dianne.
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