On Tue, 21 May 2024 12:47:00 +0200 Jiri Slaby <jirislaby@xxxxxxxxxx> wrote: > I believe it is a correct behavior after all. As you use pty for > testing, the "framing" is lost during the pty-to-pty pass on the > flush to ldisc path (receive_buf()). That might be what's happening, but I don't think it matches the documentation in n_hdlc.c. If you read the comment block near the top of the file, it says this: * All HDLC data is frame oriented which means: * * 1. tty write calls represent one complete transmit frame of data * The device driver should accept the complete frame or none of * the frame (busy) in the write method. Each write call should have * a byte count in the range of 2-65535 bytes (2 is min HDLC frame * with 1 addr byte and 1 ctrl byte). The max byte count of 65535 * should include any crc bytes required. For example, when using * CCITT CRC32, 4 crc bytes are required, so the maximum size frame * the application may transmit is limited to 65531 bytes. For CCITT * CRC16, the maximum application frame size would be 65533. * * * 2. receive callbacks from the device driver represents * one received frame. The device driver should bypass * the tty flip buffer and call the line discipline receive * callback directly to avoid fragmenting or concatenating * multiple frames into a single receive callback. * * The HDLC line discipline queues the receive frames in separate * buffers so complete receive frames can be returned by the * tty read calls. * * 3. tty read calls returns an entire frame of data or nothing. [...] */ Point 2 says that the driver should avoid fragmenting frames, or concatenating frames into a single receive callback. Doesn't this imply that frame boundaries should be preserved when you read() data, which happens reliably when you add a small delay between write()'s? Regards, Dianne.
