On 21. 05. 24, 16:15, Dianne Skoll wrote:
> On Tue, 21 May 2024 12:47:00 +0200
> Jiri Slaby <[email protected]> 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?
The driver definitely behaves as described. If the ldisc is used on a
real HW. ptys are a different story -- it's not guaranteed there. Does
it make sense to use nhdlc on a pty pair? I believe not.
thanks,
--
js
suse labs