On 21. 05. 24, 16:15, Dianne Skoll wrote:
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?
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
