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linuxlinux-kernellinux-device-drivertty

Linux tty flip buffer lock when reading part of available data

I have a driver that builds on the new serdev bus in the linux kernel.

In my driver I receive messages from an external device, all messages ends with a null byte (0x00) and the protocol ensures that there are no null bytes in my data (COBS). Now I try to have the TTY layer hand me full messages by scanning for zeros in my input and if there are none I'll just return zero in the callback that is called from the tty layer when bytes are available.

This kind of works. Or rather it works for some messages. After a while though it locks up and the tty layer keeps sending the same size of received bytes indefinitely. My guess is that this happens when one half of the tty flip buffer is full and the rest of my message is in the other half.

I have two questions:

  1. Am I correct in that the tty layer can "hang" until I read out all data in one half of the flip buffer?

  2. If that is so, is there some way to prevent this from happening? I'd rather not implement my own buffering scheme on top of the tty buffer already available.

Thanks

Solution

  • It looks like (drivers/tty/tty_buffer.c and the function flush_to_ldisc) that it is not possible to do what I attempted to do. When the tty buffer is about to flip over the consumer will have to do a read and buffer any half messages.

    That is, returning zero and hoping for a larger chunk of data in your callback next time will only work up until the end of the first part of the buffer then the last bit of data must be read.

    This is not a problem in userspace because a read call will have an argument that is the most bytes you want but read is free to return fewer bytes than requested.