Re: [Qemu-devel] Two question on serial port emulation

[Fabrice Bellard <fabrice@bellard.org>]

Jiang, Yunhong wrote:
> Hi, all
>     I'm using qemu on linux, and I have two questions on the serial port
> emulation. The serial port means the hardware serial port (/dev/ttyS0 on
> Linux).
> 
>     1) Currently tty_serial_init()  set the c_oflags as following:
>         tty.c_oflag |= OPOST
>   but it didn't clear the ONLCR/ONLRET etc. This may cause some problem
> on binary transfer. For example, when transfer "0x0d 0x0a" for "0x0a".
> Following patch may fix this problem, although I'm not sure if we clear
> OPOST, do we still need to clear ONLCR/OCRNL etc bits.
>  
> --- vl.c        2006-05-04 04:32:58.000000000 +0800
> +++ ../qemu-0.8.1_bogus/vl.c    2006-06-01 22:34:09.000000000 +0800
> @@ -1586,7 +1586,8 @@
>  
>      tty.c_iflag &= ~(IGNBRK|BRKINT|PARMRK|ISTRIP
>                            |INLCR|IGNCR|ICRNL|IXON);
> -    tty.c_oflag |= OPOST;
> +    tty.c_oflag &= ~(ONLCR | OCRNL | ONOCR |ONLRET) ;
> +    tty.c_oflag &= ~OPOST;
>      tty.c_lflag &= ~(ECHO|ECHONL|ICANON|IEXTEN|ISIG);
>      tty.c_cflag &= ~(CSIZE|PARENB|PARODD|CRTSCTS);
>      switch(data_bits) {

So forcing all the flags to known values may be even better !

> 2) Currently all write to the serial port will through unix_write().
> Considering : a) if the return value is -EAGAIN, it will not try to
> re-write this again. b) all write will return immetiately. So when guest
> try to write a lot of data continuously, like using zmodem to transfer
> file or using serial port for debugger, it may cause application failure
> (the data is write successfuly from guest point of view, while it failed
> on the host environment). I changed the write process to multi-thread,
> but I don't think it is a good method, so how do you think of these?

You should use the callback based I/Os. An output FIFO must be 
maintained in the serial driver itself. Unfortunately an API is missing 
on the QEMU char devices to do proper asynchronous outputs.

Fabrice.