Re: [PATCH 1/2] tty: add bits to manage multidrop mode

[Richard Genoud <richard.genoud@gmail.com>]

Hi Rodolfo,

You could also add these mailing lists:
- linux-serial@vger.kernel.org
- linux-arm-kernel@lists.infradead.org
- linux-kernel@vger.kernel.org

For reference, I've dug the conversation we had 2 years ago:
0/2 : https://marc.info/?t=149192176100002&r=1&w=2
1/2 : https://marc.info/?t=149192176300001&r=1&w=2
2/2 : https://marc.info/?t=149192176700001&r=1&w=2

And, like I said at that time, one problem I see with this
implementation (setting a SENDA bit that automatically unsets itself)
will break the way tcgetattr() is working:
https://marc.info/?l=linux-serial&m=149209522108027&w=2


Le 12/09/2019 à 10:40, Rodolfo Giometti a écrit :
> From: Rodolfo Giometti <giometti@linux.it>
> 
> Multidrop mode differentiates the data characters and the address
> characters. Data is transmitted with the parity bit to 0 and addresses
> are transmitted with the parity bit to 1. However this usually slow
> down communication by adding a delay between the first byte and the
> others.
> 
> This patch defines two non-stadard bits PARMD (that enables multidrop)
> and SENDA (that marks the next transmitted byte as address) that can
> be used to completely remove the delay during transmission by
> correctly managing the parity bit generation in hardware.
> 
> A simple example code about how to set up it is reported below:
> 
>     struct termios term;
> 
>     tcgetattr(fd, &term);
> 
>     /* Transmission: enable parity multidrop and mark 1st byte as address */
>     term.c_cflag |= PARENB | CMSPAR | PARMD | SENDA;
>     /* Reception: enable parity multidrop and parity check */
>     term.c_iflag |= PARENB | PARMD | INPCK;
> 
>     tcsetattr(fd, TCSADRAIN, &term);
> 
> After that we can start 9 bits data transmission.
> 
> Signed-off-by: Rodolfo Giometti <giometti@linux.it>
> Signed-off-by: Joshua Henderson <joshua.henderson@microchip.com>
> ---
>  include/linux/tty.h                 | 2 ++
>  include/uapi/asm-generic/termbits.h | 2 ++
>  2 files changed, 4 insertions(+)
> 
> diff --git a/include/linux/tty.h b/include/linux/tty.h
> index bfa4e2ee94a9..66a25294f125 100644
> --- a/include/linux/tty.h
> +++ b/include/linux/tty.h
> @@ -168,6 +168,8 @@ struct tty_bufhead {
>  #define C_CIBAUD(tty)	_C_FLAG((tty), CIBAUD)
>  #define C_CRTSCTS(tty)	_C_FLAG((tty), CRTSCTS)
>  #define C_CMSPAR(tty)	_C_FLAG((tty), CMSPAR)
> +#define C_PARMD(tty)	_C_FLAG((tty), PARMD)
> +#define C_SENDA(tty)	_C_FLAG((tty), SENDA)
>  
>  #define L_ISIG(tty)	_L_FLAG((tty), ISIG)
>  #define L_ICANON(tty)	_L_FLAG((tty), ICANON)
> diff --git a/include/uapi/asm-generic/termbits.h b/include/uapi/asm-generic/termbits.h
> index 2fbaf9ae89dd..ead5eaebdd3b 100644
> --- a/include/uapi/asm-generic/termbits.h
> +++ b/include/uapi/asm-generic/termbits.h
> @@ -141,6 +141,8 @@ struct ktermios {
>  #define HUPCL	0002000
>  #define CLOCAL	0004000
>  #define CBAUDEX 0010000
> +#define PARMD   040000000
> +#define SENDA   0100000000
>  #define    BOTHER 0010000
>  #define    B57600 0010001
>  #define   B115200 0010002
>