On 14/04/2017 16:28, Rodolfo Giometti wrote:
> On 04/14/17 16:24, Richard Genoud wrote:
>> On 13/04/2017 17:12, Rodolfo Giometti wrote:
>>> On 04/13/17 16:53, Richard Genoud wrote:
>>>> On 13/04/2017 14:19, Rodolfo Giometti wrote:
>>>>> On 04/13/17 12:43, Richard Genoud wrote:
>>>>>> On 11/04/2017 22:12, Rodolfo Giometti wrote:
>>>>>>> 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.
>>>>>>>
>>>>>>> Signed-off-by: Rodolfo Giometti <giometti@xxxxxxxx>
>>>>>>> ---
>>>>>>> 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 83b264c..85238ff 100644
>>>>>>> --- a/include/linux/tty.h
>>>>>>> +++ b/include/linux/tty.h
>>>>>>> @@ -166,6 +166,8 @@ 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 232b478..7e82859 100644
>>>>>>> --- a/include/uapi/asm-generic/termbits.h
>>>>>>> +++ b/include/uapi/asm-generic/termbits.h
>>>>>>> @@ -140,6 +140,8 @@ struct ktermios {
>>>>>>> #define HUPCL 0002000
>>>>>>> #define CLOCAL 0004000
>>>>>>> #define CBAUDEX 0010000
>>>>>>> +#define PARMD 0100000
>>>>>>> +#define SENDA 0200000
>>>>>>> #define BOTHER 0010000
>>>>>>> #define B57600 0010001
>>>>>>> #define B115200 0010002
>>>>>>>
>>>>>> I guess there's a problem with SENDA (bit 16)
>>>>>> It's overlapping with CIBAUD (002003600000 (bits 28,19,18,17,16))
>>>>>
>>>>> You're right! In this case which bit should I use for SENDA? :(
>>>>>
>>>>> Since it is referred to output data maybe can we move it into c_oflag
>>>>> bits?
>>>>
>>>> Well, bits 23 and 24 are free in asm-generic/c_cflag, and since
>>>> multidrop uses the parity bit, I'd say it fits better in c_cflag
>>>> than in
>>>> c_oflag.
>>>
>>> I see, but while PARMD defines a special parity usage the SENDA bit is
>>> dedicated to output data only... however if you prefer I use bits 23 and
>>> 24 it's OK for me! :-)
>>>
>>>> But honestly, I find the SENDA bit logic disturbing and not compliant
>>>> with the way tc{g,s}etattr() works.
>>>>
>>>> Because, usually, a tcsetattr() call is followed by a tcgetattr() call
>>>> to check if everything went well. (tcsetattr return 0 if *any* of the
>>>> requested change succeeded).
>>>> So, with the proposed implementation, we will have
>>>
>>>> struct termios term, check;
>>>>
>>>> ret = tcgetattr(fd, &term);
>>>> if (ret < 0) {
>>>> perror("tcgetattr");
>>>> exit(-1);
>>>> }
>>>> term.c_cflag |= PARENB | CMSPAR | PARMD | SENDA;
>>>> ret = tcsetattr(fd, TCSADRAIN, &term);
>>>> if (ret < 0) {
>>>> perror("tcsetattr");
>>>> exit(-1);
>>>> }
>>>> ret = tcgetattr(fd, &check);
>>>> if (term.c_cflag != check.c_cflag) {
>>>> /* fail to set all c_cflags */
>>>> exit(-1);
>>>> }
>>>> => it will always fail.
>>>
>>> This should be another reason because we should use c_oflag instead of
>>> c_cflag! :-P
>> Actually it isn't, I gave the example of c_cflag, but it's the same with
>> c_{i,o,l}flag
>
> I see... so any advice about how to manage SENDA bit in order to avoid
> this drawback?
I'm not a tty guru, but I think the proper way to implement this would
be with a new line discipline.
cf:
http://www.linux-mag.com/id/1891/
http://www.linusakesson.net/programming/tty/
Regards,
Richard.
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