Re: [PATCH v3 2/5] tty: Introduce SER_RS485_SOFTWARE read-only flag for struct serial_rs485

Matwey V. Kornilov
2015-11-13T20:03:50Z

2015-11-12 22:57 GMT+03:00 One Thousand Gnomes <gnomes@lxorguk.ukuu.org.uk>:
> On Thu, 12 Nov 2015 17:33:53 +0300
> "Matwey V. Kornilov" <matwey@sai.msu.ru> wrote:
>
>> This flag is supposed to be used by uart drivers using software rs485 direction control.
>>
>> Signed-off-by: Matwey V. Kornilov <matwey@sai.msu.ru>
>> ---
>> include/uapi/linux/serial.h | 3 +++
>> 1 file changed, 3 insertions(+)
>>
>> diff --git a/include/uapi/linux/serial.h b/include/uapi/linux/serial.h
>> index 25331f9..95b15ca 100644
>> --- a/include/uapi/linux/serial.h
>> +++ b/include/uapi/linux/serial.h
>> @@ -121,6 +121,9 @@ struct serial_rs485 {
>> #define SER_RS485_RTS_AFTER_SEND (1 << 2) /* Logical level for
>> RTS pin after sent*/
>> #define SER_RS485_RX_DURING_TX (1 << 4)
>> +#define SER_RS485_SOFTWARE (1 << 5) /* Software
>> + implementation is
>> + being used */
>
> I've only got one question here - why do we need this flag. Why does the
> application care whether the timer is in the kernel or in the chip. In
> particular think about cases where some combinations of features require
> software fallback and others don't. What would the flag indicate then.
>

Peter asked for it, I respect his experience.
Only two lines are required to implement this, so it is easy to add,
easy to drop.

> The patches look nice but I'd strongly favour not having a software flag.
> It should never matter as the kernel API is the same in all cases and we
> should therefore discourage application code from trying to know things
> it doesn't need to worry about.
>
> Alan
>

--
With best regards,
Matwey V. Kornilov.
Sternberg Astronomical Institute, Lomonosov Moscow State University, Russia
119991, Moscow, Universitetsky pr-k 13, +7 (495) 9392382