Re: [PATCH v3] ioctl_tty.2: Add example how to get or set baudrate on the serial port

["Alejandro Colomar (man-pages)" <alx.manpages@gmail.com>]

Hi Pali,

On 8/1/21 3:51 PM, Pali Rohár wrote:
> Signed-off-by: Pali Rohár <pali@kernel.org>
> 
> ---
> Changes in v3:
> * Check support for custom baudrate only based on BOTHER macro
> * Use TCGETS/TCSETS/termios when TCGETS2/TCSETS2/termios2 is not available
> 
> Changes in v2:
> * Use \e for backslash
> * Use exit(EXIT_*) instead of return num
> * Sort includes
> * Add comment about possible fallback
> ---
> 
> Hello Alejandro!
> 
> I found out that this stuff is more complicated as I originally thought.
> And seems that additional documentation on this topic is needed...
> 
> For setting custom baudrate it is needed to set BOTHER flag in c_cflag
> field and baudrate value itself in c_ospeed and c_ispeed fields.
> 
> So when BOTHER flag is not provided by <asm/termbits.h> then setting custom
> baudrate is not possible, fields c_ospeed and c_ispeed do not exist (and
> only some predefined Bnnn baudrate values are supported). This applies when
> compiling application with older version of header files (prior support for
> custom baudrate was introduced into header files).
> 
> First caveat: BOTHER constant is different for different architectures.
> So it is not possible to provide fallback #ifndef..#define BOTHER.
> 
> And now the biggest issue: Some architectures have these c_ospeed and
> c_ispeed fields in struct termios and some in struct termios2.
> 
> TCGETS/TCSETS ioctls use struct termios and TCGETS/TCSETS2 use
> struct termios2.
> 
> Some architectures (e.g. amd64) provide both struct termios and struct
> termios2, but c_ospeed and c_ispeed are only in struct termios2.
> 
> Some other architectures (e.g. alpha) provide both struct termios and struct
> termios2 and both have c_ospeed and c_ispeed fields.
> 
> And some other architectures (e.g. powerpc) provide only struct termios
> (no struct termios2) and it has c_ospeed and c_ispeed fields.
> 
> So basically to support all architectures it is needed to use
> struct termios2 when TCGETS2/TCSETS2 is supported. Otherwise it is needed
> to use struct termios with TCGETS/TCSETS (case for e.g. powerpc).

When you send v4, please include the above text (or something similar) 
to the commit message.

Thanks,

Alex

> 
> I updated v3 patch to handle this logic.
> ---


-- 
Alejandro Colomar
Linux man-pages comaintainer; https://www.kernel.org/doc/man-pages/
http://www.alejandro-colomar.es/