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

["Pali Rohár via Libc-alpha" <libc-alpha@sourceware.org>]

On Thursday 05 August 2021 07:52:03 Greg Kroah-Hartman wrote:
> On Thu, Aug 05, 2021 at 12:08:08AM +0200, Pali Rohár wrote:
> > + linux-serial
> > + Greg
> > 
> > Greg, could I ask you for reviewing this documentation manpage patch?
> 
> If it is submitted in a format I can review, sure (i.e. not top-post...)
> 
> But I will dig down below to say one thing...
> 
> > 
> > On Sunday 01 August 2021 15:51:45 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).
> > > 
> > > I updated v3 patch to handle this logic.
> > > ---
> > >  man2/ioctl_tty.2 | 73 ++++++++++++++++++++++++++++++++++++++++++++++++
> > >  1 file changed, 73 insertions(+)
> > > 
> > > diff --git a/man2/ioctl_tty.2 b/man2/ioctl_tty.2
> > > index 3020f9984872..d83cbd17225b 100644
> > > --- a/man2/ioctl_tty.2
> > > +++ b/man2/ioctl_tty.2
> > > @@ -764,6 +764,79 @@ main(void)
> > >      close(fd);
> > >  }
> > >  .EE
> > > +.PP
> > > +Get or set arbitrary baudrate on the serial port.
> > > +.PP
> > > +.EX
> > > +#include <asm/termbits.h>
> > > +#include <fcntl.h>
> > > +#include <stdio.h>
> > > +#include <stdlib.h>
> > > +#include <sys/ioctl.h>
> > > +#include <sys/types.h>
> > > +#include <unistd.h>
> > > +
> > > +int
> > > +main(int argc, char *argv[])
> > > +{
> > > +#ifndef BOTHER
> > > +    fprintf(stderr, "BOTHER is unsupported\en");
> > > +    /* Program may fallback to TCGETS/TCSETS with Bnnn constants */
> > > +    exit(EXIT_FAILURE);
> 
> So this is a BOTHER test only?

Yes.

> What is the goal of this program?  Don't throw a bunch of #ifdef in here
> for no good reason.  These options should all be present on all normal
> kernels, why wouldn't they be?

I wanted to provide complete example which compiles fine on all Linux
systems, even with older include header files. I do not know right now
in which kernel version was introduced BOTHER support for all
architectures.

If BOHTER is not supported then it is possible to still use Bnnn
constants to get / set baudrate. Just it is needed to write long code
for converting number to suitable Bnnn constant.

Do you think that this BOTHER check is not useful in this case?

> thanks,
> 
> greg k-h