Re: [PATCH v4] 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 8/31/21 10:34 PM, Pali Rohár wrote:
> On Tuesday 10 August 2021 21:49:28 Pali Rohár wrote:
>> Setting custom baudrate for which is not defined Bnnn constant is possible
>> via BOTHER flag and then filling speed in c_ospeed and c_ispeed fields.
>>
>> These two fields are either in struct termios or struct termios2. Former
>> belongs to TCGETS/TCSETS ioctls, latter to TCGETS2/TCSETS2 ioctls.
>>
>> BOTHER flag with these two fields and new struct termios2 is not supported
>> by older versions of include header files.
>>
>> 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).
>>
>> Setting input baudrate is done via IBSHIFT macro.
>>
>> Signed-off-by: Pali Rohár <pali@kernel.org>
> 
> Hello! Do you have any comments on this patch?

Sorry, I started to fix the licensing issues Greg pointed out, and 
forgot about this.

I assume it's good since you have been doing a lot of related work in 
recent patches.  I'll only point out a cosmetic issue with the 
preprocessor stuff:  I'd indent with a single space after '#' to clarify 
preprocessor #if #else #endif relationships and improve readability.  I 
also prefer '#if [!]defined()' rather than '#if[n]def' (except for 
include guards); and there seems to be a mix in the existing pages, so 
I'll standardize that way from now on.

For example:

#if !defined a
#else
# if defined b
# endif
#endif

However, I applied those changes myself in a following patch, so you 
don't worry about them.

Patch applied!

Thanks,

Alex

> 
>> ---
>> Changes in v4:
>> * Add SPDX-License-Identifier
>> * Correctly process split baudrates (separate output and input) via IBSHIFT
>> * Update commit message
>>
>> 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
>> ---
>>   man2/ioctl_tty.2 | 100 +++++++++++++++++++++++++++++++++++++++++++++++
>>   1 file changed, 100 insertions(+)
>>
>> diff --git a/man2/ioctl_tty.2 b/man2/ioctl_tty.2
>> index abfdc1e21fe9..7b2b03ff6757 100644
>> --- a/man2/ioctl_tty.2
>> +++ b/man2/ioctl_tty.2
>> @@ -796,6 +796,106 @@ main(void)
>>       close(fd);
>>   }
>>   .EE
>> +.PP
>> +Get or set arbitrary baudrate on the serial port.
>> +.PP
>> +.EX
>> +/* SPDX-License-Identifier: GPL-2.0-or-later */
>> +
>> +#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);
>> +#else
>> +    /* Declare tio structure, its type depends on supported ioctl */
>> +#ifdef TCGETS2
>> +    struct termios2 tio;
>> +#else
>> +    struct termios tio;
>> +#endif
>> +    int fd, rc;
>> +
>> +    if (argc != 2 && argc != 3 && argc != 4) {
>> +        fprintf(stderr, "Usage: %s device [output [input] ]\en", argv[0]);
>> +        exit(EXIT_FAILURE);
>> +    }
>> +
>> +    fd = open(argv[1], O_RDWR | O_NONBLOCK | O_NOCTTY);
>> +    if (fd < 0) {
>> +        perror("open");
>> +        exit(EXIT_FAILURE);
>> +    }
>> +
>> +    /* Get the current serial port settings via supported ioctl */
>> +#ifdef TCGETS2
>> +    rc = ioctl(fd, TCGETS2, &tio);
>> +#else
>> +    rc = ioctl(fd, TCGETS, &tio);
>> +#endif
>> +    if (rc) {
>> +        perror("TCGETS");
>> +        close(fd);
>> +        exit(EXIT_FAILURE);
>> +    }
>> +
>> +    /* Change baud rate when more arguments were provided */
>> +    if (argc == 3 || argc == 4) {
>> +        /* Clear the current output baud rate and fill a new value */
>> +        tio.c_cflag &= ~CBAUD;
>> +        tio.c_cflag |= BOTHER;
>> +        tio.c_ospeed = atoi(argv[2]);
>> +
>> +        /* Clear the current input baud rate and fill a new value */
>> +        tio.c_cflag &= ~(CBAUD << IBSHIFT);
>> +        tio.c_cflag |= BOTHER << IBSHIFT;
>> +        /* When 4th argument is not provided reuse output baud rate */
>> +        tio.c_ispeed = (argc == 4) ? atoi(argv[3]) : atoi(argv[2]);
>> +
>> +        /* Set new serial port settings via supported ioctl */
>> +#ifdef TCSETS2
>> +        rc = ioctl(fd, TCSETS2, &tio);
>> +#else
>> +        rc = ioctl(fd, TCSETS, &tio);
>> +#endif
>> +        if (rc) {
>> +            perror("TCSETS");
>> +            close(fd);
>> +            exit(EXIT_FAILURE);
>> +        }
>> +
>> +        /* And get new values which were really configured */
>> +#ifdef TCGETS2
>> +        rc = ioctl(fd, TCGETS2, &tio);
>> +#else
>> +        rc = ioctl(fd, TCGETS, &tio);
>> +#endif
>> +        if (rc) {
>> +            perror("TCGETS");
>> +            close(fd);
>> +            exit(EXIT_FAILURE);
>> +        }
>> +    }
>> +
>> +    close(fd);
>> +
>> +    printf("output baud rate: %u\en", tio.c_ospeed);
>> +    printf("input baud rate: %u\en", tio.c_ispeed);
>> +
>> +    exit(EXIT_SUCCESS);
>> +#endif
>> +}
>> +.EE
>>   .SH SEE ALSO
>>   .BR ldattach (1),
>>   .BR ioctl (2),
>> -- 
>> 2.20.1
>>


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