linux: implement arbitrary and split speeds in termios

Linux has supported arbitrary speeds and split speeds in the kernel
since 2008 on all platforms except Alpha (fixed in 2020), but glibc
was never updated to match. This is further complicated by POSIX uses
of macros for the cf[gs]et[io]speed interfaces, rather than plain
numbers, as it really ought to have.

On most platforms, the glibc ABI includes the c_[io]speed fields in
struct termios, but they are incorrectly used. On MIPS and SPARC, they
are entirely missing.

For backwards compatibility, the kernel will still use the legacy
speed fields unless they are set to BOTHER, and will use the legacy
output speed as the input speed if the latter is 0 (== B0). However,
the specific encoding used is visible to user space applications,
including ones other than the one running.

- SPARC and MIPS get a new struct termios, and tc[gs]etattr() is
  versioned accordingly. However, the new struct termios is set to be
  a strict extension of the old one, which means that cf* interfaces
  other than the speed-related ones do not need versioning.
- The Bxxx constants are redefined as equivalent to their integer
  values and the legacy Bxxx constants are renamed __Bxxx.
- cf[gs]et[io]speed() and cfsetspeed() are versioned accordingly.
- tcgetattr() and cfset[io]speed() are adjusted to always keep the
  c_[io]speed fields correct (unlike earlier versions), but to
  canonicalize the representation to ALSO configure the legacy fields
  if a valid legacy representation exists.
- tcsetattr(), too, canonicalizes the representation in this way
  before passing it to the kernel, to maximize compatibility with
  older applications/tools.
- The old IBAUD0 hack is removed; it is no longer necessary since
  even the legacy c_cflag baud rate fields have had separate input
  values for a long time.

Signed-off-by: H. Peter Anvin (Intel) <hpa@zytor.com>
Reviewed-by: Adhemerval Zanella  <adhemerval.zanella@linaro.org>

sysdeps/unix/sysv/linux/bits/termios.h

@@ -24,35 +24,41 @@ typedef unsigned char	cc_t;
 typedef unsigned int	speed_t;
 typedef unsigned int	tcflag_t;
 
-#include <bits/termios-struct.h>
+#ifdef _TERMIOS_H
+# include <bits/termios-struct.h>
+#endif
+
 #include <bits/termios-c_cc.h>
 #include <bits/termios-c_iflag.h>
 #include <bits/termios-c_oflag.h>
 
 /* c_cflag bit meaning */
-#define  B0	0000000		/* hang up */
-#define  B50	0000001
-#define  B75	0000002
-#define  B110	0000003
-#define  B134	0000004
-#define  B150	0000005
-#define  B200	0000006
-#define  B300	0000007
-#define  B600	0000010
-#define  B1200	0000011
-#define  B1800	0000012
-#define  B2400	0000013
-#define  B4800	0000014
-#define  B9600	0000015
-#define  B19200	0000016
-#define  B38400	0000017
-#ifdef __USE_MISC
-# define EXTA B19200
-# define EXTB B38400
-#endif
-#include <bits/termios-baud.h>
-
 #include <bits/termios-c_cflag.h>
+
+#define __B0	 0000000	/* hang up */
+#define __B50	 0000001
+#define __B75	 0000002
+#define __B110	 0000003
+#define __B134	 0000004
+#define __B150	 0000005
+#define __B200	 0000006
+#define __B300	 0000007
+#define __B600	 0000010
+#define __B1200	 0000011
+#define __B1800	 0000012
+#define __B2400	 0000013
+#define __B4800	 0000014
+#define __B9600  0000015
+#define __B19200 0000016
+#define __B38400 0000017
+#include <bits/termios-cbaud.h>
+
+#ifdef __USE_MISC
+# define EXTA	 __B19200
+# define EXTB	 __B38400
+# define BOTHER  __BOTHER
+#endif
+
 #include <bits/termios-c_lflag.h>
 
 #ifdef __USE_MISC
@@ -74,3 +80,5 @@ typedef unsigned int	tcflag_t;
 #include <bits/termios-tcflow.h>
 
 #include <bits/termios-misc.h>
+
+#include <bits/termios-baud.h>