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b1691bd049
Contrary to what SMSC documentation says and unlike NS16C550A UARTs the SMSC Super I/O IC claims compatibility with the SMSC UART implementation does not support dividing the internal PLL clock by any divisor from 1 to 65535[1], with the exception of two magic divisors of 32769 and 32770 used respectively to select the high-speed data rates of 460800bps and 230400bps[2] if enabled on a port-by-port basis in with the Serial Port Mode Register in the Device Configuration Space[3][4]. Instead empirical evidence indicates that the divisor, as stored in the DLL and DLM register pair, has the range of 1 to 32767 only, and bit 7 of the DLM register (bit 15 of the divisor) effectively serves as a selection bit for the prescaler from the base frequency of 7.3728MHz, either 4 if the bit is 0, or 1 if the bit is 1 and high-speed operation has been enabled with the Serial Port Mode Register. So if high-speed operation has not been enabled, then say the values of 1 and 32769 (0x8001) written into the combined DLL and DLM register pair both select the divisor of 1 and the baud rate of 115200bps. [1] "FDC37M81x, PC98/99 Compliant Enhanced Super I/O Controller with Keyboard/Mouse Wake-Up", Standard Microsystems Corporation, Rev. 03/27/2000, Section "Programmable Baud Rate Generator (and Divisor Latches DLH, DLL)", p. 75 [2] same, Table 31 - "Baud Rates", p. 77 [3] same, Table 60 - "Serial Port 1, Logical Device 4 [Logical Device Number = 0x04]", p. 153 [4] same, Table 61 - "Serial Port 2, Logical Device 5 [Logical Device Number = 0x05]", p. 153 Signed-off-by: Maciej W. Rozycki <macro@orcam.me.uk> Link: https://lore.kernel.org/r/alpine.DEB.2.21.2106092330530.5469@angie.orcam.me.uk Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org> |
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.. | ||
hvc | ||
ipwireless | ||
serdev | ||
serial | ||
vt | ||
amiserial.c | ||
ehv_bytechan.c | ||
goldfish.c | ||
Kconfig | ||
Makefile | ||
mips_ejtag_fdc.c | ||
moxa.c | ||
moxa.h | ||
mxser.c | ||
mxser.h | ||
n_gsm.c | ||
n_hdlc.c | ||
n_null.c | ||
n_tty.c | ||
nozomi.c | ||
pty.c | ||
synclink_gt.c | ||
sysrq.c | ||
tty.h | ||
tty_audit.c | ||
tty_baudrate.c | ||
tty_buffer.c | ||
tty_io.c | ||
tty_ioctl.c | ||
tty_jobctrl.c | ||
tty_ldisc.c | ||
tty_ldsem.c | ||
tty_mutex.c | ||
tty_port.c | ||
ttynull.c | ||
vcc.c |