iMX8M set serial baudrate in Linux

[gavinl]

We would like to take advantage of the high baudrate that iMX8M supports in Linux. What I found out is the stty can only set baudrate to 921600 (stty -F /dev/ttymxc0 921600), it can't set any baudrate higher than it.
I tried to use devmem to directly set /dev/ttymxc0, but the write value doesn't match the read back value. Please help give me a clue. Thanks

echo "devmem write/read to verify"
devmem 0x30860080 32 1
devmem 0x30860084 32 0x4127
devmem 0x3086008c 32 0x7c00
devmem 0x30860090 32 0xa9e
devmem 0x308600a4 32 3
devmem 0x308600a8 32 4
devmem 0x30860080 32 0x2001
devmem 0x308600b8 32 0xa50b
echo "----------"
devmem 0x30860080
devmem 0x30860084
devmem 0x3086008c
devmem 0x30860090
devmem 0x308600a4
devmem 0x308600a8
devmem 0x30860080
devmem 0x308600b8

[nxf63675]

Hi Gavinl,

Modify the baud rate from userspace is not possible, this need to be modifying on the kernel from the core code, you may find helpful the next post:

https://community.nxp.com/t5/i-MX-Processors-Knowledge-Base/Building-Linux-Kernel/ta-p/1110576#toc-h...

Hope this helps you.

Regards,

Israel.

[gavinl]

I know I can change it in kernel space, but don't understand why can't I modified NXP registers in userspace, I can do it in the ARM production from other companies. What makes NXP products can't be modified in userspace?

[gavinl]

Found the supported baudrate in drivers/tty/tty_baudrate.c

static const speed_t baud_table[] = {
	0, 50, 75, 110, 134, 150, 200, 300, 600, 1200, 1800, 2400,
	4800, 9600, 19200, 38400, 57600, 115200, 230400, 460800,
#ifdef __sparc__
	76800, 153600, 307200, 614400, 921600, 500000, 576000,
	1000000, 1152000, 1500000, 2000000
#else
	500000, 576000, 921600, 1000000, 1152000, 1500000, 2000000,
	2500000, 3000000, 3500000, 4000000
#endif
};

But still don't understand why devmem can't write to memory mapped registers

[gavinl]

with stty -F /dev/ttymxc0 the_baudrate, I can use devmem to verify the UBIR and UBMR registers are programmed.
Don't know why devmem can't write to those registers, I even write a Linux user side program to write memory mapped registers and I can't write to those registers also.
Is there anyone know the reason? I have my program attached.