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Kernel lockup/hang when attempting to reuse console UART

Question asked by Seth Richards on Jan 30, 2015
Latest reply on Nov 7, 2017 by Fabio Estevam



I'm integrating a Linux 3.0.35 kernel on an i.MX6D platform (based loosely on the SABRE SDB design), and am encountering a kernel hang/lockup related to the UARTs. We're attempting to disable the serial console by specifying "console=" on the u-boot kernel command line, and then reuse the serial port for other purposes. However, we occasionally encounter locks or hangs when we attempt to open the UART from userspace.


It appears to be very similar to the problem described in this thread, but I don't see a clear resolution there:


I've attempted debugging this issue by adding a USB-serial converter to the platform and using that as a serial console, and it looks like it may be getting hung up in the imx_startup function in the i.MX serial driver; it appears that things may go off the rails during configuration of the UARTs registers. I dumped the registers to the USB console and it appears there are some leftover configuration changes from either U-boot or from kernel console setup that are apparently causing problems.


I attempted resetting the UART by writing to the SRST bit in UCR2 and it doesn't seem to have any effect. Does this bit not clear out the config registers and any interrupts? Has anyone else run into similar problems?