How to blacklist built-in Kernel module 8250_pci

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If the 8250_pci driver is built into the kernel, modprobe.blacklist will have no effect at all to it, since modprobe will not be involved at all.

Instead, you could pipe the PCI bus ID of the device in question to /sys/bus/pci/drivers/serial_pci/unbind to unplug the driver from a particular device. (The pathname might be .../serial/unbind instead, I cannot check at the moment.)

Example:

echo 0000:03:00.0 > /sys/bus/pci/drivers/serial_pci/unbind

If there is a remove_id in the same directory, you might also pipe the vendor & product IDs (use lspci -nn to see them) of the device in question into it, so the driver will not be picking the device up again if the serial device auto-detection is triggered for any reason.

Example:

echo "1415 c158" > /sys/bus/pci/drivers/serial_pci/remove_id

Once you find the right value(s) to pipe into the pseudo-file(s), you might want to add them into /etc/rc.local or write your own small start-up script (or a .service file if applicable) to run the commands at every boot.