[Bug 235016] ASIX AX99100 based uart controllers broken on 12.0

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=235016

--- Comment #2 from david at dcrosstech.com ---
Created attachment 221648
  --> https://bugs.freebsd.org/bugzilla/attachment.cgi?id=221648&action=edit
Patch to fix uart_bus_pci.c

This is my working patch.  After consulting with the pci_alloc_msi and freebsd
MSI page as well as looking at other drivers I think this is probably the
'best' patch at this point.

Given: https://people.freebsd.org/~grehan/msi_api.txt  (yes, it looks a bit
dated), that makes it explicitly clear that if you get back fewer MSI resources
than the device expects the driver is responsible for ensuring the device
doesn't generate MSI greater than has been allocated.. There is no such code in
the UART space.  Additionally there are numerous other drivers that expect a
single MSI count and if they get it establish MSI for that, and if not fall
back to legacy IRQ.

I couldn't get anything resembling "real" documentation out of ASIX for what is
going on, but given a few facts: That MSI interrupts must be allocated in
powers of 2, that NO MSI interrupts are visible on this device when it is
enabled (that I have ben able to find), and there are 5 UART interrupt
registers (tx, rx, sigchange, overrun, break) I am *guessing* they map 5 MSI
interrupts to each of those individual serial interrupt registers.  I briefly
contemplated attempting to wire these directly to the uart_intr_* calls, or all
of them to uart_intr itself, but there were too many questions about the right
way to pass this along, to abstract it, and the concurrency (multiple
interrupts firing at once on different cores).. given that, i think this is the
best patch.

Thanks!

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