Slow performance of VUART on AST2500 with 5.15.5

[Oskar Senft]

Hi everyone

I'm doing some more validation work with meta-tyan
(https://gerrit.openbmc-project.xyz/c/openbmc/openbmc/+/49181) on the
current 5.15.5 kernel. I noticed that when using ttyS0 from the host
(being the VUART in the AST2500), that the transfer host->BMC is
really slow (like 300 baud slow). This is true even after stopping
obmc-console-server and just doing `cat /dev/ttyVUART0`, so I figured
it must be a kernel problem. When I then tried kernel 5.2.11 (with the
DTS from 5.15.5 minus the uart_routing node), then VUART behaved
normally. After having done this comparison, I think that 5.15.5 is
generally just much slower.

Is anyone aware of AST2500 VUART (or something else that would affect
performance on an AST2500) having gotten broken somewhere between
5.2.11 and 5.15.5?

Thanks
Oskar.