Per the i2c spec, a slave device can stretch SCL idefinitely, so 25ms is
a bit arbitrary in general. smbus does specify an optional timeout
recovery mechanism to be done at about 25~35ms, but the IPMI SSIF spec
says that BMCs don't have any obligation to implement that.
The BMC on Altra seems to mostly respond within 25ms, but occasionally
will stretch SCL for ~300 msec.
Also, the count_us mechanism seems to actually timeout around 25%
earlier than it would claim (timeout really happening around 19ms
instead of 25ms).
Sponsored by: Ampere Computing LLC
Submitted by: Klara Inc.
Reviewed by: manu, imp
Differential Revision: https://reviews.freebsd.org/D28747
37ef8d4cd6