I'm unable to talk to the modem on ttyS1 with just 2.4.19-pre7.
2.4.19-pre6 and 2.4.19-pre7-ac2 are fine.
The later two sources used were generated with diffs against
2.4.19-pre6, and the same initial .config for make oldconfig,
and CONFIG_IDE_TASKFILE_IO=y
CONFIG_ZLIB_INFLATE=m
CONFIG_ZLIB_DEFLATE=m
the only new items for ac2.