sstp: add ppposeq transport to avoid userspace HDLC framingsstp-ppposeq

A pty is a byte stream, so the tty flip buffer merges frames written
back to back and sstp has to re-delimit them with async HDLC escaping
and a CRC-16 FCS. On a 1452-byte payload that is ~3600 ns per frame,
most of it spent on the FCS.

PPPOSEQ is a pppox protocol whose socket is the ppp endpoint itself,
so one datagram is one frame and no framing is needed at all. The
same payload takes ~380 ns per frame, about 9 times less. Requires
kernel 2.6.37, the first with PX_MAX_PROTO 3, whose remaining slot
it claims.
Supported kernels are from 2.6.37 to 7.2.

The new ppp-mode option selects the transport; auto, the default,
falls back to async when the module is unavailable, so hosts with
prebuilt kernels are unaffected.

PPP_SYNC is removed, being disabled and unfixable over a pty: frame
boundaries cannot be recovered from the stream, and coalescing cannot
be prevented since frames arrive from the network stack.

1 files changed, 4 insertions, 0 deletions

diff --git a/drivers/ppposeq/Makefile b/drivers/ppposeq/Makefile
new file mode 100644
index 00000000..f66096c6
--- /dev/null
+++ b/drivers/ppposeq/Makefile
@@ -0,0 +1,4 @@
+obj-m += ppposeq.o
+
+default:
+	make -C $(KDIR) M=$(PWD) modules