selftests: drv-net: Test tail-adjustment support

Add test to validate support for the two cases of tail adjustment: 1)
tail extension, and 2) tail shrinking across different frame sizes and
offset values. For each of the two cases, test both the single and
multi-buffer cases by choosing appropriate packet size.

The negative offset value result in growing of tailroom (shrinking of
payload) while the positive offset result in shrinking of tailroom
(growing of payload).

Since the support for tail adjustment varies across drivers, classify the
test as pass if at least one combination of packet size and offset from a
pre-selected list results in a successful run. In case of an unsuccessful
run, report the failure and highlight the packet size and offset values
that caused the test to fail, as well as the values that resulted in the
last successful run.

Note: The growing part of this test for netdevsim may appear flaky when
the offset value is larger than 1. This behavior occurs because tailroom
is not explicitly reserved for netdevsim, with 1 being the typical
tailroom value. However, in certain cases, such as payload being the last
in the page with additional available space, the truesize is expanded.
This also result increases the tailroom causing the test to pass
intermittently. In contrast, when tailrrom is explicitly reserved, such
as in the of fbnic, the test results are deterministic.

./drivers/net/xdp.py
TAP version 13
1..7
ok 1 xdp.test_xdp_native_pass_sb
ok 2 xdp.test_xdp_native_pass_mb
ok 3 xdp.test_xdp_native_drop_sb
ok 4 xdp.test_xdp_native_drop_mb
ok 5 xdp.test_xdp_native_tx_mb
\# Failed run: ... successful run: ... offset 1. Reason: Adjustment failed
ok 6 xdp.test_xdp_native_adjst_tail_grow_data
ok 7 xdp.test_xdp_native_adjst_tail_shrnk_data
\# Totals: pass:7 fail:0 xfail:0 xpass:0 skip:0 error:0

Signed-off-by: Mohsin Bashir <mohsin.bashr@gmail.com>
Link: https://patch.msgid.link/20250719083059.3209169-5-mohsin.bashr@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

0 files changed, 0 insertions, 0 deletions