doc: update driver TX algorithm in timestamping.txt
Since cd4d8fdad1f1 ("net: kernel panic in dev_hard_start_xmit:
remove faulty software TX time stamping") dev_hard_start_xmit()
will not provide software timestamps. It's a responsibility of
the drivers to call skb_tx_timestamp() at the right time.
Cc: linux-doc@vger.kernel.org
Signed-off-by: Jakub Kicinski <kubakici@wp.pl>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/Documentation/networking/timestamping.txt b/Documentation/networking/timestamping.txt
index 048c92b..bc35541 100644
--- a/Documentation/networking/timestamping.txt
+++ b/Documentation/networking/timestamping.txt
@@ -202,6 +202,9 @@
and not free the skb. A driver not supporting hardware time stamping doesn't
do that. A driver must never touch sk_buff::tstamp! It is used to store
software generated time stamps by the network subsystem.
+- Driver should call skb_tx_timestamp() as close to passing sk_buff to hardware
+ as possible. skb_tx_timestamp() provides a software time stamp if requested
+ and hardware timestamping is not possible (SKBTX_IN_PROGRESS not set).
- As soon as the driver has sent the packet and/or obtained a
hardware time stamp for it, it passes the time stamp back by
calling skb_hwtstamp_tx() with the original skb, the raw
@@ -212,6 +215,3 @@
this would occur at a later time in the processing pipeline than other
software time stamping and therefore could lead to unexpected deltas
between time stamps.
-- If the driver did not set the SKBTX_IN_PROGRESS flag (see above), then
- dev_hard_start_xmit() checks whether software time stamping
- is wanted as fallback and potentially generates the time stamp.