netdev - [PATCH net-next 3/3] docs: networking: timestamping: add a set of frequently asked questions

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-Id: <20200717161027.1408240-4-olteanv@gmail.com>
Date:   Fri, 17 Jul 2020 19:10:27 +0300
From:   Vladimir Oltean <olteanv@...il.com>
To:     kuba@...nel.org, davem@...emloft.net, netdev@...r.kernel.org
Cc:     richardcochran@...il.com, sorganov@...il.com,
        linux-doc@...r.kernel.org
Subject: [PATCH net-next 3/3] docs: networking: timestamping: add a set of frequently asked questions

These are some questions I had while trying to explain the behavior of
some drivers with respect to software timestamping. Answered with the
help of Richard Cochran.

Signed-off-by: Vladimir Oltean <olteanv@...il.com>
---
 Documentation/networking/timestamping.rst | 26 +++++++++++++++++++++++
 1 file changed, 26 insertions(+)

diff --git a/Documentation/networking/timestamping.rst b/Documentation/networking/timestamping.rst
index 4004c5d2771d..e01ec01179fe 100644
--- a/Documentation/networking/timestamping.rst
+++ b/Documentation/networking/timestamping.rst
@@ -791,3 +791,29 @@ The correct solution to this problem is to implement the PHY timestamping
 requirements in the MAC driver found broken, and submit as a bug fix patch to
 netdev@...r.kernel.org. See :ref:`Documentation/process/stable-kernel-rules.rst
 <stable_kernel_rules>` for more details.
+
+3.4 Frequently asked questions
+------------------------------
+
+Q: When should drivers set SKBTX_IN_PROGRESS?
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+When the interface they represent offers both ``SOF_TIMESTAMPING_TX_HARDWARE``
+and ``SOF_TIMESTAMPING_TX_SOFTWARE``.
+Originally, the network stack could deliver either a hardware or a software
+time stamp, but not both. This flag prevents software timestamp delivery.
+This restriction was eventually lifted via the ``SOF_TIMESTAMPING_OPT_TX_SWHW``
+option, but still the original behavior is preserved as the default.
+
+Q: Should drivers that don't offer SOF_TIMESTAMPING_TX_SOFTWARE call skb_tx_timestamp()?
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+The ``skb_clone_tx_timestamp()`` function from its body helps with propagation
+of TX timestamps from PTP PHYs, and the required placement of this call is the
+same as for software TX timestamping.
+Additionally, since PTP is broken on ports with timestamping PHYs and unmet
+requirements, the consequence is that any driver which may be ever coupled to
+a timestamping-capable PHY in ``netdev->phydev`` should call at least
+``skb_clone_tx_timestamp()``. However, calling the higher-level
+``skb_tx_timestamp()`` instead achieves the same purpose, but also offers
+additional compliance to ``SOF_TIMESTAMPING_TX_SOFTWARE``.
-- 
2.25.1