linux-kernel - [PATCH net-next] net/packet: Improve the comment about LL header visibility criteria

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Message-Id: <20210205224124.21345-1-xie.he.0141@gmail.com>
Date:   Fri,  5 Feb 2021 14:41:24 -0800
From:   Xie He <xie.he.0141@...il.com>
To:     "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Willem de Bruijn <willemb@...gle.com>,
        Eric Dumazet <edumazet@...gle.com>,
        John Ogness <john.ogness@...utronix.de>,
        Tanner Love <tannerlove@...gle.com>,
        Eyal Birger <eyal.birger@...il.com>, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org
Cc:     Xie He <xie.he.0141@...il.com>
Subject: [PATCH net-next] net/packet: Improve the comment about LL header visibility criteria

The "dev_has_header" function, recently added in
commit d549699048b4 ("net/packet: fix packet receive on L3 devices
without visible hard header"),
is more accurate as criteria for determining whether a device exposes
the LL header to upper layers, because in addition to dev->header_ops,
it also checks for dev->header_ops->create.

When transmitting an skb on a device, dev_hard_header can be called to
generate an LL header. dev_hard_header will only generate a header if
dev->header_ops->create is present.

Signed-off-by: Xie He <xie.he.0141@...il.com>
---
 net/packet/af_packet.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/net/packet/af_packet.c b/net/packet/af_packet.c
index 6bbc7a448593..e24b2841c643 100644
--- a/net/packet/af_packet.c
+++ b/net/packet/af_packet.c
@@ -132,17 +132,17 @@ Resume
     because it is invisible to us.
 
 
 On transmit:
 ------------
 
-dev->header_ops != NULL
+dev_has_header(dev) == true
    mac_header -> ll header
    data       -> ll header
 
-dev->header_ops == NULL (ll header is invisible to us)
+dev_has_header(dev) == false (ll header is invisible to us)
    mac_header -> data
    data       -> data
 
    We should set network_header on output to the correct position,
    packet classifier depends on it.
  */
-- 
2.27.0