On Wed, 2 Sep 2020 17:06:58 -0700 Xie He wrote:
> PVC devices are virtual devices in this driver stacked on top of the
> actual HDLC device. They are the devices normal users would use.
> PVC devices have two types: normal PVC devices and Ethernet-emulating
> PVC devices.
>
> When transmitting data with PVC devices, the ndo_start_xmit function
> will prepend a header of 4 or 10 bytes. Currently this driver requests
> this headroom to be reserved for normal PVC devices by setting their
> hard_header_len to 10. However, this does not work when these devices
> are used with AF_PACKET/RAW sockets. Also, this driver does not request
> this headroom for Ethernet-emulating PVC devices (but deals with this
> problem by reallocating the skb when needed, which is not optimal).
>
> This patch replaces hard_header_len with needed_headroom, and set
> needed_headroom for Ethernet-emulating PVC devices, too. This makes
> the driver to request headroom for all PVC devices in all cases.
Since this is a tunnel protocol on top of HDLC interfaces, and
hdlc_setup_dev() sets dev->hard_header_len = 16; should we actually
set the needed_headroom to 10 + 16 = 26? I'm not clear on where/if
hdlc devices actually prepend 16 bytes of header, though.
CC: Willem as he was reviewing your similar patch recently.
> diff --git a/drivers/net/wan/hdlc_fr.c b/drivers/net/wan/hdlc_fr.c
> index 9acad651ea1f..12b35404cd8e 100644
> --- a/drivers/net/wan/hdlc_fr.c
> +++ b/drivers/net/wan/hdlc_fr.c
> @@ -1041,7 +1041,7 @@ static void pvc_setup(struct net_device *dev)
> {
> dev->type = ARPHRD_DLCI;
> dev->flags = IFF_POINTOPOINT;
> - dev->hard_header_len = 10;
> + dev->hard_header_len = 0;
Is there a need to set this to 0? Will it not be zero after allocation?
> dev->addr_len = 2;
> netif_keep_dst(dev);
> }
> @@ -1093,6 +1093,7 @@ static int fr_add_pvc(struct net_device *frad, unsigned
> int dlci, int type)
> dev->mtu = HDLC_MAX_MTU;
> dev->min_mtu = 68;
> dev->max_mtu = HDLC_MAX_MTU;
> + dev->needed_headroom = 10;
> dev->priv_flags |= IFF_NO_QUEUE;
> dev->ml_priv = pvc;
>
