Re: [PATCH net v2] drivers/net/wan/hdlc_fr: Add needed_headroom for PVC devices

[Xie He <xie.he.0141@gmail.com>]

On Fri, Sep 4, 2020 at 6:28 PM Xie He <xie.he.0141@gmail.com> wrote:
>
> The HDLC device is not actually prepending any header when it is used
> with this driver. When the PVC device has prepended its header and
> handed over the skb to the HDLC device, the HDLC device just hands it
> over to the hardware driver for transmission without prepending any
> header.
>
> If we grep "header_ops" and "skb_push" in "hdlc.c" and "hdlc_fr.c", we
> can see there is no "header_ops" implemented in these two files and
> all "skb_push" happen in the PVC device in hdlc_fr.c.

I want to provide a little more information about the flow after an
HDLC device's ndo_start_xmit is called.

An HDLC hardware driver's ndo_start_xmit is required to point to
hdlc_start_xmit in hdlc.c. When a HDLC device receives a call to its
ndo_start_xmit, hdlc_start_xmit will check if the protocol driver has
provided a xmit function. If it has provided this function,
hdlc_start_xmit will call it to start transmission. If it has not,
hdlc_start_xmit will directly call the hardware driver's function to
start transmission. This driver (hdlc_fr) has not provided a xmit
function in its hdlc_proto struct, so hdlc_start_xmit will directly
call the hardware driver's function to transmit.

So no header will be prepended after ndo_start_xmit is called.

There would not be any header prepended before ndo_start_xmit is
called, either, because there is no header_ops implemented in either
hdlc.c or hdlc_fr.c.

On Fri, Sep 4, 2020 at 6:28 PM Xie He <xie.he.0141@gmail.com> wrote:
>
> Thank you for your email, Jakub!
>
> On Fri, Sep 4, 2020 at 3:14 PM Jakub Kicinski <kuba@kernel.org> wrote:
> >
> > 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.
>
> The HDLC device is not actually prepending any header when it is used
> with this driver. When the PVC device has prepended its header and
> handed over the skb to the HDLC device, the HDLC device just hands it
> over to the hardware driver for transmission without prepending any
> header.
>
> If we grep "header_ops" and "skb_push" in "hdlc.c" and "hdlc_fr.c", we
> can see there is no "header_ops" implemented in these two files and
> all "skb_push" happen in the PVC device in hdlc_fr.c.
>
> For this reason, I have previously submitted a patch to change the
> value of hard_header_len of the HDLC device from 16 to 0, because it
> is not actually used.
>
> See:
> 2b7bcd967a0f (drivers/net/wan/hdlc: Change the default of hard_header_len to 0)
>
> > > 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?
>
> Oh. I understand your point. Theoretically we don't need to set it to
> 0 because it already has the default value of 0. I'm setting it to 0
> only because I want to tell future developers that this value is
> intentionally set to 0, and it is not carelessly missed out.