Re: [RFC 3/7] Bluetooth: Add FCS awareness to L2CAP HCI fragmentation.

["Gustavo F. Padovan" <gustavo@padovan.org>]

Hi Mat,

* Gustavo F. Padovan <gustavo@padovan.org> [2010-08-11 00:35:41 -0300]:

> * Mat Martineau <mathewm@codeaurora.org> [2010-08-10 12:15:00 -0700]:
> 
> > In order to not limit ERTM and streaming mode PDUs to the HCI MTU
> > size, L2CAP must be able to split PDUs in to multple HCI fragments.
> > This is done by allocating space for the FCS in the last fragment.
> > 
> > Signed-off-by: Mat Martineau <mathewm@codeaurora.org>
> > ---
> >  net/bluetooth/l2cap.c |   39 ++++++++++++++++++++++++++++++++++-----
> >  1 files changed, 34 insertions(+), 5 deletions(-)
> > 
> > diff --git a/net/bluetooth/l2cap.c b/net/bluetooth/l2cap.c
> > index aa69c84..b485c4a 100644
> > --- a/net/bluetooth/l2cap.c
> > +++ b/net/bluetooth/l2cap.c
> > @@ -1664,31 +1664,63 @@ static inline int l2cap_skbuff_fromiovec(struct sock *sk, struct msghdr *msg, in
> >  {
> >  	struct l2cap_conn *conn = l2cap_pi(sk)->conn;
> >  	struct sk_buff **frag;
> > +	struct sk_buff *final;
> >  	int err, sent = 0;
> >  
> > +	BT_DBG("sk %p, msg %p, len %d, count %d, skb %p", sk,
> > +		msg, (int)len, (int)count, skb);
> > +
> >  	if (memcpy_fromiovec(skb_put(skb, count), msg->msg_iov, count))
> >  		return -EFAULT;
> >  
> >  	sent += count;
> >  	len  -= count;
> > +	final = skb;
> >  
> >  	/* Continuation fragments (no L2CAP header) */
> >  	frag = &skb_shinfo(skb)->frag_list;
> >  	while (len) {
> > +		int skblen;
> >  		count = min_t(unsigned int, conn->mtu, len);
> >  
> > -		*frag = bt_skb_send_alloc(sk, count, msg->msg_flags & MSG_DONTWAIT, &err);
> > +		/* Add room for the FCS if it fits */
> > +		if (l2cap_pi(sk)->fcs == L2CAP_FCS_CRC16 &&
> > +		    len + L2CAP_FCS_SIZE <= conn->mtu)
> 
> You don't need to check for (len + L2CAP_FCS_SIZE <= conn=mtu) here.
> Section 5.1 point that: 
> 
> "Unlike the B-Frame length field, the I-frame length field may be greater
> than the configured MTU because it includes the octet lengths of the
> Control, L2CAP SDU Length (when present), and frame check sequence
> fields as well as the Information octets."
> 
> From that I understand "len <=" and not "len + L2CAP_FCS_SIZE <="

So here you might want to the check if we support FCS, and then add 2 to
skblen.

> 
> 
> > +			skblen = count + L2CAP_FCS_SIZE;
> > +		else
> > +			skblen = count;
> > +
> > +		*frag = bt_skb_send_alloc(sk, skblen,
> > +					msg->msg_flags & MSG_DONTWAIT, &err);
> >  		if (!*frag)
> >  			return -EFAULT;
> > -		if (memcpy_fromiovec(skb_put(*frag, count), msg->msg_iov, count))
> > +
> > +		if (memcpy_fromiovec(skb_put(*frag, count),
> > +				msg->msg_iov, count))
> >  			return -EFAULT;
> >  
> >  		sent += count;
> >  		len  -= count;
> >  
> > +		final = *frag;
> > +
> >  		frag = &(*frag)->next;
> >  	}
> >  
> > +	if (l2cap_pi(sk)->fcs == L2CAP_FCS_CRC16) {
> > +		if (skb_tailroom(final) < L2CAP_FCS_SIZE) {
> > +			*frag = bt_skb_send_alloc(sk, L2CAP_FCS_SIZE,
> > +						msg->msg_flags & MSG_DONTWAIT,
> > +						&err);

Why do we need to check for FCS again? We already added it required
space to the last fragment.


-- 
Gustavo F. Padovan
http://padovan.org