Subject: HDLC driver - dev_free_skb_irq causes Segfault

From: "Russell McGuire" <[email-masked]>
Date: Sat, 9 Feb 2008 18:32:16 -0800

All,

So I am in the process of debugging my newly established HDLC driver.
More or less modeled after a simplified gianfar / ucc_geth idea.

However, after loading, etc. and using the following commands

-> insmod hdlc-8360.ko
-> sethdlc hdlc0 hdlc-eth
-> ifconfig hdlc0 up 192.168.1.100

All is well, and I am seeing IDL interrupts. Great.

Now I go to ping an address like, 
ping 192.168.1.101

I can see that I get the start_xmit function, the IRQ from the QE comes back
and reports the TXBD as successfully sent.

Here is the problem, when I goto free the skb in the tx_handler, I get a 
'Unable to Handle Kernel Paging Request for data at address 0x00000000'
Even though for the life of me, I can't see any pointers that are at address
zero.

I have checked the pointer value I am passing in, and indeed it is the exact
same pointer I am receiving from the original 
start-xmit call..

Are we supposed to copy the skb? And free it immediately in the start_xmit?
Some special way to store the pointer? 

I have tried
txbd->buf = skb->data;
txbd->buf = virt_to_phys(skb->data);
etc.. and various other ways to save that I have seen in  the gianfar and
ucc_geth drivers.

My tx_sk_buff** is identical as alloced the same way. 

Anyone have any ideas?

-Russ