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Re: Error on TX descriptor free

To: Thomas Petazzoni <thomas.petazzoni@enix.org>
Subject: Re: Error on TX descriptor free
From: Manish Lachwani <mlachwani@mvista.com>
Date: Fri, 22 Oct 2004 09:58:57 -0700
Cc: linux-mips@linux-mips.org
In-reply-to: <417907A2.3030000@enix.org>
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References: <417907A2.3030000@enix.org>
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Hello Thomas

Thats a bug. I dont have a board to try this on. But, can you?

Thomas Petazzoni wrote:
Hello,

I'm currently using the MV643xx Ethernet driver on my board. When I run the command :

# ifconfig eth0 down

I see the following message :

eth0: Error on Tx descriptor free - could not free 1 descriptors

I went through the code, and saw that this message is displayed in mv64340_eth_free_tx_rings(). This function frees all remaining skbs (registered in the mp->tx_skb array), and decrement mp->tx_ring_skbs. Then, it checks if it reached 0. In my case, it is still 1.

In fact, mp->tx_ring_skbs is initialized to 0 and then incremented in mv64340_eth_start_xmit() (when a transmission starts), and is decremented in mv64340_eth_free_tx_queue (when the transmission is done). But the decrementation only occurs if mp->tx_ring_skbs is different from one. I don't understand why.

At the end of mv64340_eth_free_tx_queue(), the following code makes sure that the number of skbs did not reach 0 :

    if (mp->tx_ring_skbs == 0)
      panic("ERROR - TX outstanding SKBs counter is corrupted");

Well, my question is simply : why can't we decrement the mp->tx_ring_skbs counter to 0 ?

What needs to be fixed ? The decrementation of the counter, or the function that frees the TX queue when the interface is stopped ?

Decrement the counter to zero as you suggested. And remove the panic("..") in mv64340_eth_free_tx_queue(). Let me know if it works fine

Thanks
Manish Lachwani




I've seen similar code in the Titan GE driver.

Do not hesitate to ask for further details,

Thanks,

Thomas



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