Via-Rhine Driver - questions for D. Becker.

Via-Rhine Driver - questions for D. Becker.

[Ivan G.]

I have been trying to debug the kernel Via-Rhine driver but I do not have 
much information and I'm forced to guess about many things.

Since you are the original author, I wanted to ask you some questions about 
the operation of the driver. If you have time, your help would be greatly 
appreciated.

(btw. I am aware that you have your own version of the driver.
However most issues will probably apply to that driver as well. )

Here are my questions:

- 1) "Something wicked happened"]
What is the precise purpose of the trap - what should be trapped and what 
shouldn't. Is it even necessary? There are a definite number of interrupts to 
call via_rhine_error - why not simply handle each one of them? I've seen 3 
different versions of the "wicked" code. 

- 2) CmdTxDemand should be issued to handle which error interrupts?

- 3) Missing interrupts in setting the bitmask....
Is there a reason or is it an error? some of those are used later in the 
code... RxNoBuf for example, RxOverflow is another (actually your driver 
includes that but never uses it), TxAborted is missing in both kernel and 
your version.

- 4) RxOverflow is not handled. Does it need to be and how?

-5 ) What is a good resource on those things? I read the datasheets.
They give useful numbers but little explanation on how different interrupts 
should be handled. What info did you have when you were writing the driver.  

-6) TxUnderrun - it increases threshold and then issues CmdTxDemand in  the 
"wicked error" trap. The linuxfet VIA driver, additionally  does the 
following: Sets descriptor bit, CmdTxOn, CmdTxDemand

if (txstatus & 0x0800) {
                    /* uderrun happen */
                    np->tx_ring[entry].tx_status = cpu_to_le32(DescOwn);
                    writel(virt_to_bus(&np->tx_ring[entry]), ioaddr + 
TxRingPtr);
                    /* Turn on Tx On*/
                    writew(CmdTxOn | np->chip_cmd, dev->base_addr + ChipCmd); 
  

                    /* Stats counted in Tx-done handler, just restart Tx. */
                    writew(CmdTxDemand | np->chip_cmd, dev->base_addr + 
ChipCmd)
;
                    if (debug > 1)
                       printk(KERN_ERR "Underrun happen");

Is it necessary to do the above on underruns?

That's all for now.
I have tried to figure those things out on my own but
the main problem is lack of any resource to help me.
I haven't written any network drivers so I am unfamiliar with 
what every interrupt does and how. if you can point me to some documentation 
for help maybe I could figure it out myself. 

Thank you for your help.
This message will be cc'd to LKML as well.



 

Re: Via-Rhine Driver - questions for D. Becker.

[Donald Becker]

On Wed, 24 Apr 2002, Ivan G. wrote:

> I have been trying to debug the kernel Via-Rhine driver but I do not have
> much information and I'm forced to guess about many things.

There are datasheets available for both the Rhine and Rhine-II chips.

> - 1) "Something wicked happened"]
> What is the precise purpose of the trap - what should be trapped and what
> shouldn't. Is it even necessary? There are a definite number of interrupts to
> call via_rhine_error - why not simply handle each one of them? I've seen 3
> different versions of the "wicked" code.

The earlier versions of the driver produced that messsage for most
unusual events.  The problem was that normal events combined with
uncommon events would trigger the message.  The driver at scyld.com
cleaned up the check for events that trigger the message long ago.

See
 http://www.scyld.com/network/via-rhine.html
    ftp://www.scyld.com/pub/network/via-rhine.c

> - 2) CmdTxDemand should be issued to handle which error interrupts?

It's used to wake an idle Tx unit.
There is a race condition when adding a new descriptor to the Tx
descriptor list.  The driver might add the descriptor just a cycle too
late, leaving the just-queued packet untransmitted.  It's safe to make
extra calls to CmdTxDemand, so the driver does this after every packet
is queued.

CmdTxDemand is also called when there is a transmit abort due to
excessive collisions, FIFO underrun, or lost carrier.

Aagin, see the code at scyld.com

> - 3) Missing interrupts in setting the bitmask....
> Is there a reason or is it an error? some of those are used later in the
> code... RxNoBuf for example, RxOverflow is another (actually your driver
> includes that but never uses it), TxAborted is missing in both kernel and
> your version.

RxNoBuf doesn't apply.  In our case it will always occur in conjunction
with IntrRxEmpty, which is the event we handle.

Compare IntrTxAbort (which we handle) with the similar IntrTxAborted
(which the driver does not explicitly mention).

> - 4) RxOverflow is not handled. Does it need to be and how?

See the datasheet.
We handle it implicitly by refilling the Rx descriptors.

> -5 ) What is a good resource on those things? I read the datasheets.
> They give useful numbers but little explanation on how different interrupts
> should be handled. What info did you have when you were writing the driver.

>From the datasheet.  As with most datasheets, it is written as if no
similar chip exists but can't be understood unless you know how other
Ethernet NICs works.  After all, they can't just come out and say "it's
just like the LANCE/Tulip/etc but we shuffled the bit fields and tweaked
this interface".

> -6) TxUnderrun - it increases threshold and then issues CmdTxDemand in  the
> "wicked error" trap. The linuxfet VIA driver, additionally  does the
> following: Sets descriptor bit, CmdTxOn, CmdTxDemand

Hmmmm, I'm not certain that the "underrun" comment is correct here.

> if (txstatus & 0x0800) {
>              /* uderrun happen */
>               np->tx_ring[entry].tx_status = cpu_to_le32(DescOwn);
>               writel(virt_to_bus(&np->tx_ring[entry]), ioaddr + TxRingPtr);

This is backing up the Tx pointer to retransmit the failed packet.
My original driver did not do this -- I believe that it's semantically
wrong.

I write my drivers to always fail the current transmit attempt on Tx
Abort.  Ethernet uses dropped packets on severe overload (16 collision
abort) to provide flow control to the protocol layers.  If you back up
and attempt to retransmit the same packet you defeat this element of the
protocol.  It's an atypical case, but Ethernet works where Aloha fails
because it gets the rare cases right.

>               /* Turn on Tx On*/
>               writew(CmdTxOn | np->chip_cmd, dev->base_addr + ChipCmd);

Note: My driver doesn't turn the Tx back on as a separate step.

>                /* Stats counted in Tx-done handler, just restart Tx. */
>               writew(CmdTxDemand | np->chip_cmd, dev->base_addr + ChipCmd);
>               if (debug > 1)
>                        printk(KERN_ERR "Underrun happen");

This is code from my driver, but someone didn't get the erorr message right.
The error message must prefix the device e.g. "eth0".

	if (np->msg_level & NETIF_MSG_TX_ERR)
		printk(KERN_INFO "%s: Transmitter underrun, increasing Tx "
		    "threshold setting to %2.2x.\n", dev->name, np->tx_thresh);


-- 
Donald Becker				becker@scyld.com
Scyld Computing Corporation		http://www.scyld.com
410 Severn Ave. Suite 210		Second Generation Beowulf Clusters
Annapolis MD 21403			410-990-9993

Re: Via-Rhine Driver - questions for D. Becker.

[Ivan G.]

> The earlier versions of the driver produced that messsage for most
> unusual events.  The problem was that normal events combined with
> uncommon events would trigger the message.  The driver at scyld.com
> cleaned up the check for events that trigger the message long ago.

Yes, I saw that problem. How do you define
"unusual events". Right now your "Wicked" trap will catch
IntrTxUnderrun, IntrPCIErr, and any other interrupts not in
IntrNormalSummary that occur along MIIChange,StatsMax,LinkChange,
Underrun, Abort, or PCIErr, and issue a CmdTxDemand on each.
Was this the intention?


> There is a race condition when adding a new descriptor to the Tx
> descriptor list.  The driver might add the descriptor just a cycle too
> late, leaving the just-queued packet untransmitted.  It's safe to make
> extra calls to CmdTxDemand, so the driver does this after every packet
> is queued.

This is interesting. The reason why I started messing with the driver in
the first place was because my card was stalling. Some investigation
showed that 1 Tx Done interrupt clears 2 descriptor ownership bits
after which the card stalls. hmm.

> RxNoBuf doesn't apply.  In our case it will always occur in conjunction
> with IntrRxEmpty, which is the event we handle.

Okay, but you use it to call netdev_rx while you haven't included it
when setting the interrupt mask. The kernel driver does the same.
The same is true for IntrTxAbort and IntrRxWakeUp.
Also, why are there two interrupts that do the same thing according
to documentation (aborted and abort).

> See the datasheet.
> We handle it implicitly by refilling the Rx descriptors.

The kernel driver includes it in the interrupt mask
and never uses it explicitly. Should it be removed?

----------------------------------------------
Some additional interrupt questions:
IntrMIIChange is defined as Transmit Descriptor Underflow by the docs (??)
IntrRxDropped is defined as FIFO Overflow (??)