On Wed, 27 Aug 2003, Doug Alcorn wrote:
> Oddly enough, my problem has already been fixed. That's the good
> news. Thanks for the work you guys have done! Now the catch is that
> I need some documentation on what the fix was for our quality system.
> I've been developing a driver using the 2.4.20 kernel. Since our
> product that uses this driver is controlled according to FDA
> guidelines, I can't just make arbitrary changes without
sounds interesting - would you mind sharing some experiences about how FDA
deals with OSS?
> justification. So, I'm hunting for some more details to justify the
> switch from kernel 2.4.20 to 2.4.22.
>
> Here's the setup. The userland applications initiates an ioctl call
> to my driver. This could either be to read data from my device or to
> write data out to it. I've only been able to reproduce the hang when
> writing data out. Each ioctl call from userland initiates four urb
> transfers: three bulk outs and one bulk in. That last bulk in is
> reading status information from my device to determine how well the
> command worked. The ioctl handler in the driver initiates the first
> bulk out urb asynchronously. The ioclt handler then uses
> wait_event_interruptible to determine when all four urbs have been
OK, this macro is expected to be race-safe, in contrast to sleep_on and
friend. What is the exact condition you are waiting on, something related
to urb->status?
> transfered. The completion handler for each urb in turn submits the
> next urb. So, three times out of four usb_submit_urb is called from
> interrupt context.
Sounds fair to me.
> To produce the hang, I repeat this ioctl loop several thousand times:
> write, write, write, read. I'm not exactly sure how many times I need
> to repeat before the hang. It seems somewhat consistent although
> there is some variance. The actual hang is between the third write
> and the final read. I have pretty good diagnostics on my device. I
> can tell quite clearly that the device received the third urb from the
> host and has prepared the status packet to be read. It is sitting
> and waiting for the host to read that status packet. In my driver the
Any dependency on host system, I mean hardware (*HCI) or driver (uhci vs.
usb-uhci) f.e.?
> code looks pretty much like this:
>
> printk("reading status packet");
> usb_fill_bulk_urb (.....);
> rc = usb_submit_urb(...);
> if (rc) {
> printk("error reading status packet");
> }
>
> When the hang occurs I see the first printk saying it's getting ready
> to read the status packet. I don't get the error printk. However, I
> don't know what happens after that. My completion handler is never
> called. I don't have a bus analyzer, so I can't confirm whether or
> not that last transaction is initiated by the kernel.
Do you have access to the device's firwware? Maybe on could change the
it to signal completion of the status transaction somehow - using some LED
or whatever? It would be really important to be sure about this.
> Here's the tricky part. If I add an "else printk (....)" to the
> pseudo-code above, I'm unable to trigger the hang. In other words,
> this seems to me to be an ugly timing issue. Take the last debug
Yep. Sounds like a race somewhere. My guess would be with the printk the
final urb tends to complete before the wait_on_event is reached.
> statement out, it hangs. Put it back in, it won't hang. When I
> upgraded to 2.4.22 (using make oldconfig on the .config from 2.4.20) I
> couldn't get it to hang. What I don't know is if there was a bug in
> 2.4.20 usb core that was fixed in 2.4.22 or if the timing in 2.4.22
> was changed enough to mask my problem. So, any guesses what this
> might be? Any recommendations?
>
> BTW, you can find the full source to my driver at
> http://lathi.net/view/Main/DataPlay.
Nope, 404 ;-)
Martin
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