kqueue for usb_dev

[Kohji Okuno]

Hi John-Mark,

Thank you for you comment.

From: John-Mark Gurney <jmg at funkthat.com>
> Kohji Okuno wrote this message on Thu, Feb 27, 2014 at 14:26 +0900:
>> I tried add kqueue I/F to usb_dev.c. I attached my patch.
>> What do you think about my patch?
> 
> A few comments...
> 
> 1) You should just drop the use of flag_iskevent and just
> unconditionally call KNOTE... since you have the lock already held,
> the cost is minimal (and w/ modern branch prediction, may be cheaper)...

Should we set the use of flag_iskevent, when usb_filter_read() and
usb_filter_write() return `0'?


> 2) Why do you try to start read/write transfers in the _filter?  You
> should just check to see if data is available and not do work..  This
> is also important since kqueue calls the filter just before delivering
> the knote to userland to verify that there is still data, and it will
> call your _event function for each knote on the fd...  The work should
> be started through other mechanisms, like read/write syscall or
> interrupt or timeout/callout...  If it's required to get results from
> USB_IF_POLL, then it's fine..

I copied from usb_poll().
Should we try to start read/write transfers in usb_kqfilter()?
Or should not we try to start read/write transfers in poll and kqueue?

> 3) I don't see any calls to knlist_destroy... These calls are needed
> to clean up the knlist...

I understood.

> Obviously the #if 1's will need to go...
> 
> Also, I don't think your change is against HEAD..  The line numbers
> in my version of usb_dev.c are different...

I'm sorry.

Many thanks,
 Kohji Okuno