svn commit: r191037 - head/sys/net

[Robert Watson]

On Tue, 14 Apr 2009, Kip Macy wrote:

>> The commit message should perhaps read:
>>
>>  Call default if_qflush on ifq if there are still packets left in the
>>  default queue after calling the driver's flush method.
>>
>> However, this seems a bit odd: what if the driver uses if_snd as its queue 
>> but implements other differences in the transmit routine?  In that case, we 
>> might impose default queue properties on if_snd even though the driver 
>> doesn't use them.  Could you talk a bit about the circumstances under which 
>> the driver provides an if_qflush that doesn't drain its queues properly?
>
> I'm afraid I don't understand the question. If a driver only uses if_snd for 
> its transmit routine then there is no reason for it to override the default 
> if_qflush implementation.

In the past (and possibly still) device driver and link layers have used 
if_snd plus their own queues for differentiated traffic.  For example, if_slip 
used an additional "fast" queue for interactive traffic, and if_snd for "slow" 
traffic.  It would still need to implement a flush method itself since it 
maintains additional queues.

What I'd like to see is two modes of operation:

(1) Historic mode: the ifnet framework provides all queueing support,
     enqueuing using the standard macros to if_snd, and the default
     implementation of if_qflush in place to flush the queue.  This means
     implementing neither if_transmit nor if_qflush methods on the ifnet.

(2) Modern mode: the driver provides all queueing support, possibly by
     invoking "library"  routines from the stack, and the ifnet queue stays
     entirely out of the way.  This means implementing both if_transmit and
     if_qflush methods on the ifnet.

To support the (1) scenario above, simply calling ifp->if_qflush() does the 
trick fine, since it's pre-initialized to if_qflush().  For the (2) scenario, 
the driver might choose to call if_qflush() to perform some or all flushing, 
but it shouldn't happen by itself unless the device driver author wants that.

Robert N M Watson
Computer Laboratory
University of Cambridge