svn commit: r191037 - head/sys/net

Kip Macy kmacy at freebsd.org
Thu Apr 16 23:08:54 UTC 2009


see 191161 - I think it addresses everything

-Kip

On Thu, Apr 16, 2009 at 3:51 PM, Robert Watson <rwatson at freebsd.org> wrote:
> 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



-- 
All that is necessary for the triumph of evil is that good men do nothing.
    Edmund Burke


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