linux-kernel - Re: [PATCH RFC] mac80211: Use IFF_ECHO to force delivery of tx

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Message-Id: <40145B82-DCFB-40EC-88BD-BA71DA0CB86F@gmx.net>
Date:   Sat, 2 Mar 2019 16:16:31 +0100
From:   Julius Niedworok <julius.n@....net>
To:     Johannes Berg <johannes@...solutions.net>
Cc:     Oliver Hartkopp <socketcan@...tkopp.net>,
        linux-wireless@...r.kernel.org, ga58taw@...um.de,
        David Hildenbrand <david@...hat.com>, nc@....in.tum.de,
        "David S. Miller" <davem@...emloft.net>,
        Edward Cree <ecree@...arflare.com>,
        Jiri Pirko <jiri@...lanox.com>,
        Ido Schimmel <idosch@...lanox.com>,
        Petr Machata <petrm@...lanox.com>,
        Kirill Tkhai <ktkhai@...tuozzo.com>,
        Alexander Duyck <alexander.h.duyck@...el.com>,
        Amritha Nambiar <amritha.nambiar@...el.com>,
        Li RongQing <lirongqing@...du.com>, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org, Julius Niedworok <julius.n@....net>
Subject: Re: [PATCH RFC] mac80211: Use IFF_ECHO to force delivery of tx_status
 frames


> On 01.03.2019 09:32, Johannes Berg wrote:
> 
> Thus, I don't think this was ever intended for any cross-interface
> behaviour, even if it may be on the same physical NIC.

Now we got your point. We are sorry for the confusion - it seems we 
understood that wrong.

> Not all drivers can and do this, I believe. Some things don't work very
> well if they don't do it, but I _think_ you've just been lucky and used
> hardware that does in fact support it.

If the drivers do not adhere to the API, this is a problem of the drivers,
right? Because, how can we rely on *any* functionality of the drivers when
we assume that they do not adhere to the documented interfaces?

> Also note that for some hardware that does support this, there's
> sometimes significant overhead - not just the performance overhead of
> actually reporting the frames, but sometimes also overhead in how the
> hardware is programmed and used, and how TX status is extracted.

The default of this option will be disabled. If it is turned on manually
for debugging, the performance impact is probably acceptable.

> I suppose it could be in mac80211 (perhaps debugfs?) too. I just really
> don’t think IFF_ECHO is the right approach.

We see your point. So you think we should use debugfs to enable/disable
our functionality? If so, we are happy to see that we find a way to force
REQ_TX_STATUS to one from there.

Thank you,
Julius and Charlie