Subject: Re: [PATCH 5/5] mac80211: set NETIF_F_LLTX when using intermediate tx
 queues
To:     Johannes Berg <johannes@sipsolutions.net>,
        Herbert Xu <herbert@gondor.apana.org.au>
Cc:     =?UTF-8?Q?Toke_H=c3=b8iland-J=c3=b8rgensen?= <toke@redhat.com>,
        Felix Fietkau <nbd@nbd.name>, linux-wireless@vger.kernel.org
References: <20190316170634.13125-1-nbd@nbd.name>
 <20190316170634.13125-5-nbd@nbd.name> <87sgvmvg9g.fsf@toke.dk>
 <773e4dff-29fd-22b4-e4bc-cd5a94c66dc2@broadcom.com>
 <20190416074444.pdubbh6fbibdnhi7@gondor.apana.org.au>
 <73b18131-2777-da5c-a6ee-9d9b3e13cd06@broadcom.com>
 <20190416083636.5ttvezqyhzr2worg@gondor.apana.org.au>
 <eab39dd804405854997e50102f3d0ff925feb8d2.camel@sipsolutions.net>
From:   Arend Van Spriel <arend.vanspriel@broadcom.com>
Message-ID: <cd0715bb-b75c-63b0-0ae4-858d14afd500@broadcom.com>
Date:   Tue, 16 Apr 2019 11:17:53 +0200
User-Agent: Mozilla/5.0 (Windows NT 10.0; WOW64; rv:60.0) Gecko/20100101
 Thunderbird/60.6.1
MIME-Version: 1.0
In-Reply-To: <eab39dd804405854997e50102f3d0ff925feb8d2.camel@sipsolutions.net>
Content-Type: text/plain; charset=utf-8; format=flowed
Content-Language: en-US
Content-Transfer-Encoding: 7bit
Sender: linux-wireless-owner@vger.kernel.org
Precedence: bulk


On 4/16/2019 10:37 AM, Johannes Berg wrote:
> On Tue, 2019-04-16 at 16:36 +0800, Herbert Xu wrote:
>> On Tue, Apr 16, 2019 at 10:04:24AM +0200, Arend Van Spriel wrote:
>>>
>>> I was just writing up an email clarifying my question. But let me summarize
>>> this email thread. The patch from Felix adds this flag in mac80211 for
>>> drivers that indicate to support pulling packets from the internal TXQ in
>>> mac80211. I found it is deprecated, but as Felix mentioned it is used in
>>> various parts of the network subsystem, ie. batman-adv, bridge, vlan, tunnel
>>> implementations. So its use seems to be restricted rather than deprecated.
>>> Given your response above I guess my question would be to get details about
>>> what you call "proper design" as I think you are saying with that it is not
>>> needed, right?
>>
>> Essentially the only time it would be OK to use LLTX in its current
>> form is if you have no TX queue/congestion feedback which is clearly
>> not the case with wireless drivers.
> 
> It is true because we have an entire buffering layer in mac80211 (in
> this case at least) and never push back to the stack.

Ok, so the crux is the "never push back to the stack" part? Well, the 
internal TXQ and how that is used is obviously enabling that ;-)

Regards,
Arend