Subject: Re: [PATCH 5/5] mac80211: set NETIF_F_LLTX when using intermediate tx
 queues
To:     Arend Van Spriel <arend.vanspriel@broadcom.com>,
        =?UTF-8?Q?Toke_H=c3=b8iland-J=c3=b8rgensen?= <toke@redhat.com>,
        linux-wireless@vger.kernel.org
Cc:     johannes@sipsolutions.net, Herbert Xu <herbert@gondor.apana.org.au>
References: <20190316170634.13125-1-nbd@nbd.name>
 <20190316170634.13125-5-nbd@nbd.name> <87sgvmvg9g.fsf@toke.dk>
 <773e4dff-29fd-22b4-e4bc-cd5a94c66dc2@broadcom.com>
From:   Felix Fietkau <nbd@nbd.name>
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Message-ID: <e912d736-a3ad-1e21-0cdd-6a065df96fbb@nbd.name>
Date:   Sun, 14 Apr 2019 13:19:43 +0200
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On 2019-04-14 11:44, Arend Van Spriel wrote:
> + Herbert
> 
> On 3/16/2019 7:14 PM, Toke Høiland-Jørgensen wrote:
>> Felix Fietkau <nbd@nbd.name> writes:
>> 
>>> When using iTXQ, tx sequence number allocation and statistics are run at
>>> dequeue time. Because of that, it is safe to enable NETIF_F_LLTX, which
>>> allows tx handlers to run on multiple CPUs in parallel.
>> 
>> Cool, didn't know about that flag.
> 
> It is water under the bridge as this patch got applied already, but I 
> stumbled upon it just recently and didn't know about that flag either. 
> So I looked for more information about it and found the definition [1], 
> but the comment seemed important enough to send this reply.
> 
> 	NETIF_F_LLTX_BIT,	/* LockLess TX - deprecated. Please */
> 				/* do not use LLTX in new drivers */
> 
> Here is the commit that marked it deprecated:
> 
> commit e24eb521fbf2a350ce879dfc1d8e56d4ffa2aa22
> Author: Christian Borntraeger <borntraeger@de.ibm.com>
> Date:   Tue Sep 25 19:42:02 2007 -0700
> 
>      [NET]: note that NETIF_F_LLTX is deprecated
> 
> So I am not sure we should really do this in mac80211. Maybe Herbert can 
> comment although it has been over a decade ago.
There is a lot of comparable code that also uses this flag, e.g.
batman-adv, bridge, vlan, various tunnel implementations. I think
mac80211 fits well with those kinds of use cases.
If I remember correctly, the deprecation was added to avoid quirky
custom locking schemes in ethernet drivers.

- Felix