linux-kernel - Re: [PATCH 01/12] net: mediatek: fix DQL support

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Message-ID: <CALx6S357KoKbS6tMKUgvHHLowXauZqAXib=bGuJBprwMDFkQ2g@mail.gmail.com>
Date:	Tue, 7 Jun 2016 16:20:33 -0700
From:	Tom Herbert <tom@...bertland.com>
To:	David Miller <davem@...emloft.net>
Cc:	john@...ozen.org,
	Linux Kernel Network Developers <netdev@...r.kernel.org>,
	linux-mediatek@...ts.infradead.org, keyhaede@...il.com,
	LKML <linux-kernel@...r.kernel.org>, nbd@....name
Subject: Re: [PATCH 01/12] net: mediatek: fix DQL support

On Tue, Jun 7, 2016 at 4:01 PM, David Miller <davem@...emloft.net> wrote:
> From: John Crispin <john@...ozen.org>
> Date: Mon, 6 Jun 2016 08:43:13 +0200
>
>> i think one solution would be to add some code to have 2 devices share
>> the same dql instance. would that be an acceptable solution ?
>
> You still need to address the issue of synchronization.
>
> dql purposefully doesn't use locking, always because a higher level
> object (in this case the netdev TX queue) it is contained within
> provides the synchronization.
>
> That breaks apart once you share the dql between two netdevs, as you
> are proposing here.  You'll have to add locking, which is expensive.
>
> That's why I'm trying to encourage you to think out of the box and
> find some way to solve the issue without having to access shared
> state shared between multiple devices.
>

I think you guys mean mean BQL not DQL :-)

If two netdevs share the same DMA ring then is using two netdevs the
right approach. Seems like this would have other consequences beyond
BQL...

Tom

> Thanks.
>