Subject: Re: [PATCH 1/3] ath9k: Decrease skb size to fit into one page.
From: Eric Dumazet <eric.dumazet@gmail.com>
To: "Luis R. Rodriguez" <mcgrof@gmail.com>
Cc: Christian Lamparter <chunkeey@googlemail.com>,
	greearb@candelatech.com, linux-wireless@vger.kernel.org,
	ath9k-devel@venema.h4ckr.net
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References: <1294361165-15308-1-git-send-email-greearb@candelatech.com>
	 <201101070205.00886.chunkeey@googlemail.com>
	 <1294363427.2704.23.camel@edumazet-laptop>
	 <AANLkTinQ42y9Tu=Ugb92-R+56BP4U+cDBzd4tQQCES1W@mail.gmail.com>
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Date: Fri, 07 Jan 2011 03:07:52 +0100
Message-ID: <1294366072.2704.32.camel@edumazet-laptop>
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Sender: linux-wireless-owner@vger.kernel.org

Le jeudi 06 janvier 2011 à 17:57 -0800, Luis R. Rodriguez a écrit :
> On Thu, Jan 6, 2011 at 5:23 PM, Eric Dumazet <eric.dumazet@gmail.com> wrote:
> > Le vendredi 07 janvier 2011 à 02:04 +0100, Christian Lamparter a écrit :
> >> On Friday 07 January 2011 01:46:03 greearb@candelatech.com wrote:
> >> > From: Ben Greear <greearb@candelatech.com>
> >> >
> >> > Patch is from Eric Dumazet, as described here:
> >> > https://patchwork.kernel.org/patch/104271/
> >> >
> >> > Reported-by: Michael Guntsche <mike@it-loops.com>
> >> > Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
> >> > Signed-off-by: Ben Greear <greearb@candelatech.com>
> >> > ---
> >> >
> >> > NOTE:  This needs review by ath9k and/or other informed
> >> > people.
> >>
> >> Does the hardware support vector-i/o for rx (like for instance iwlagn)?
> >> Else, this change would break A-MSDU rx - which is a mandatory feature
> >> (although, not very popular) of 802.11n -
> >>
> >> See for example 802.11n-2009 9.7c:
> >>
> >> "Support for the reception of an A-MSDU, where [...], is mandatory for
> >> an HT STA"
> >>
> >> And 7.1.2 "The maximum frame body size is determined by the maximum
> >> MSDU size (2304 octets) OR the maximum A-MSDU (3839 or 7935 octets,
> >> depending upon the STA's capability), plus any overhead from security
> >> encapsulation.
> >
> > Then, only solution is to mark this broken, and perform a copy of each
> > received frame, to keep a order-1 buffer(s) allocated for hardware.
> 
> -ENOTPOSSIBLE -- its an WFA requirement to RX AMSDU.
> 
> > Its too easy to have memory allocation failures for high order pages and
> > freeze the card.
> 
> Can't we us paged RX skbs, which mac80211 supports now?
> 
> See 2f301227a1ede57504694e1f64839839f5737cac and friends.
> 

Maybe you dont understand the point. A fix is needed for stable kernels.
paged RX skbs is probably a bit complex, even if copy/pasted from other
drivers.

If the hardware needs 8192 bytes (or 16384) buffers to perform its
operation, it should not give them back to linux, because there is no
guarantee it can allocate fresh ones for the next frames.