Subject: Re: [PATCH/RFT] iwlagn: fix RX skb alignment
From: Johannes Berg <johannes@sipsolutions.net>
To: John Linville <linville@tuxdriver.com>
Cc: Tomas Winkler <tomasw@gmail.com>,
	reinette chatre <reinette.chatre@intel.com>,
	Marcel Holtmann <marcel@holtmann.org>,
	"Luis R. Rodriguez" <mcgrof@gmail.com>,
	"Rafael J. Wysocki" <rjw@sisk.pl>, Matt Mackall <mpm@selenic.com>,
	Christophe Dumez <dchris@gmail.com>, Zhu Yi <yi.zhu@intel.com>,
	Jan =?UTF-8?Q?V=C4=8Del=C3=A1k?= <vcelak.jan@seznam.cz>,
	Thomas Witt <witt_th@gmx.de>,
	linux-wireless <linux-wireless@vger.kernel.org>,
	Andres Freund <andres@anarazel.de>
In-Reply-To: <1226969241.4014.24.camel@johannes.berg> (sfid-20081118_014839_339263_738F563C)
References: <1226969241.4014.24.camel@johannes.berg>
	 (sfid-20081118_014839_339263_738F563C)
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Date: Tue, 18 Nov 2008 10:43:10 +0100
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On Tue, 2008-11-18 at 01:47 +0100, Johannes Berg wrote:

> So I dug deeper into the DMA problems I had with iwlagn and a kind soul
> helped me in that he said something about pci-e alignment and mentioned
> the iwl_rx_allocate function to check for crossing 4KB boundaries. Since
> there's 8KB A-MPDU support, crossing 4k boundaries didn't seem like
> something the device would fail with

Come to think of it, maybe it does. One common failure case seems to be
"this goes wrong with an 11n AP, but not on a regular one", so one
explanation for that would be that the hardware designers expect the
driver to split up the DMA into multiple DMA descriptors at 4K
boundaries, but iwlagn uses only one and undefined behaviour results.
That's something you'll need to ask the HW designers though or look up
in the datasheet.

johannes

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