linux-kernel - RE: [PATCH v2] dmaengine: Simple DMA memcpy test client

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Date:	Fri, 14 Dec 2007 09:45:26 -0800
From:	"Nelson, Shannon" <shannon.nelson@...el.com>
To:	"Olof Johansson" <olof@...om.net>,
	"Haavard Skinnemoen" <hskinnemoen@...el.com>
Cc:	"Williams, Dan J" <dan.j.williams@...el.com>,
	<linux-kernel@...r.kernel.org>, "Sam Ravnborg" <sam@...nborg.org>
Subject: RE: [PATCH v2] dmaengine: Simple DMA memcpy test client

>From: Olof Johansson [mailto:olof@...om.net] 
>Sent: Thursday, December 13, 2007 7:32 PM
>To: Haavard Skinnemoen
>Cc: Nelson, Shannon; Williams, Dan J; 
>linux-kernel@...r.kernel.org; Sam Ravnborg
>Subject: Re: [PATCH v2] dmaengine: Simple DMA memcpy test client
>
>On Fri, Nov 23, 2007 at 04:34:36PM +0100, Haavard Skinnemoen wrote:
>> This client tests DMA memcpy using various lengths and 
>various offsets
>> into the source and destination buffers. It will initialize both
>> buffers with a know pattern and verify that the DMA engine copies the
>> requested region and nothing more.
>> 
>> Signed-off-by: Haavard Skinnemoen <hskinnemoen@...el.com>
>
>Hi,
>
>First of all: Thanks for sharing this, it's quite useful! I've been
>playing around with this a bit today, and I've been seeing some odd
>behaviour.
>
>It seems that once a channel is allocated to dmatest, it will never be
>freed, i.e. the drivers device_free_chan_resources will never be called
>on it. Looking at the dma stack, I'm not sure just where it's expected
>to happen. Once the channel is allocated, the dma_chan_get/put 
>calls all
>just modify the per-cpu variables, and nothing will ever boil down to a
>call to kref_put() of the refcount until the _driver_ is deregistered.
>Not even deregistering the client seems to do it.
>
>Or am I missing something here? Shannon? Dan?
>
>I happened to catch it due to a BUG_ON() in my 
>device_alloc_chan_resources
>that checked to make sure it wasn't allocated twice without a free
>inbetween. It hit on the second load of the dmatest module, since they
>were never freed on unload.
>
>
>-Olof
>

No, you're not missing anything, you've read it right.  Notice at the
top of the ioatdma's ioat_dma_alloc_chan_resources() we check to see if
we've already been here.

I suspect this is a hangover from the earlier dmaengine design where TCP
was the only client and it never released things, so the only time we
needed to free the resources was when ioatdma was unloaded.

sln
--
======================================================================
Mr. Shannon Nelson                 LAN Access Division, Intel Corp.
Shannon.Nelson@...el.com                I don't speak for Intel
(503) 712-7659                    Parents can't afford to be squeamish.

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