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

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Date:	Thu, 13 Dec 2007 21:32:21 -0600
From:	Olof Johansson <olof@...om.net>
To:	Haavard Skinnemoen <hskinnemoen@...el.com>
Cc:	Shannon Nelson <shannon.nelson@...el.com>,
	Dan Williams <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

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
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