From: "Dan Williams" <dan.j.williams@intel.com>
To: "Guennadi Liakhovetski" <g.liakhovetski@gmx.de>
Cc: linux-kernel@vger.kernel.org, netdev@vger.kernel.org,
maciej.sosnowski@intel.com, hskinnemoen@atmel.com,
nicolas.ferre@atmel.com
Subject: Re: [PATCH 03/13] dmaengine: up-level reference counting to the module level
Date: Thu, 4 Dec 2008 11:51:53 -0700 [thread overview]
Message-ID: <e9c3a7c20812041051l44bd8086o97c12d77a116f62@mail.gmail.com> (raw)
In-Reply-To: <Pine.LNX.4.64.0812041746400.5079@axis700.grange>
On Thu, Dec 4, 2008 at 9:56 AM, Guennadi Liakhovetski
<g.liakhovetski@gmx.de> wrote:
>> +
>> + /* try to grab channels */
>> + list_for_each_entry_safe(device, _d, &dma_device_list, global_node)
>> + list_for_each_entry(chan, &device->channels, device_node) {
>> + err = dma_chan_get(chan);
>
> Dan, could you please explain this: dma_chan_get() takes a reference on
> the channel _and_ calls .device_alloc_chan_resources() on first invocation
> for a specific channel. I now see three locations in dmaengine.c, where
> dma_chan_get() is called: in dma_request_channel() - logical, but also in
> dmaengine_get() and dma_async_device_register(), and these latter two I
> don't understand. I do not understand why we have to grab references and
> allocate resources for all (public) channels on all controllers in the
> system if someone just called dmaengine_get()?
Consider the case where a subsystem that consumes engines like
async_tx or net_dma loads before engines are available in the system.
They will have taken a reference against dmaengine, but calls to
dma_find_channel will return NULL. Once a channel driver is loaded
dma_find_channel() can start returning results, and it is at this
point that resources must be allocated and the backing module pinned.
So dmaengine_get() means "I am interested in offloading stuff, if you
see an offload resource grab it and prep it so I can discover it with
dma_find_channel()".
>
>> @@ -420,6 +443,19 @@ int dma_async_device_register(struct dma_device *device)
>> }
>>
>> mutex_lock(&dma_list_mutex);
>> + list_for_each_entry(chan, &device->channels, device_node) {
>> + /* if clients are already waiting for channels we need to
>> + * take references on their behalf
>> + */
>> + if (dmaengine_ref_count && dma_chan_get(chan) == -ENODEV) {
>> + /* note we can only get here for the first
>> + * channel as the remaining channels are
>> + * guaranteed to get a reference
>> + */
>
> This is the second location - where and how are clients waiting for
> channels? In the old implementation clients had notification callbacks,
> which were called as new channels became available. Now clients are gone,
> so, what is meant here?
>
The assumption is that mem-to-mem offload clients poll dma_find_channel().
Regards,
Dan
next prev parent reply other threads:[~2008-12-04 18:52 UTC|newest]
Thread overview: 52+ messages / expand[flat|nested] mbox.gz Atom feed top
2008-11-14 21:34 [PATCH 00/13] dmaengine redux Dan Williams
2008-11-14 21:34 ` [PATCH 01/13] async_tx, dmaengine: document channel allocation and api rework Dan Williams
2008-11-14 21:34 ` [PATCH 02/13] dmaengine: remove dependency on async_tx Dan Williams
2008-11-15 6:02 ` Andrew Morton
2008-11-17 23:44 ` Dan Williams
2008-11-14 21:34 ` [PATCH 03/13] dmaengine: up-level reference counting to the module level Dan Williams
2008-11-15 6:08 ` Andrew Morton
2008-11-18 3:42 ` Dan Williams
2008-12-04 16:56 ` Guennadi Liakhovetski
2008-12-04 18:51 ` Dan Williams [this message]
2008-12-04 19:28 ` Guennadi Liakhovetski
2008-12-08 22:39 ` Dan Williams
2008-12-12 14:28 ` Sosnowski, Maciej
2008-12-15 22:12 ` Dan Williams
2008-12-18 14:26 ` Sosnowski, Maciej
2008-11-14 21:34 ` [PATCH 04/13] dmaengine: centralize channel allocation, introduce dma_find_channel Dan Williams
2008-11-15 6:14 ` Andrew Morton
2008-11-18 5:59 ` Dan Williams
2008-11-14 21:34 ` [PATCH 05/13] dmaengine: provide a common 'issue_pending_all' implementation Dan Williams
2008-11-14 21:34 ` [PATCH 06/13] net_dma: convert to dma_find_channel Dan Williams
2008-11-14 21:34 ` [PATCH 07/13] dmaengine: introduce dma_request_channel and private channels Dan Williams
2008-12-02 15:52 ` Guennadi Liakhovetski
2008-12-02 17:16 ` Dan Williams
2008-12-02 17:27 ` Guennadi Liakhovetski
2008-12-02 19:10 ` Dan Williams
2008-12-02 21:28 ` Guennadi Liakhovetski
2009-01-30 17:03 ` Atsushi Nemoto
2009-01-30 23:13 ` Dan Williams
2009-01-30 23:27 ` Guennadi Liakhovetski
2009-01-31 12:18 ` Atsushi Nemoto
2008-12-02 17:26 ` Nicolas Ferre
2008-12-12 14:29 ` Sosnowski, Maciej
2008-12-15 23:55 ` Dan Williams
2008-12-18 14:33 ` Sosnowski, Maciej
2008-12-18 17:27 ` Dan Williams
2009-02-06 16:58 ` Atsushi Nemoto
2008-11-14 21:34 ` [PATCH 08/13] dmatest: convert to dma_request_channel Dan Williams
2008-11-15 6:17 ` Andrew Morton
2008-11-18 18:24 ` Dan Williams
2008-11-18 20:58 ` Andrew Morton
2008-11-18 22:19 ` Dan Williams
2008-11-14 21:35 ` [PATCH 09/13] atmel-mci: convert to dma_request_channel and down-level dma_slave Dan Williams
2009-01-30 16:40 ` Atsushi Nemoto
2009-01-30 23:02 ` Dan Williams
2008-11-14 21:35 ` [PATCH 10/13] dmaengine: replace dma_async_client_register with dmaengine_get Dan Williams
2008-11-14 21:35 ` [PATCH 11/13] dmaengine: kill struct dma_client and supporting infrastructure Dan Williams
2008-12-12 14:29 ` Sosnowski, Maciej
2008-12-16 0:09 ` Dan Williams
2008-12-18 14:34 ` Sosnowski, Maciej
2008-11-14 21:35 ` [PATCH 12/13] dmaengine: remove 'bigref' infrastructure Dan Williams
2008-11-14 21:35 ` [PATCH 13/13] dmaengine: kill enum dma_state_client Dan Williams
2008-12-12 14:27 ` [PATCH 00/13] dmaengine redux Sosnowski, Maciej
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