linux-kernel - RE: [PATCH 11/13] dmaengine: kill struct dma

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Message-ID: <129600E5E5FB004392DDC3FB599660D70C984F6E@irsmsx504.ger.corp.intel.com>
Date:	Thu, 18 Dec 2008 14:34:18 +0000
From:	"Sosnowski, Maciej" <maciej.sosnowski@...el.com>
To:	"Williams, Dan J" <dan.j.williams@...el.com>
CC:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"hskinnemoen@...el.com" <hskinnemoen@...el.com>,
	"g.liakhovetski@....de" <g.liakhovetski@....de>,
	"nicolas.ferre@...el.com" <nicolas.ferre@...el.com>
Subject: RE: [PATCH 11/13] dmaengine: kill struct dma_client and supporting
 infrastructure

Dan Williams wrote:
> On Fri, Dec 12, 2008 at 7:29 AM, Sosnowski, Maciej
> <maciej.sosnowski@...el.com> wrote:
>> Williams, Dan J wrote:
>>> All users have been converted to either the general-purpose
>>> allocator, dma_find_channel, or dma_request_channel.
>>> 
>>> Signed-off-by: Dan Williams <dan.j.williams@...el.com>
>>> ---
>> (...)
>>>  /**
>>> - * dma_chans_notify_available - broadcast available channels to
>>> the clients 
>>> - */
>>> -static void dma_clients_notify_available(void)
>>> -{
>>> -     struct dma_client *client;
>>> -
>>> -     mutex_lock(&dma_list_mutex);
>>> -
>>> -     list_for_each_entry(client, &dma_client_list, global_node)
>>> -             dma_client_chan_alloc(client);
>>> -
>>> -     mutex_unlock(&dma_list_mutex);
>>> -}
>> 
>> I agree with Guennadi's concern about removing clients' notification
>> of new devices available in the system.
>> I understand that this design is based on polling instead,
>> however polling is always less efficient approach.
>> Do you think that restoring notifications in this redesigned
>> dmaengine would be more painful than limiting clients to polling
>> solution? 
>> 
> 
> You are missing that net_dma has always "polled".  Consider the case
> of how net_dma currently operates prior to ioatdma.ko being loaded.
> It periodically calls get_softnet_dma() to see if a channel is
> available, if it does not find one it falls back to a cpu copy.  All
> that has changed is replacing this custom channel allocation routine
> with the unified dma_find_channel() interface.  Channel notifications
> are not required.  Either everything is built-in to guarantee that an
> engine is available upon request[1], or the client is smart enough to
> run without a channel for while like net_dma and raid offload.

My comment here was rather more general than ioatdma/net_dma specific.
You are of course right that net_dma has been always based on polling.
I am rather worrying a bit that this change limits all current and future clients
to polling approach only, not letting them base on notifications anymore.

> 
> Regards,
> Dan
> 
> [1] http://marc.info/?l=linux-kernel&m=122835195303213&w=2

Regards,
Maciej--
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