linux-kernel - Re: [PATCH 2/2] ioatdma: add DMA

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Date:	Wed, 19 Jun 2013 13:44:08 -0700
From:	Dan Williams <djbw@...com>
To:	Jon Mason <jon.mason@...el.com>
Cc:	Vinod Koul <vinod.koul@...el.com>,
	Dave Jiang <dave.jiang@...el.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/2] ioatdma: add DMA_PRIVATE capabilities flag

On Wed, Jun 19, 2013 at 1:10 PM, Jon Mason <jon.mason@...el.com> wrote:
> On Wed, Jun 19, 2013 at 11:56:38AM -0700, Dan Williams wrote:
>> On Wed, Jun 19, 2013 at 10:52 AM, Jon Mason <jon.mason@...el.com> wrote:
>> >> DMA_PRIVATE here keeps all channels private, so they couldn't be used
>> >> elsewhere, for example raid offload.  Do you need a private allocation
>> >> or can you get away with a dynamically assigned channel?
>> >
>> > I would like to have a dedicated DMA engine.  async_tx could cause the
>> > copies to complete out of order.  Do I need to add infrastructure to
>> > allow for private channel usage, and when unused allow the channel to
>> > be used by async_tx?
>>
>> Can NTB just call dma_find_channel() once and be done?  Also, async_tx
>> will only get things out of order if you allow it to pick a new
>> channel for every operation, but as long as you specify a dependency
>> chain it will keep things in order (same as remembering the result of
>> dma_find_channel).
>
> Neither of these allow a device exclusive access to a channel.  Is
> this not something that is desired by other users of DMA Engines, or
> do I have a unique usage model?
>

Yes, ioatdma has never been used in a mixed model.  Other platforms
are either using devicetree bindings for specifying channels to their
master devices or are exclusively slave channels.  Other slave usage
models require use of a dedicated channel, since NTB is alternating
between cpu and dma it seems it could get away with using a "public"
channel.
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