linux-kernel - Re: dw_dmac driver questions

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Message-Id: <20090109.230626.126760668.anemo@mba.ocn.ne.jp>
Date:	Fri, 09 Jan 2009 23:06:26 +0900 (JST)
From:	Atsushi Nemoto <anemo@....ocn.ne.jp>
To:	haavard.skinnemoen@...el.com
Cc:	linux-kernel@...r.kernel.org, maciej.sosnowski@...el.com,
	dan.j.williams@...el.com
Subject: Re: dw_dmac driver questions

On Mon, 05 Jan 2009 23:02:30 +0900 (JST), Atsushi Nemoto <anemo@....ocn.ne.jp> wrote:
> On Mon, 29 Dec 2008 23:39:32 +0900 (JST), Atsushi Nemoto <anemo@....ocn.ne.jp> wrote:
> > Hi.  I'm writing a new DMA driver, using dw_dmac driver in kernel
> > 2.6.28 as a reference implementation.
> > 
> > I can see an outline of the dw_dmac driver and framework (well,
> > hopefully), but have some questions in details.
> 
> One more.

One more again.

5. dwc_issue_pending()

Is this needed?  The dwc_tx_submit might queue the descriptor, but it
should be issued automatically after completion of all descriptors in
active list.

It yes, it seems implementation like this would be possible:

* dwc_tx_submit() just always add the descriptor to queue list even if
  active list was empty.

* dwc_issue_pending() call dwc_dostart() and move queued descriptor(s)
  to active list, if the active list was empty.

But I'm not sure "chain as many as possible at a time" is really
better then "start engine as soon as possible".

---
Atsushi Nemoto
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