Date: Tue, 06 Jan 2009 23:31:10 +0900 (JST)
Message-Id: <20090106.233110.106262211.anemo@mba.ocn.ne.jp>
To: haavard.skinnemoen@atmel.com
Cc: linux-kernel@vger.kernel.org, maciej.sosnowski@intel.com,
       dan.j.williams@intel.com
Subject: Re: dw_dmac driver questions
From: Atsushi Nemoto <anemo@mba.ocn.ne.jp>
In-Reply-To: <20090106111403.2bdc0f83@hskinnemoen-d830>
References: <20090105164847.6fbca375@hskinnemoen-d830>
	<20090106.103344.134344452.nemoto@toshiba-tops.co.jp>
	<20090106111403.2bdc0f83@hskinnemoen-d830>
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On Tue, 6 Jan 2009 11:14:03 +0100, Haavard Skinnemoen <haavard.skinnemoen@atmel.com> wrote:
> > 1) driver enqueue desc A to active list and start DMA engine
> > 2) driver enqueue desc B to queue list
> > 3) driver enqueue desc C to queue list  (desc C contains bad address)
> > 4) DMA engine finish desc A and raise interrupt
> > 5) tasklet remove desc A from active list and move desc B and C to active list
> > 6) DMA engine finish desc B and raise interrupt
> > 7) DMA engine abort desc C and raise interrupt
> > 8) tasklet detect error
> > 
> > The point is the order of (7) and (8) cannot be expected.  If (7)
> > comes first, the head of the active list contains desc B at (8) and
> > that is not a bad descriptor.
> 
> The tasklet won't detect any errors unless the DMA controller flags it,
> so (7) must happen before (8). That does not necessarily mean that the
> interrupts from (6) and (7) get handled before (8), but I don't think
> it matters because dwc_handle_error() calls dwc_scan_descriptors(),
> which will remove desc B from the active list if it is finished. And it
> must be finished if desc C has failed, so the failed descriptor will
> always be at the head of the queue after dwc_scan_descriptors() returns.

Oh I understand.  I overlooked dwc_scan_descriptors() call at
beginning of dwc_handle_error().  Thank you for explanation.

---
Atsushi Nemoto
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