linux-kernel - Wrong "nollp" DW DMAC parameter value on ARC SDP.

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Date:	Fri, 12 Aug 2016 08:03:14 +0000
From:	Eugeniy Paltsev <Eugeniy.Paltsev@...opsys.com>
To:	"dmaengine@...r.kernel.org" <dmaengine@...r.kernel.org>,
	"andriy.shevchenko@...ux.intel.com" 
	<andriy.shevchenko@...ux.intel.com>
CC:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"Nelson Pereira" <Nelson.Pereira@...opsys.com>,
	"viresh.kumar@...aro.org" <viresh.kumar@...aro.org>,
	"vinod.koul@...el.com" <vinod.koul@...el.com>,
	"linux-snps-arc@...ts.infradead.org" 
	<linux-snps-arc@...ts.infradead.org>
Subject: Wrong "nollp" DW DMAC parameter value on ARC SDP.

Hi,

"nollp" parameter defines if DW DMAC channel supports multi block
transfer or not.

It is calculated in runtime, but differently depending on on
availability of pdata. If pdata is absent "nollp" is calculated using
autoconfig hardware registers. Otherwise "nollp" is calculated using
the next code construction:
channel_writel(dwc, LLP, DWC_LLP_LOC(0xffffffff));
dwc->nollp = DWC_LLP_LOC(channel_readl(dwc, LLP)) == 0;
channel_writel(dwc, LLP, 0);

I realized that these methods give different results.
For example on ARC AXS101 SDP in case of using autoconfig "nollp" was
calculated as "true" (and DMAC works fine), 
otherwise "nollp" was calculated as "false" (and DMAC doesn't work).

So I'm wondering how the code in question really works?
From DW AHB DMAC databook I wasn't able to find anything relevant to
this tricky implementation. Could you please clarify a little but what
happens here?
Maybe we should add "nollp" field in pdata structure and receive it
from pdata/device tree (like we use "is_private" or "is_memcpu" fields)

-- 
 Paltsev Eugeniy