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

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Date:	Fri, 12 Aug 2016 13:59:44 +0300
From:	Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To:	Eugeniy Paltsev <Eugeniy.Paltsev@...opsys.com>,
	"dmaengine@...r.kernel.org" <dmaengine@...r.kernel.org>
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: Re: Wrong "nollp" DW DMAC parameter value on ARC SDP.

On Fri, 2016-08-12 at 08:03 +0000, Eugeniy Paltsev wrote:
> 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).

Can you show out what the value you read back?

> 
> 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?

"Table 4-1:
...
Hardcode Channel x LLP register to 0?
...
Description: If set to 1, hardcodes channel x Linked List Pointer
register to 0 (LLPx.LOC == 0), ..."


> 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)

Yeah, perhaps we can remove that trick since we need this flag to be set
on Intel Quark which might have the same issue as your case [1].

[1] http://www.spinics.net/lists/linux-serial/msg22948.html

-- 
Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
Intel Finland Oy