Wrong "nollp" DW DMAC parameter value on ARC SDP.

[Andy Shevchenko]

On Fri, 2016-08-12 at 13:36 +0000, Eugeniy Paltsev wrote:
> On Fri, 2016-08-12 at 13:59 +0300, Andy Shevchenko wrote:
> > 
> > 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?
> 
> channel_readl(dwc, LLP) return 0xfffffffc

Nice.

Oh, forgot to ask, what are the DW_PARAMS and DWC_PARAMS[x] are on the
same hardware?

I assume we are talking about that one which has no hardware LLP
support.

> > > 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), ..."

So, any comment on this one? I suppose you may have an access to some
internal Synopsys documentation which might shed a light. Or maybe I
missed something else which should be considered.

> > > 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
> > 
> 
> In which tree I can find this patch applied, so I may base my work on
> it?

The series is under review. I'm preparing v10, so, I would like to re-
make this patch with regarding to your input.

For now I would prefer just to remove the trick, but I still wonder what
the circumstances are to bring it not working.

-- 
Andy Shevchenko <andriy.shevchenko at linux.intel.com>
Intel Finland Oy