linux-kernel - Re: [PATCH 2/2] dw_dmac: return proper residue value

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Date:	Tue, 22 Jan 2013 09:24:51 +0200
From:	Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To:	Vinod Koul <vinod.koul@...el.com>
Cc:	Andy Shevchenko <andriy.shevchenko@...ux.jf.intel.com>,
	Viresh Kumar <viresh.linux@...il.com>,
	linux-kernel@...r.kernel.org, spear-devel <spear-devel@...t.st.com>
Subject: Re: [PATCH 2/2] dw_dmac: return proper residue value

On Mon, 2013-01-21 at 06:22 -0800, Vinod Koul wrote: 
> On Mon, Jan 21, 2013 at 11:45:51AM +0200, Andy Shevchenko wrote:
> > > > +	return 0;
> > > hmmm, why not use BLOCK_TS value. That way you dont need to look at direction
> > > and along with burst can easily calculate residue...
> > 
> > Do you mean to read CTL hi/lo and do
> > 
> > desc->len - ctlhi.block_ts * ctllo.src_tr_width?
> > 
> Yes
> > I think it could be not precise when memory-to-peripheral transfer is
> > going on. In that case you probably will have src_tr_width like 32 bits,
> > meanwhile peripheral may receive only byte stream.
> Nope that is not the case.
> SAR/DAR is always incremented in src/dstn_tr_width granularity. For example if
> you are using MEM to DMA, then SAR will always increment in case of x86 in 4byte
> granularity as we will read bursts not singles.
> 
> Also if check the spec, it says "Once the transfer starts, the read-back value is the
> total number of data items already read from the source peripheral, regardless of
> what is the flow controller"
> 
> So basically you get what is read from buffer in case of MEM->PER and get what
> is read from FIFO in case of PER->MEM which IMO gives you better or equal results
> than your calulation.

I will try this. Indeed I don't like usage of direction as well, and
your solution seems much clear in that sense.

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