Re: PXA270 SSP DMA Corruption

From: Ned Forrester <nforrester-/d+BM93fTQY@public.gmane.org>
To: "J. Scott Merritt" <merrij3-IL7dBOYR4Vg@public.gmane.org>
Cc: spi-devel
	<spi-devel-general-5NWGOfrQmneRv+LV9MX5uipxlwaOVQ5f@public.gmane.org>
Subject: Re: PXA270 SSP DMA Corruption
Date: Wed, 12 Nov 2008 22:19:49 -0500	[thread overview]
Message-ID: <491B9CD5.3030908@whoi.edu> (raw)
In-Reply-To: <20081112213403.402948b9.merrij3-IL7dBOYR4Vg@public.gmane.org>

J. Scott Merritt wrote:
> On Wed, 12 Nov 2008 18:10:01 -0500
> It appears that the timeout is computed based upon the "Peripheral Clock
> frequency" on the PXA270 - which would appear to be 312Mhz, (or something
> divided down from there).  If it is 312Mhz, then for my SSP speed of
> 300K, I guess I need something around 10,000.

It says "peripheral clock frequency" without ever defining that phrase
(at least in the 1/2006 version of the developer's manual).  Oddly on
PXA255 this frequency appears to be runclock/4 = 99.5MHz for a 400MHz
machine.  I bet it is not as high as 312MHz, but you can measure it by
setting very long times, forcing the timeout (no tx data in pio mode, I
think), and perhaps using GPIO probes to trigger a scope.  I did
something like that on my system.  It would sure be nice to know the
answer to that, as no one using PXA270 has ever answered the question.

> Table 8-4 in my PXA manual is titled "TFT and RFT values with possible
> DMA Burst Sizes" ... for 8 bit data, it says ... for TFT > 7
> "Do not use DMA" ...  Am I misinterpreting this ?  I -think- that means
> that 8/8 is not a good choice ...

The table you refer to lists the values loaded into the TFT and RFT bit
fields, which are the desired threshold-1.  The values passed to
pxa2xx_spi are the desired thresholds, so TFT>7 is the same as threshold>8.

> With respect to the pxa2xx_spi patch, please proceed - I will certainly
> not attempt to generate one myself.

I'll get something out in the next couple of days.

-- 
Ned Forrester                                       nforrester-/d+BM93fTQY@public.gmane.org
Oceanographic Systems Lab                                  508-289-2226
Applied Ocean Physics and Engineering Dept.
Woods Hole Oceanographic Institution          Woods Hole, MA 02543, USA
http://www.whoi.edu/sbl/liteSite.do?litesiteid=7212
http://www.whoi.edu/hpb/Site.do?id=1532
http://www.whoi.edu/page.do?pid=10079

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