linux-kernel - Re: [spi-devel-general] atmel

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Message-Id: <20080219.235123.07645009.anemo@mba.ocn.ne.jp>
Date:	Tue, 19 Feb 2008 23:51:23 +0900 (JST)
From:	Atsushi Nemoto <anemo@....ocn.ne.jp>
To:	hskinnemoen@...el.com
Cc:	david-b@...bell.net, spi-devel-general@...ts.sourceforge.net,
	linux-kernel@...r.kernel.org
Subject: Re: [spi-devel-general] atmel_spi clock polarity

On Mon, 18 Feb 2008 23:49:18 +0100, Haavard Skinnemoen <hskinnemoen@...el.com> wrote:
> > > CLK     ______________________|~|___|~~~|___|~~~|___|~~~|___
> > 
> > ... and at T1 CPOL is changed??  That's wrong.  There should
> > never be a partial clock period while a chipselect is active.
> > While it's inactive, sure -- no chip should care.
> 
> ...but what I'm afraid of is that since we're using GPIO chipselects,
> the controller may _think_ that no chip is selected and change the
> clock polarity just before it would pull the chipselect low if we were
> using automatic chipselects...

Yes.  That's I suppose.

> If that's the case, it would be a bit strange if it actually helped to
> initialize all the CSRn registers, but it would also offer us a nice
> workaround...

I suppose the clock state of the AT91 just follows _last_ transfer.
My patch (setting all CSRn.POL for next transfer) was based on that
assumption.

> Atsushi, do you by any chance have debugging enabled? That would
> explain the long delay from CS activation to the change of clock
> polarity. Compared to printk() the DMA setup takes almost no time at
> all.

No, I did not enabled debugging.

> If you can confirm that my patch helps, I think that's the one we want
> in mainline.

Unfortunately I had no time to try it today.  Hopefully tomorrow...

---
Atsushi Nemoto
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