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From: David Brownell <david-b@pacbell.net>
To: nforrester@whoi.edu
Cc: spi-devel-general@lists.sourceforge.net, marc.pignat@hevs.ch,
	linux-kernel@vger.kernel.org, anemo@mba.ocn.ne.jp
Subject: Re: [spi-devel-general] [PATCH] atmel_spi: support zero length	 transfer
Date: Fri, 22 Feb 2008 11:02:28 -0800	[thread overview]
Message-ID: <20080222190228.2B0CB28E363@adsl-69-226-248-13.dsl.pltn13.pacbell.net> (raw)
In-Reply-To: <47BED734.5030002@whoi.edu>

> >> David, do you think writing 0 bytes is a valid use of this API?
> >
> > Just a zero byte transfer ... no, though it depends what you mean
> > by "valid".  (I'm not sure I'd expect all controller drivers to
> > reject such requests.)  That has no effect on bits-on-the-wire,
> > and would make trouble for various DMA engines.
>
> FWIW, the pxa2xx_spi driver does not, near as I can tell, reject zero
> length transfers, it will go through the motions, the same as for any
> other transfer.

Makes sense to me ... although:

> However, if the transfer is by DMA, note that the PXA255 and PXA270
> Developer's Manuals have the following language regarding DMA lengths:
>
> 	LEN = 0 means zero bytes for descriptor-fetch transactions.
> 	LEN = 0 is an invalid setting for no-descriptor-fetch
> 	transactions. ...
>
> Because the pxa2xx_spi driver does not currently use DMA descriptors,
> zero length DMAs are invalid.

In that case the pxa2xx_spi driver should add a special case to
avoid starting such transfers in DMA mode.


> > Passing zero bytes to get an inline delay at an exact spot in the
> > overall protocol message ... I don't see why not.  Better than
> > adding delay fields for every spot it might be needed by various
> > oddball devices, for sure!!
>
> I agree with Marc: any such delay will be undefined, in the general
> case.  It might work for a specific driver implementation.

Is that what Marc said?  I couldn't tell.  In any case, I disagree;
the semantics of that delay are clearly defined.

- Dave


  reply	other threads:[~2008-02-22 19:02 UTC|newest]

Thread overview: 18+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2008-02-20 15:54 Atsushi Nemoto
2008-02-20 17:55 ` Marc Pignat
2008-02-21  1:52   ` Atsushi Nemoto
2008-02-21  9:26     ` Marc Pignat
2008-02-21 19:23       ` David Brownell
2008-02-22  9:30         ` Marc Pignat
2008-02-22 14:15           ` Atsushi Nemoto
2008-02-22 14:28             ` [spi-devel-general] " Ned Forrester
2008-02-22 19:06               ` David Brownell
2008-02-22 19:52                 ` Ned Forrester
2008-02-22 18:58             ` David Brownell
2008-02-23  2:55           ` David Brownell
2008-02-25  8:15             ` Marc Pignat
2008-02-22 14:07         ` [spi-devel-general] " Ned Forrester
2008-02-22 19:02           ` David Brownell [this message]
2008-02-22 19:36             ` Ned Forrester
2008-02-23  2:37               ` David Brownell
2008-02-25  0:25               ` Atsushi Nemoto

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