linux-kernel - Re: [PATCH] atmel_spi: support zero length transfer

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Message-Id: <20080221.105233.41199605.nemoto@toshiba-tops.co.jp>
Date:	Thu, 21 Feb 2008 10:52:33 +0900 (JST)
From:	Atsushi Nemoto <anemo@....ocn.ne.jp>
To:	marc.pignat@...s.ch
Cc:	hskinnemoen@...el.com, david-b@...bell.net,
	spi-devel-general@...ts.sourceforge.net,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] atmel_spi: support zero length transfer

On Wed, 20 Feb 2008 18:55:01 +0100, Marc Pignat <marc.pignat@...s.ch> wrote:
> > A spi transfer with zero length is not invalid.  Such transfer can be
> > used to achieve delay before first CLK edge after chipselect assertion.
> How long will be that delay?

My funny custom device requires 100us or so.  Unfortunately atmel_spi
can not use such a slow bitrate.

> If they are really users of that kind of thing, this should be fixed by adding
> a "delay_us_before_xfer" field in the  struct spi_transfer.

Yes, it would be an another way to achieve it.  But as long as zero
length transfer is legal on this API, I don't want to add other
fields.

> Have you tested it? I think if you start a transfer with 0 len, the ENDRX bit
> will never rise, however, I'm not sure about this.

Yes.  I tested it on AT91SAM9260 and it seems ENDRX rises soon.
Though it can be possible to avoid starting DMA for zero length
transfer, I think it is not worth to optimize for such a rare case.

---
Atsushi Nemoto
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