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

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Date:	Sun, 30 Mar 2008 21:29:06 +0900 (JST)
From:	Atsushi Nemoto <anemo@....ocn.ne.jp>
To:	haavard.skinnemoen@...el.com
Cc:	david-b@...bell.net, spi-devel-general@...ts.sourceforge.net,
	linux-kernel@...r.kernel.org, akpm@...ux-foundation.org,
	marc.pignat@...s.ch, nforrester@...i.edu
Subject: Re: [PATCH] atmel_spi: support zero length transfer (resend)

On Sat, 29 Mar 2008 17:56:51 +0100, Haavard Skinnemoen <haavard.skinnemoen@...el.com> wrote:
> > Though some discussion about zero length transfer were raised by this
> > patch last time, I think there were no explicit objection to this
> > patch itself.
> 
> Well, the patch doesn't seem to do any harm, but I can't see much of a
> point to it either if zero-length transfers aren't going to be
> allowed...

Well, some wired device might want long delay before first CLK edge.
I think most device do not have such constraint, but ...

Anyway, SPI framework does not reject it.  So I think the controller
driver should accept it unless it is hard to support on that hardware.

> Also, if the length is zero, the driver will end up doing a memset with
> length zero at some point. Is that allowed?

I believe memset() with zero length should be allowed.  It seems
ambiguous that dma_sync_single_for_device() with zero length is legal
or not.  Actually, it seems OK on ARM and AVR32.

---
Atsushi Nemoto
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