linux-kernel - Re: [PATCH -mm v4 7/9] atmel

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Date:	Mon, 28 Jan 2008 12:41:25 +0100
From:	Haavard Skinnemoen <hskinnemoen@...el.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	linux@...im.org.za, linux@...mer.net, coldwell@...hat.com,
	marc.pignat@...s.ch, david-b@...bell.net,
	linux-kernel@...r.kernel.org, alan@...rguk.ukuu.org.uk
Subject: Re: [PATCH -mm v4 7/9] atmel_serial: Add DMA support

On Mon, 28 Jan 2008 02:20:00 -0800
Andrew Morton <akpm@...ux-foundation.org> wrote:

> On Mon, 28 Jan 2008 10:59:09 +0100 Haavard Skinnemoen <hskinnemoen@...el.com> wrote:
> 
> > > ho-hum.  The generic uart buffer-handling code does ringbuffers the wrong
> > > way.  Maybe it has to handle non-power-of-two buffer sizes.
> > 
> > Hmm...I don't understand. What does it do wrong?
> 
> An faq ;) If the buffer size is a power-of-two it's better to allow the
> head and tail indices wrap through 0xffffffff and only mask them when
> subscripting.  It ends up faster (usually) and you can use all of the
> elements of the buffer (rather than all-1) and you get nice things like:
> 
> is_empty = (head == tail)
> is_full = (tail - head == size)
> nr_items_in_ring = (tail - head)

Ok, that makes sense. Thanks for explaining. Not sure if want to start
improving things right now, although I'm pretty sure the circ stuff
can't handle non-power-of-two buffer size currently so it should be
possible.

> > > All those uart_circ_*() macros reference their arg more than once and ... 
> > > you know the deal.
> > 
> > Yeah. Would you like a patch that inline-ifies <linux/circ.h>?
> 
> uh, if you're feeling especially keen.  We have bigger problems than this.

Well, if you put it like that; no, not really.

I'll post a fix for the other things you pointed out shortly.

Haavard
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