netdev - RE: [PATCH v3 06/10] net/macb: clean up ring buffer logic

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Date:	Wed, 31 Oct 2012 08:57:43 -0000
From:	"David Laight" <David.Laight@...LAB.COM>
To:	Håvard Skinnemoen <havard@...nnemoen.net>
Cc:	"Nicolas Ferre" <nicolas.ferre@...el.com>,
	<netdev@...r.kernel.org>, <davem@...emloft.net>,
	<bhutchings@...arflare.com>,
	<linux-arm-kernel@...ts.infradead.org>, <plagnioj@...osoft.com>,
	<patrice.vilchez@...el.com>, <linux-kernel@...r.kernel.org>,
	<manabian@...il.com>
Subject: RE: [PATCH v3 06/10] net/macb: clean up ring buffer logic

> On Tue, Oct 30, 2012 at 4:12 AM, David Laight <David.Laight@...lab.com> wrote:
> >> Instead of masking head and tail every time we increment them, just let them
> >> wrap through UINT_MAX and mask them when subscripting. Add simple accessor
> >> functions to do the subscripting properly to minimize the chances of messing
> >> this up.
> > ...
> >> +static unsigned int macb_tx_ring_avail(struct macb *bp)
> >> +{
> >> +     return TX_RING_SIZE - (bp->tx_head - bp->tx_tail);
> >> +}
> >
> > That one doesn't look quite right to me.
> > Surely it should be masking with 'TX_RING_SIZE - 1'
> 
> Why is that? head and tail can never be more than TX_RING_SIZE apart,
> so it shouldn't make any difference.

It's a ring buffer (I presume) the pointers can be in either order.

	David



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