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

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Date:	Wed, 31 Oct 2012 11:39:49 +0100
From:	Nicolas Ferre <nicolas.ferre@...el.com>
To:	Håvard Skinnemoen <havard@...nnemoen.net>,
	David Laight <David.Laight@...lab.com>
CC:	<netdev@...r.kernel.org>, <manabian@...il.com>,
	<patrice.vilchez@...el.com>, <linux-kernel@...r.kernel.org>,
	<bhutchings@...arflare.com>, <plagnioj@...osoft.com>,
	<davem@...emloft.net>, <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH v3 06/10] net/macb: clean up ring buffer logic

On 10/31/2012 10:59 AM, Nicolas Ferre :
> On 10/30/2012 07:22 PM, Håvard Skinnemoen :
>> 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.
> 
> Absolutely.

Well not so absolute, after having thinking twice ;-)

We should move to:

static unsigned int macb_tx_ring_avail(struct macb *bp)
{
	return (TX_RING_SIZE - (bp->tx_head - bp->tx_tail) & (TX_RING_SIZE - 1));
}

Thanks David!

(sorry for the noise) Bye,
-- 
Nicolas Ferre
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