netdev - Re: [PATCH (net-next.git) 13/18] stmmac: perf, remove modulo in stmmac

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Message-ID: <5669096B.6050505@st.com>
Date:	Thu, 10 Dec 2015 06:11:07 +0100
From:	Giuseppe CAVALLARO <peppe.cavallaro@...com>
To:	David Laight <David.Laight@...LAB.COM>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>
CC:	"alexandre.torgue@...com" <alexandre.torgue@...com>,
	Fabrice Gasnier <fabrice.gasnier@...com>
Subject: Re: [PATCH (net-next.git) 13/18] stmmac: perf, remove modulo in stmmac_rx()

On 12/9/2015 6:21 PM, David Laight wrote:
> From: Giuseppe Cavallaro
>> Sent: 09 December 2015 08:38
>> The indexes into the ring buffer are always incremented, and
>> the entry is accessed via doing a modulo to find the "real" index.
>> Modulo is an expensive operation.
>>
>> This patch replaces the modulo with a simple if clamp.
>> It helps improve stmmac RX path as it's being called inside RX loop.
>
> Is dma_rx_size always a power of 2 ?

no

> If so you can replace the % by & and remove the conditionals.
>
> If you have changed the read and write values to indexes
> then you need to be certain that the 'ring full' and 'ring empty'
> cases are correctly distinguished.

this is implicitly managed by dirty and curr index.

thx
peppe

>
> 	David
>

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