linux-kernel - Re: [PATCH 8/8] dmaengine/dw_dmac: Add support for DMA_SLAVE

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Date:	Tue, 17 Jan 2012 15:40:44 +0530
From:	Jassi Brar <jaswinder.singh@...aro.org>
To:	Viresh Kumar <viresh.kumar@...com>
Cc:	"vinod.koul@...el.com" <vinod.koul@...el.com>,
	Pratyush ANAND <pratyush.anand@...com>,
	Rajeev KUMAR <rajeev-dlh.kumar@...com>,
	"linux@....linux.org.uk" <linux@....linux.org.uk>,
	Bhupesh SHARMA <bhupesh.sharma@...com>,
	Armando VISCONTI <armando.visconti@...com>,
	"linus.walleij@...aro.org" <linus.walleij@...aro.org>,
	Mirko GARDI <mirko.gardi@...com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Vipin KUMAR <vipin.kumar@...com>,
	Shiraz HASHIM <shiraz.hashim@...com>,
	Amit VIRDI <Amit.VIRDI@...com>,
	Vipul Kumar SAMAR <vipulkumar.samar@...com>,
	"viresh.linux@...il.com" <viresh.linux@...il.com>,
	Deepak SIKRI <deepak.sikri@...com>,
	Bhavna YADAV <bhavna.yadav@...com>,
	"dan.j.williams@...el.com" <dan.j.williams@...el.com>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	Vincenzo FRASCINO <Vincenzo.FRASCINO@...com>
Subject: Re: [PATCH 8/8] dmaengine/dw_dmac: Add support for DMA_SLAVE_CONFIG

On 16 January 2012 17:12, Viresh Kumar <viresh.kumar@...com> wrote:
> On 1/16/2012 3:20 PM, Viresh KUMAR wrote:
>> +     /*
>> +      * Fix sconfig's burst size according to dw_dmac. We need to convert
>> +      * them as: 1 -> 0, 2 -> 1, 4 -> 2, 8 -> 3, 16 -> 4.
>> +      *
>> +      * This can be done by findiding least significant bit set: n & (n - 1)
>> +      */
>> +     sconfig->src_maxburst &= sconfig->src_maxburst - 1;
>> +     sconfig->dst_maxburst &= sconfig->dst_maxburst - 1;
>
> Perhaps, this looks incorrect. It will always return 0. :(
> Can somebody suggest any inbuild function to do this, i think
>
Calculating log2(n) which isn't very tidy. For that reason usually we make do
the other way around by saving values as power of 2 and simply left shifting
1 by that value.
In case at hand since most likely only valid values are power of 2,
maybe you can do
    maxburst = ffs(maxburst) - 1

-j
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