linux-kernel - Re: [PATCH] [MTD] [NAND] pxa3xx: fix ns2cycle equation

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Date:	Mon, 16 Aug 2010 20:14:27 +0800
From:	Eric Miao <eric.y.miao@...il.com>
To:	David Woodhouse <dwmw2@...radead.org>
Cc:	Axel Lin <axel.lin@...il.com>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Matt Reimer <mreimer@...p.net>, linux-mtd@...ts.infradead.org
Subject: Re: [PATCH] [MTD] [NAND] pxa3xx: fix ns2cycle equation

On Mon, Aug 16, 2010 at 6:32 PM, David Woodhouse <dwmw2@...radead.org> wrote:
> On Mon, 2010-08-16 at 16:09 +0800, Axel Lin wrote:
>> Test on a PXA310 platform with Samsung K9F2G08X0B NAND flash,
>> with tCH=5 and clk is 156MHz, ns2cycle(5, 156000000) returns -1.
>>
>> ns2cycle returns negtive value will break NDTR0_tXX macros.
>>
>> After checking the commit log, I found the problem is introduced by
>> commit 5b0d4d7c8a67c5ba3d35e6ceb0c5530cc6846db7
>> "[MTD] [NAND] pxa3xx: convert from ns to clock ticks more accurately"
>>
>> To get num of clock cycles, we use below equation:
>> num of clock cycles = time (ns) / one clock cycle (ns) + 1
>> We need to add 1 cycle here because integer division will truncate the result.
>> It is possible the developers set the Min values in SPEC for timing settings.
>> Thus the truncate may cause problem, and it is safe to add an extra cycle here.
>>
>> The various fields in NDTR{01} are in units of clock ticks minus one,
>> thus we should subtract 1 cycle then.
>>
>> Thus the correct equation should be:
>> num of clock cycles = time (ns) / one clock cycle (ns) + 1 - 1
>>                     = time (ns) / one clock cycle (ns)
>
> Looks sane to me; thanks. Wants to go to stable@ too, yes?
>
> Can I have an Acked-By: from Matt and/or Eric?
>

Acked-by: Eric Miao <eric.y.miao@...il.com>
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