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

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Date:	Mon, 16 Aug 2010 11:32:18 +0100
From:	David Woodhouse <dwmw2@...radead.org>
To:	Axel Lin <axel.lin@...il.com>
Cc:	linux-kernel <linux-kernel@...r.kernel.org>,
	Matt Reimer <mreimer@...p.net>,
	Eric Miao <eric.y.miao@...il.com>,
	linux-mtd@...ts.infradead.org
Subject: Re: [PATCH] [MTD] [NAND] pxa3xx: fix ns2cycle equation

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?

-- 
David Woodhouse                            Open Source Technology Centre
David.Woodhouse@...el.com                              Intel Corporation

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