From mboxrd@z Thu Jan 1 00:00:00 1970 Subject: Re: [PATCH] [MTD] [NAND] pxa3xx: fix ns2cycle equation From: David Woodhouse To: Axel Lin In-Reply-To: <1281946149.10299.1.camel@mola> References: <1281946149.10299.1.camel@mola> Content-Type: text/plain; charset="UTF-8" Date: Mon, 16 Aug 2010 11:32:18 +0100 Message-ID: <1281954738.21549.65.camel@localhost> Mime-Version: 1.0 Content-Transfer-Encoding: 7bit Cc: linux-mtd@lists.infradead.org, Matt Reimer , Eric Miao , linux-kernel List-Id: Linux MTD discussion mailing list List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , 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@intel.com Intel Corporation