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 for linux-mtd@lists.infradead.org; Wed, 30 Sep 2015 21:18:21 +0000
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Date: Thu, 01 Oct 2015 00:17:57 +0300
From: Vladimir Zapolskiy <vz@mleia.com>
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To: Brian Norris <computersforpeace@gmail.com>
CC: David Woodhouse <dwmw2@infradead.org>, linux-mtd@lists.infradead.org,
 Roland Stigge <stigge@antcom.de>
Subject: Re: [PATCH 1/5] mtd: nand: lpc32xx_slc: fix calculation of timing
 arcs from given values
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 <20150930211253.GK143959@google.com>
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Hi Brian,

On 01.10.2015 00:12, Brian Norris wrote:
> + Roland, who authored the driver
> 
> On Mon, Sep 28, 2015 at 07:38:58PM +0300, Vladimir Zapolskiy wrote:
>> According to LPC32xx User's Manual all values measured in clock cycles
>> are programmable from 1 to 16 clocks (4 bits) starting from 0 in
>> bitfield. Correctness of 0 bitfield value (i.e. programmed 1 clock
>> timing) is proven with actual NAND chip devices.
>>
>> Signed-off-by: Vladimir Zapolskiy <vz@mleia.com>
>> ---
>>  drivers/mtd/nand/lpc32xx_slc.c | 12 ++++++------
>>  1 file changed, 6 insertions(+), 6 deletions(-)
>>
>> diff --git a/drivers/mtd/nand/lpc32xx_slc.c b/drivers/mtd/nand/lpc32xx_slc.c
>> index abfec13..5a3c6e0 100644
>> --- a/drivers/mtd/nand/lpc32xx_slc.c
>> +++ b/drivers/mtd/nand/lpc32xx_slc.c
>> @@ -240,13 +240,13 @@ static void lpc32xx_nand_setup(struct lpc32xx_nand_host *host)
>>  
>>  	/* Compute clock setup values */
>>  	tmp = SLCTAC_WDR(host->ncfg->wdr_clks) |
>> -		SLCTAC_WWIDTH(1 + (clkrate / host->ncfg->wwidth)) |
>> -		SLCTAC_WHOLD(1 + (clkrate / host->ncfg->whold)) |
>> -		SLCTAC_WSETUP(1 + (clkrate / host->ncfg->wsetup)) |
>> +		SLCTAC_WWIDTH(clkrate / host->ncfg->wwidth) |
>> +		SLCTAC_WHOLD(clkrate / host->ncfg->whold) |
>> +		SLCTAC_WSETUP(clkrate / host->ncfg->wsetup) |
>>  		SLCTAC_RDR(host->ncfg->rdr_clks) |
>> -		SLCTAC_RWIDTH(1 + (clkrate / host->ncfg->rwidth)) |
>> -		SLCTAC_RHOLD(1 + (clkrate / host->ncfg->rhold)) |
>> -		SLCTAC_RSETUP(1 + (clkrate / host->ncfg->rsetup));
>> +		SLCTAC_RWIDTH(clkrate / host->ncfg->rwidth) |
>> +		SLCTAC_RHOLD(clkrate / host->ncfg->rhold) |
>> +		SLCTAC_RSETUP(clkrate / host->ncfg->rsetup);
>>  	writel(tmp, SLC_TAC(host->io_base));
>>  }
>>  
> 
> Hmm, I'm guessing this was done to ensure we didn't round down too much.
> Perhaps this should be using DIV_ROUND_UP() instead, so we can get an
> exact match where possible, but not set too low of a clock rate by
> rounding down?

unfortunately bare DIV_ROUND_UP() can not help here, because 0 is a
valid resulting bitfield value here, if some timing is lower than clkrate.

--
With best wishes,
Vladimir