RE: [PATCH] mmc: dw_mmc: rewrite CLKDIV computation

[Seungwon Jeon]

On Wednesday, March 27, 2013, Grant Grundler wrote:
> Last year Seungwon Jeon (Samsung) fixed a bug in CLKDIV computation.
For easily identifying, it would be good to point the commit id and subject.
> But when debugging a related issue (http://crbug.com/221828) I found
It is not easy to catch up issue in your site. What problem is bothering you?
Could you describe the problem and conditions you have found?

Thanks,
Seungwon Jeon

> the code unreadable.  This rewrite simplifies the computation and
> explains each step.
> 
> Signed-off-by: Grant Grundler <grundler@xxxxxxxxxxxx>
> ---
> Tested on Samsung Series 3 Chromebook (exynos 5250 chipset) using
> ChromeOS 3.4 kernel (not 3.9-rc3 which this patch is based against).
> 
> I've written a test program to confirm this patch generates the
> same correct values and will share that separately.
> 
>  drivers/mmc/host/dw_mmc.c | 22 +++++++++++++++-------
>  1 file changed, 15 insertions(+), 7 deletions(-)
> 
> 
> diff --git a/drivers/mmc/host/dw_mmc.c b/drivers/mmc/host/dw_mmc.c
> index 9834221..6fdf55b 100644
> --- a/drivers/mmc/host/dw_mmc.c
> +++ b/drivers/mmc/host/dw_mmc.c
> @@ -631,14 +631,22 @@ static void dw_mci_setup_bus(struct dw_mci_slot *slot, bool force_clkinit)
> 
>  	if (slot->clock != host->current_speed || force_clkinit) {
>  		div = host->bus_hz / slot->clock;
> -		if (host->bus_hz % slot->clock && host->bus_hz > slot->clock)
> -			/*
> -			 * move the + 1 after the divide to prevent
> -			 * over-clocking the card.
> +		if (host->bus_hz > slot->clock) {
> +			/* don't overclock due to integer math losses */
> +			if ((div * slot->clock) < host->bus_hz)
> +				div++;
> +
> +			/* don't overclock due to resolution of HW */
> +			if (div & 1)
> +				div++;
> +
> +			/* See 6.2.3 CLKDIV in "Mobile Storage Host Databook"
> +			 * Look for dwc_mobile_storage_db.pdf from Synopsys.
> +			 * CLKDIV value 0 means divisor 1, val 1 -> 2, ...
>  			 */
> -			div += 1;
> -
> -		div = (host->bus_hz != slot->clock) ? DIV_ROUND_UP(div, 2) : 0;
> +			div /= 2;
> +		} else
> +			div = 0;        /* use bus_hz */
> 
>  		dev_info(&slot->mmc->class_dev,
>  			 "Bus speed (slot %d) = %dHz (slot req %dHz, actual %dHZ"
> --
> 1.8.1.3
> 
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