Re: [PATCH] fix mpc83xx spi prescale modulus calculate

[Anton Vorontsov <avorontsov@ru.mvista.com>]

On Fri, Jun 20, 2008 at 05:38:17PM +0800, Chen Gong wrote:
> Signed-off-by: Chen Gong <g.chen@freescale.com>

The patch lacks any description. E.g. at which frequencies the current
math breaks, and why?

> ---
>  drivers/spi/spi_mpc83xx.c |   29 ++++++++++++++++-------------
>  1 files changed, 16 insertions(+), 13 deletions(-)
> 
> diff --git a/drivers/spi/spi_mpc83xx.c b/drivers/spi/spi_mpc83xx.c
> index 6832da6..070c621 100644
> --- a/drivers/spi/spi_mpc83xx.c
> +++ b/drivers/spi/spi_mpc83xx.c
> @@ -266,21 +266,24 @@ int mpc83xx_spi_setup_transfer(struct spi_device *spi, struct spi_transfer *t)
>  
>  	cs->hw_mode |= SPMODE_LEN(bits_per_word);
>  
> -	if ((mpc83xx_spi->spibrg / hz) >= 64) {
> -		pm = mpc83xx_spi->spibrg / (hz * 64) - 1;
> -		if (pm > 0x0f) {
> -			dev_err(&spi->dev, "Requested speed is too "
> -				"low: %d Hz. Will use %d Hz instead.\n",
> -				hz, mpc83xx_spi->spibrg / 1024);
> -			pm = 0x0f;
> +	if ((mpc83xx_spi->spibrg / hz) > 64) {
> +		pm = mpc83xx_spi->spibrg / (hz * 64);
> +		if (pm > 16) {

Suppose this didn't happen. So we calculated:
pm = mpc83xx_spi->spibrg / (hz * 64);
And we didn't set any hw flag.

> +			cs->hw_mode |= SPMODE_DIV16;
> +			pm /= 16;
> +			if (pm > 16) {
> +				dev_err(&spi->dev, "Requested speed is too "
> +					"low: %d Hz. Will use %d Hz instead.\n",
> +					hz, mpc83xx_spi->spibrg / 1024);
> +				pm = 16;
> +			}
>  		}
> -		cs->hw_mode |= SPMODE_PM(pm) | SPMODE_DIV16;
> -	} else {
> +	} else
>  		pm = mpc83xx_spi->spibrg / (hz * 4);

Now suppose this did happen. So we calculated:
pm = mpc83xx_spi->spibrg / (hz * 4);
And we didn't set any hw flag either.

So, we used two different formulas with the same hw flags.

Maybe I'm missing something, but this looks quite wrong.

> -		if (pm)
> -			pm--;
> -		cs->hw_mode |= SPMODE_PM(pm);
> -	}
> +	if (pm)
> +		pm--;
> +
> +	cs->hw_mode |= SPMODE_PM(pm);
>  	regval =  mpc83xx_spi_read_reg(&mpc83xx_spi->base->mode);
>  	if (cs->hw_mode != regval) {
>  		unsigned long flags;
> -- 
> 1.5.4

-- 
Anton Vorontsov
email: cbouatmailru@gmail.com
irc://irc.freenode.net/bd2