Re: [PATCH v2 1/2] spi: sunxi: fix CDR2 calculation

[Andre Przywara <andre.przywara@arm.com>]

On Thu, 18 Jul 2024 22:42:52 +0200
Michael Walle <mwalle@kernel.org> wrote:

Hi,

> The CDR2 divider calculation always yield a frequency greater than the
> requested one. Use DIV_ROUND_UP() to keep the frequency equal or below
> the requested one. This way, we can also drop the "if div > 0" check
> because we know for a fact that div cannot be zero.
> 
> FWIW, this aligns the CDR2 calculation with the linux driver.

Thanks, that looks alright now. I tested some corner cases, and it seems
to do the right thing (TM) now.

> Suggested-by: Andre Przywara <andre.przywara@arm.com>
> Signed-off-by: Michael Walle <mwalle@kernel.org>

Reviewed-by: Andre Przywara <andre.przywara@arm.com>

Applied to sunxi/master.

Cheers,
Andre.

> ---
>  drivers/spi/spi-sunxi.c | 11 ++++-------
>  1 file changed, 4 insertions(+), 7 deletions(-)
> 
> diff --git a/drivers/spi/spi-sunxi.c b/drivers/spi/spi-sunxi.c
> index bfb402902b8..f110a8b7658 100644
> --- a/drivers/spi/spi-sunxi.c
> +++ b/drivers/spi/spi-sunxi.c
> @@ -233,7 +233,7 @@ err_ahb:
>  static void sun4i_spi_set_speed_mode(struct udevice *dev)
>  {
>  	struct sun4i_spi_priv *priv = dev_get_priv(dev);
> -	unsigned int div;
> +	unsigned int div, div_cdr2;
>  	u32 reg;
>  
>  	/*
> @@ -259,15 +259,12 @@ static void sun4i_spi_set_speed_mode(struct udevice *dev)
>  	 */
>  
>  	div = DIV_ROUND_UP(SUNXI_INPUT_CLOCK, priv->freq);
> +	div_cdr2 = DIV_ROUND_UP(div, 2);
>  	reg = readl(SPI_REG(priv, SPI_CCR));
>  
> -	if ((div / 2) <= (SUN4I_CLK_CTL_CDR2_MASK + 1)) {
> -		div /= 2;
> -		if (div > 0)
> -			div--;
> -
> +	if (div_cdr2 <= (SUN4I_CLK_CTL_CDR2_MASK + 1)) {
>  		reg &= ~(SUN4I_CLK_CTL_CDR2_MASK | SUN4I_CLK_CTL_DRS);
> -		reg |= SUN4I_CLK_CTL_CDR2(div) | SUN4I_CLK_CTL_DRS;
> +		reg |= SUN4I_CLK_CTL_CDR2(div_cdr2 - 1) | SUN4I_CLK_CTL_DRS;
>  	} else {
>  		div = fls(div - 1);
>  		/* The F1C100s encodes the divider as 2^(n+1) */