Subject: [PATCH v6 2/3] spi: sun4i: Fix clock calculations to be predictable and never exceed the requested rate

From: [email-masked] (Maxime Ripard)
Date: Sun, 27 Dec 2015 22:09:46 +0100

On Sat, Dec 26, 2015 at 04:53:05PM +0100, Marcus Weseloh wrote:
> This patch fixes multiple problems with the current clock calculations:
> 
> 1. The A10/A20 datasheet contains the formula AHB_CLK / (2^(n+1)) to
> calculate SPI_CLK from CDR1, but this formula is wrong. The actual
> formula - determined by analyzing the actual waveforms - is
> AHB_CLK / (2^n).
> 
> 2. The divisor calculations for CDR1 and CDR2 both rounded to the
> nearest integer. This could lead to a transfer speed that is higher than
> the requested speed. This patch changes both calculations to always
> round down.
> 
> 3. The mclk frequency was only ever increased, never decreased. This could
> lead to unpredictable transfer speeds, depending on the order in which
> transfers with different speeds where serviced by the SPI driver.

These 3 should probably be separate patches (and be Cc'd to stable

> Signed-off-by: Marcus Weseloh <[email-masked]>
> ---
>  drivers/spi/spi-sun4i.c | 26 ++++++++++++++++----------
>  1 file changed, 16 insertions(+), 10 deletions(-)
> 
> diff --git a/drivers/spi/spi-sun4i.c b/drivers/spi/spi-sun4i.c
> index f60a6d6..d67e142 100644
> --- a/drivers/spi/spi-sun4i.c
> +++ b/drivers/spi/spi-sun4i.c
> @@ -79,6 +79,9 @@ struct sun4i_spi {
>  	struct clk		*hclk;
>  	struct clk		*mclk;
>  
> +	int			cur_max_speed;
> +	int			cur_mclk;
> +
>  	struct completion	done;
>  
>  	const u8		*tx_buf;
> @@ -227,11 +230,17 @@ static int sun4i_spi_transfer_one(struct spi_master *master,
>  
>  	sun4i_spi_write(sspi, SUN4I_CTL_REG, reg);
>  
> -	/* Ensure that we have a parent clock fast enough */
> +	/*
> +	 * Ensure that the parent clock is set to twice the max speed
> +	 * of the spi device (possibly rounded up by the clk driver)
> +	 */
>  	mclk_rate = clk_get_rate(sspi->mclk);
> -	if (mclk_rate < (2 * tfr->speed_hz)) {
> -		clk_set_rate(sspi->mclk, 2 * tfr->speed_hz);
> +	if (spi->max_speed_hz != sspi->cur_max_speed ||
> +	    mclk_rate != sspi->cur_mclk) {

Do you need to cache the values? As far as I'm aware, you end up doing
the computation all the time anyway.

> +		clk_set_rate(sspi->mclk, 2 * spi->max_speed_hz);
>  		mclk_rate = clk_get_rate(sspi->mclk);
> +		sspi->cur_mclk = mclk_rate;
> +		sspi->cur_max_speed = spi->max_speed_hz;
>  	}
>  
>  	/*
> @@ -239,7 +248,7 @@ static int sun4i_spi_transfer_one(struct spi_master *master,
>  	 *
>  	 * We have two choices there. Either we can use the clock
>  	 * divide rate 1, which is calculated thanks to this formula:
> -	 * SPI_CLK = MOD_CLK / (2 ^ (cdr + 1))
> +	 * SPI_CLK = MOD_CLK / (2 ^ cdr)
>
>  	 * Or we can use CDR2, which is calculated with the formula:
>  	 * SPI_CLK = MOD_CLK / (2 * (cdr + 1))
>  	 * Wether we use the former or the latter is set through the
> @@ -248,14 +257,11 @@ static int sun4i_spi_transfer_one(struct spi_master *master,
>  	 * First try CDR2, and if we can't reach the expected
>  	 * frequency, fall back to CDR1.
>  	 */
> -	div = mclk_rate / (2 * tfr->speed_hz);
> -	if (div <= (SUN4I_CLK_CTL_CDR2_MASK + 1)) {
> -		if (div > 0)
> -			div--;
> -
> +	div = DIV_ROUND_UP(mclk_rate, 2 * tfr->speed_hz) - 1;

Isn't it exactly the same thing as mclk_rate / (2 * tfr->speed_hz) ?

Thanks,
Maxime

-- 
Maxime Ripard, Free Electrons
Embedded Linux, Kernel and Android engineering
http://free-electrons.com
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