diff mbox series

[v2] spi: pxa2xx: fix SCR (divisor) calculation

Message ID 1555054339-17096-1-git-send-email-f.suligoi@asem.it (mailing list archive)
State Mainlined, archived
Commit 29f2133717c527f492933b0622a4aafe0b3cbe9e
Headers show
Series [v2] spi: pxa2xx: fix SCR (divisor) calculation | expand

Commit Message

Flavio Suligoi April 12, 2019, 7:32 a.m. UTC 
Calculate the divisor for the SCR (Serial Clock Rate), avoiding
that the SSP transmission rate can be greater than the device rate.

When the division between the SSP clock and the device rate generates
a reminder, we have to increment by one the divisor.
In this way the resulting SSP clock will never be greater than the
device SPI max frequency.

For example, with:

 - ssp_clk  = 50 MHz
 - dev freq = 15 MHz

without this patch the SSP clock will be greater than 15 MHz:

 - 25 MHz for PXA25x_SSP and CE4100_SSP
 - 16,56 MHz for the others

Instead, with this patch, we have in both case an SSP clock of 12.5MHz,
so the max rate of the SPI device clock is respected.

Signed-off-by: Flavio Suligoi <f.suligoi@asem.it>
Reviewed-by: Jarkko Nikula <jarkko.nikula@linux.intel.com>
---

v2: - simplify the code using "DIV_ROUND_UP"
v1: - first version

 drivers/spi/spi-pxa2xx.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

Comments

Jarkko Nikula April 12, 2019, 12:35 p.m. UTC | #1 
On 4/12/19 10:32 AM, Flavio Suligoi wrote:
> Calculate the divisor for the SCR (Serial Clock Rate), avoiding
> that the SSP transmission rate can be greater than the device rate.
> 
> When the division between the SSP clock and the device rate generates
> a reminder, we have to increment by one the divisor.
> In this way the resulting SSP clock will never be greater than the
> device SPI max frequency.
> 
> For example, with:
> 
>   - ssp_clk  = 50 MHz
>   - dev freq = 15 MHz
> 
> without this patch the SSP clock will be greater than 15 MHz:
> 
>   - 25 MHz for PXA25x_SSP and CE4100_SSP
>   - 16,56 MHz for the others
> 
> Instead, with this patch, we have in both case an SSP clock of 12.5MHz,
> so the max rate of the SPI device clock is respected.
> 
> Signed-off-by: Flavio Suligoi <f.suligoi@asem.it>
> Reviewed-by: Jarkko Nikula <jarkko.nikula@linux.intel.com>
> ---
> 
> v2: - simplify the code using "DIV_ROUND_UP"
> v1: - first version
> 
>   drivers/spi/spi-pxa2xx.c | 8 ++++++--
>   1 file changed, 6 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/spi/spi-pxa2xx.c b/drivers/spi/spi-pxa2xx.c
> index f7068cc..a35fdcb 100644
> --- a/drivers/spi/spi-pxa2xx.c
> +++ b/drivers/spi/spi-pxa2xx.c
> @@ -884,10 +884,14 @@ static unsigned int ssp_get_clk_div(struct driver_data *drv_data, int rate)
>   
>   	rate = min_t(int, ssp_clk, rate);
>   
> +	/*
> +	 * Calculate the divisor for the SCR (Serial Clock Rate), avoiding
> +	 * that the SSP transmission rate can be greater than the device rate
> +	 */
>   	if (ssp->type == PXA25x_SSP || ssp->type == CE4100_SSP)
> -		return (ssp_clk / (2 * rate) - 1) & 0xff;
> +		return (DIV_ROUND_UP(ssp_clk, 2 * rate) - 1) & 0xff;
>   	else
> -		return (ssp_clk / rate - 1) & 0xfff;
> +		return (DIV_ROUND_UP(ssp_clk, rate) - 1)  & 0xfff;
>   }
>   
Reviewed-by: Jarkko Nikula <jarkko.nikula@linux.intel.com>
diff mbox series

Patch

diff --git a/drivers/spi/spi-pxa2xx.c b/drivers/spi/spi-pxa2xx.c
index f7068cc..a35fdcb 100644
--- a/drivers/spi/spi-pxa2xx.c
+++ b/drivers/spi/spi-pxa2xx.c
@@ -884,10 +884,14 @@  static unsigned int ssp_get_clk_div(struct driver_data *drv_data, int rate)
 
 	rate = min_t(int, ssp_clk, rate);
 
+	/*
+	 * Calculate the divisor for the SCR (Serial Clock Rate), avoiding
+	 * that the SSP transmission rate can be greater than the device rate
+	 */
 	if (ssp->type == PXA25x_SSP || ssp->type == CE4100_SSP)
-		return (ssp_clk / (2 * rate) - 1) & 0xff;
+		return (DIV_ROUND_UP(ssp_clk, 2 * rate) - 1) & 0xff;
 	else
-		return (ssp_clk / rate - 1) & 0xfff;
+		return (DIV_ROUND_UP(ssp_clk, rate) - 1)  & 0xfff;
 }
 
 static unsigned int pxa2xx_ssp_get_clk_div(struct driver_data *drv_data,