Re: [PATCH v3 3/4] media: i2c: ov5645: Set maximum leverage of external clock frequency to 24480000

[Laurent Pinchart]

Hi Prabhakar,

Thank you for the patch.

On Fri, Mar 13, 2020 at 09:12:33PM +0000, Lad Prabhakar wrote:
> While testing on Renesas RZ/G2E platform, noticed the clock frequency to
> be 24242424 as a result the probe failed. However increasing the maximum
> leverage of external clock frequency to 24480000 fixes this issue. Since
> this difference is small enough and is insignificant set the same in the
> driver.
> 
> Signed-off-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@xxxxxxxxxxxxxx>
> ---
>  drivers/media/i2c/ov5645.c | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/media/i2c/ov5645.c b/drivers/media/i2c/ov5645.c
> index 4fbabf3..b49359b 100644
> --- a/drivers/media/i2c/ov5645.c
> +++ b/drivers/media/i2c/ov5645.c
> @@ -1107,8 +1107,10 @@ static int ov5645_probe(struct i2c_client *client)
>  	}
>  
>  	xclk_freq = clk_get_rate(ov5645->xclk);
> -	/* external clock must be 24MHz, allow 1% tolerance */
> -	if (xclk_freq < 23760000 || xclk_freq > 24240000) {
> +	/* external clock must be 24MHz, allow a minimum 1% and a maximum of 2%
> +	 * tolerance

So where do these numbers come from ? I understand that 2% is what you
need to make your clock fit in the range, but why -1%/+2% instead of
-2%/+2% ? And why not 2.5 or 3% ? The sensor datasheet documents the
range of supported xvclk frequencies to be 6MHz to 54MHz. I understand
that PLL parameters depend on the clock frequency, but could they be
calculated instead of hardcoded, to avoid requiring an exact 24MHz input
frequency ?

> +	 */
> +	if (xclk_freq < 23760000 || xclk_freq > 24480000) {
>  		dev_err(dev, "external clock frequency %u is not supported\n",
>  			xclk_freq);
>  		return -EINVAL;

-- 
Regards,

Laurent Pinchart