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

From: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
To: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>
Cc: Mark Rutland <mark.rutland@arm.com>,
	devicetree@vger.kernel.org, Fabio Estevam <festevam@gmail.com>,
	Sakari Ailus <sakari.ailus@linux.intel.com>,
	Ezequiel Garcia <ezequiel@collabora.com>,
	Sascha Hauer <s.hauer@pengutronix.de>,
	Magnus Damm <magnus.damm@gmail.com>,
	linux-kernel@vger.kernel.org, linux-renesas-soc@vger.kernel.org,
	Rob Herring <robh+dt@kernel.org>,
	Geert Uytterhoeven <geert@linux-m68k.org>,
	NXP Linux Team <linux-imx@nxp.com>,
	Pengutronix Kernel Team <kernel@pengutronix.de>,
	Mauro Carvalho Chehab <mchehab@kernel.org>,
	Shawn Guo <shawnguo@kernel.org>,
	linux-arm-kernel@lists.infradead.org,
	linux-media@vger.kernel.org
Subject: Re: [PATCH v3 3/4] media: i2c: ov5645: Set maximum leverage of external clock frequency to 24480000
Date: Fri, 13 Mar 2020 23:23:45 +0200	[thread overview]
Message-ID: <20200313212345.GM4751@pendragon.ideasonboard.com> (raw)
In-Reply-To: <1584133954-6953-4-git-send-email-prabhakar.mahadev-lad.rj@bp.renesas.com>

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@bp.renesas.com>
> ---
>  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

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