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

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Message-ID: <OSBPR01MB359079EAA32E0DCBF63C6886AAFA0@OSBPR01MB3590.jpnprd01.prod.outlook.com>
Date:   Fri, 13 Mar 2020 21:31:25 +0000
From:   Prabhakar Mahadev Lad <prabhakar.mahadev-lad.rj@...renesas.com>
To:     Laurent Pinchart <laurent.pinchart@...asonboard.com>
CC:     Mauro Carvalho Chehab <mchehab@...nel.org>,
        Shawn Guo <shawnguo@...nel.org>,
        Sascha Hauer <s.hauer@...gutronix.de>,
        Pengutronix Kernel Team <kernel@...gutronix.de>,
        Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Sakari Ailus <sakari.ailus@...ux.intel.com>,
        NXP Linux Team <linux-imx@....com>,
        Magnus Damm <magnus.damm@...il.com>,
        Ezequiel Garcia <ezequiel@...labora.com>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-renesas-soc@...r.kernel.org" 
        <linux-renesas-soc@...r.kernel.org>,
        Fabio Estevam <festevam@...il.com>,
        "linux-media@...r.kernel.org" <linux-media@...r.kernel.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>
Subject: RE: [PATCH v3 3/4] media: i2c: ov5645: Set maximum leverage of
 external clock frequency to 24480000

Hi Laurent,

Thank you for the review.

> -----Original Message-----
> From: Laurent Pinchart <laurent.pinchart@...asonboard.com>
> Sent: 13 March 2020 21:24
> To: Prabhakar Mahadev Lad <prabhakar.mahadev-lad.rj@...renesas.com>
> Cc: Mauro Carvalho Chehab <mchehab@...nel.org>; Shawn Guo
> <shawnguo@...nel.org>; Sascha Hauer <s.hauer@...gutronix.de>;
> Pengutronix Kernel Team <kernel@...gutronix.de>; Rob Herring
> <robh+dt@...nel.org>; Mark Rutland <mark.rutland@....com>; Sakari
> Ailus <sakari.ailus@...ux.intel.com>; NXP Linux Team <linux-imx@....com>;
> Magnus Damm <magnus.damm@...il.com>; Ezequiel Garcia
> <ezequiel@...labora.com>; Geert Uytterhoeven <geert@...ux-m68k.org>;
> devicetree@...r.kernel.org; linux-kernel@...r.kernel.org; linux-renesas-
> soc@...r.kernel.org; Fabio Estevam <festevam@...il.com>; linux-
> media@...r.kernel.org; linux-arm-kernel@...ts.infradead.org
> Subject: Re: [PATCH v3 3/4] media: i2c: ov5645: Set maximum leverage of
> external clock frequency to 24480000
>
> 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@...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 ?
>
To be honest I don't have the datasheet for ov5645, the flyer says 6-54Mhz but the
logs/comment says 24Mhz.

Cheers,
--Prabhakar

> > + */
> > +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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