linux-kernel - Re: [PATCH v3 1/6] media: mt9p031: Read back the real clock rate

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Date:   Mon, 5 Jul 2021 07:42:01 +0000
From:   Stefan Riedmüller <S.Riedmueller@...tec.de>
To:     "sakari.ailus@...ux.intel.com" <sakari.ailus@...ux.intel.com>
CC:     "mchehab@...nel.org" <mchehab@...nel.org>,
        "laurent.pinchart@...asonboard.com" 
        <laurent.pinchart@...asonboard.com>,
        "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
        "linux-media@...r.kernel.org" <linux-media@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "robh+dt@...nel.org" <robh+dt@...nel.org>,
        "enrico.scholz@...ma-chemnitz.de" <enrico.scholz@...ma-chemnitz.de>
Subject: Re: [PATCH v3 1/6] media: mt9p031: Read back the real clock rate

Hi Sakari,

On Mon, 2021-07-05 at 10:13 +0300, Sakari Ailus wrote:
> Hi Enrico,
> 
> On Fri, Jul 02, 2021 at 11:59:17AM +0200, Stefan Riedmueller wrote:
> > From: Enrico Scholz <enrico.scholz@...ma-chemnitz.de>
> > 
> > The real and requested clock can differ and because it is used to
> > calculate PLL values, the real clock rate should be read.
> 
> Do you have a system where this happens? That suggests there's a wrong
> value in DT.

The use case here is when the clock is supplied by one of the clock outputs of
a SOC which might not hit the requested frequency exactly due to internal PLL
configuration. So to get a better pixel clock the actual clock rate is read to
calculate the PLL parameters on the sensor. At least that's the idea.

Regards,
Stefan

> 
> The preference nowadays is to rely on assigned-clock-rates, even though
> it's inherently somewhat unreliable, just as clk_set_rate(). This is an
> existing driver though. The old ones could be kept for compatibility with
> older DT binaries.
> 
> > Signed-off-by: Enrico Scholz <enrico.scholz@...ma-chemnitz.de>
> > Signed-off-by: Stefan Riedmueller <s.riedmueller@...tec.de>
> > Reviewed-by: Laurent Pinchart <laurent.pinchart@...asonboard.com>
> > ---
> >  drivers/media/i2c/mt9p031.c | 9 ++++++---
> >  1 file changed, 6 insertions(+), 3 deletions(-)
> > 
> > diff --git a/drivers/media/i2c/mt9p031.c b/drivers/media/i2c/mt9p031.c
> > index 77567341ec98..3eaaa8d44523 100644
> > --- a/drivers/media/i2c/mt9p031.c
> > +++ b/drivers/media/i2c/mt9p031.c
> > @@ -255,6 +255,7 @@ static int mt9p031_clk_setup(struct mt9p031 *mt9p031)
> >  
> >  	struct i2c_client *client = v4l2_get_subdevdata(&mt9p031->subdev);
> >  	struct mt9p031_platform_data *pdata = mt9p031->pdata;
> > +	unsigned long ext_freq;
> >  	int ret;
> >  
> >  	mt9p031->clk = devm_clk_get(&client->dev, NULL);
> > @@ -265,13 +266,15 @@ static int mt9p031_clk_setup(struct mt9p031
> > *mt9p031)
> >  	if (ret < 0)
> >  		return ret;
> >  
> > +	ext_freq = clk_get_rate(mt9p031->clk);
> > +
> >  	/* If the external clock frequency is out of bounds for the PLL use
> > the
> >  	 * pixel clock divider only and disable the PLL.
> >  	 */
> > -	if (pdata->ext_freq > limits.ext_clock_max) {
> > +	if (ext_freq > limits.ext_clock_max) {
> >  		unsigned int div;
> >  
> > -		div = DIV_ROUND_UP(pdata->ext_freq, pdata->target_freq);
> > +		div = DIV_ROUND_UP(ext_freq, pdata->target_freq);
> >  		div = roundup_pow_of_two(div) / 2;
> >  
> >  		mt9p031->clk_div = min_t(unsigned int, div, 64);
> > @@ -280,7 +283,7 @@ static int mt9p031_clk_setup(struct mt9p031 *mt9p031)
> >  		return 0;
> >  	}
> >  
> > -	mt9p031->pll.ext_clock = pdata->ext_freq;
> > +	mt9p031->pll.ext_clock = ext_freq;
> >  	mt9p031->pll.pix_clock = pdata->target_freq;
> >  	mt9p031->use_pll = true;
> >