linux-kernel - Re: [PATCH v6 2/3] media: ov8856: Add devicetree support

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Date:   Thu, 30 Apr 2020 14:07:41 +0200
From:   Marco Felsch <m.felsch@...gutronix.de>
To:     Sakari Ailus <sakari.ailus@....fi>
Cc:     Robert Foss <robert.foss@...aro.org>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Maxime Ripard <maxime@...no.tech>, linux-media@...r.kernel.org,
        devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org,
        Dongchun Zhu <dongchun.zhu@...iatek.com>,
        Fabio Estevam <festevam@...il.com>,
        Tomasz Figa <tfiga@...omium.org>
Subject: Re: [PATCH v6 2/3] media: ov8856: Add devicetree support

On 20-04-30 13:20, Sakari Ailus wrote:
> On Thu, Apr 30, 2020 at 12:11:57PM +0200, Marco Felsch wrote:
> > On 20-04-30 12:59, Sakari Ailus wrote:
> > > Hi Marco,
> > > 
> > > On Thu, Apr 30, 2020 at 11:53:32AM +0200, Marco Felsch wrote:
> > > > Hi Sakari,
> > > > 
> > > > On 20-04-30 12:45, Sakari Ailus wrote:
> > > > > Hi Marco,
> > > > > 
> > > > > On Thu, Apr 30, 2020 at 11:35:24AM +0200, Marco Felsch wrote:
> > 
> > ...
> > 
> > > > > > > -	if (mclk != OV8856_MCLK) {
> > > > > > > -		dev_err(dev, "external clock %d is not supported", mclk);
> > > > > > > -		return -EINVAL;
> > > > > > > +	if (!is_acpi_node(fwnode)) {
> > > > > > > +		ov8856->xvclk = devm_clk_get(dev, "xvclk");
> > > > > > > +		if (IS_ERR(ov8856->xvclk)) {
> > > > > > > +			dev_err(dev, "could not get xvclk clock (%pe)\n",
> > > > > > > +					ov8856->xvclk);
> > > > > > > +			return PTR_ERR(ov8856->xvclk);
> > > > > > > +		}
> > > > > > > +
> > > > > > > +		clk_set_rate(ov8856->xvclk, xvclk_rate);
> > > > > > > +		xvclk_rate = clk_get_rate(ov8856->xvclk);
> > > > > > >  	}
> > > > > > 
> > > > > > Why do we handle the clock only in DT case? Is there a problem with the
> > > > > > clock handling and ACPI?
> > > > > 
> > > > > Not really, it's just that ACPI does not provide an interface to the clocks
> > > > > as such.
> > > > 
> > > > But you will get a clk by devm_clk_get()?
> > > 
> > > No, because ACPI does not expose one to drivers. Effectively the entire
> > > power sequences are implemented in ACPI, not in the driver.
> > > 
> > 
> > Ah okay, thanks for the explanation. I'm really not into the ACPI
> > stuff.. So this means the __power_off / power_on should only be done if
> > we are using DT's?
> 
> Correct. That's why it bails out early. It could be yet earlier though,
> without doing anything.

Yes I see. For easier and error less prone handling I would prefer:

if (is_acpi_node())
	return 0;

as first instruction for __power_off/on().

Also I would refator the ov8856_check_hwcfg() so the common part can be
used by this function and by a ov8856_parse_of() function. But thats
only my opinion.

Regards,
  Marco