Re: [PATCH v7 5/7] media: i2c: add DS90UB960 driver

From: Andy Shevchenko <andriy.shevchenko@intel.com>
To: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
Cc: linux-media@vger.kernel.org, devicetree@vger.kernel.org,
	linux-kernel@vger.kernel.org, linux-i2c@vger.kernel.org,
	"Rob Herring" <robh+dt@kernel.org>,
	"Krzysztof Kozlowski" <krzysztof.kozlowski+dt@linaro.org>,
	"Wolfram Sang" <wsa@kernel.org>,
	"Luca Ceresoli" <luca.ceresoli@bootlin.com>,
	"Matti Vaittinen" <Matti.Vaittinen@fi.rohmeurope.com>,
	"Laurent Pinchart" <laurent.pinchart+renesas@ideasonboard.com>,
	"Mauro Carvalho Chehab" <mchehab@kernel.org>,
	"Peter Rosin" <peda@axentia.se>,
	"Liam Girdwood" <lgirdwood@gmail.com>,
	"Mark Brown" <broonie@kernel.org>,
	"Sakari Ailus" <sakari.ailus@linux.intel.com>,
	"Michael Tretter" <m.tretter@pengutronix.de>,
	"Shawn Tu" <shawnx.tu@intel.com>,
	"Hans Verkuil" <hverkuil@xs4all.nl>,
	"Mike Pagano" <mpagano@gentoo.org>,
	"Krzysztof Hałasa" <khalasa@piap.pl>,
	"Marek Vasut" <marex@denx.de>
Subject: Re: [PATCH v7 5/7] media: i2c: add DS90UB960 driver
Date: Wed, 25 Jan 2023 16:49:56 +0200	[thread overview]
Message-ID: <Y9FBlMl4b3l1zVck@smile.fi.intel.com> (raw)
In-Reply-To: <cad92dbb-43ef-fa8c-1962-13c4a8578899@ideasonboard.com>

On Wed, Jan 25, 2023 at 03:33:35PM +0200, Tomi Valkeinen wrote:
> On 25/01/2023 14:10, Andy Shevchenko wrote:
> > On Wed, Jan 25, 2023 at 01:15:34PM +0200, Tomi Valkeinen wrote:
> > > On 20/01/2023 18:47, Andy Shevchenko wrote:

...

> > > > > > > +	ret = fwnode_property_read_u32(link_fwnode, "ti,eq-level", &eq_level);
> > > > > > > +	if (ret) {
> > > > > > > +		if (ret != -EINVAL) {
> > > > > > > +			dev_err(dev, "rx%u: failed to read 'ti,eq-level': %d\n",
> > > > > > > +				nport, ret);
> > > > > > > +			return ret;
> > > > > > > +		}
> > > > 
> > > > This seems like trying to handle special cases, if you want it to be optional,
> > > > why not ignoring all errors?
> > > 
> > > I don't follow. Why would we ignore all errors even if the property is
> > > optional? If there's a failure in reading the property, or checking if it
> > > exists or not, surely that's an actual error to be handled, not to be
> > > ignored?
> > 
> > What the problem to ignore them?
> 
> Well, probably nothing will explode if we just ignore them. But... Why would
> we ignore them?
> 
> > But if you are really pedantic about it, perhaps the proper way is to add
> > 
> > fwnode_property_*_optional()
> > 
> > APIs to the set where you take default and return 0 in case default had been
> > used for the absent property.
> 
> Perhaps, but I don't have a default value here.

It's impossible. You have one. 0 is also can be default.

> In any case, I'm not quite sure what you are arguing here. Is it just that
> you don't think the error check is necessary and should be dropped?

Yes, I do not see the value of these complex error checking.
Dropping that makes it KISS. I.o.w. why do we care about errors
if the property is optional? Make it mandatory otherwise.

> > > > > > > +	} else if (eq_level > UB960_MAX_EQ_LEVEL) {
> > > > > > > +		dev_err(dev, "rx%u: illegal 'ti,eq-level' value: %d\n", nport,
> > > > > > > +			eq_level);
> > > > 
> > > > This part is a validation of DT again, but we discussed above this.
> > > > 
> > > > > > > +	} else {
> > > > > > > +		rxport->eq.manual_eq = true;
> > > > > > > +		rxport->eq.manual.eq_level = eq_level;
> > > > > > > +	}
> > 
> > ...
> > 
> > > > > > > +struct ds90ub9xx_platform_data {
> > > > > > > +	u32 port;
> > > > > > > +	struct i2c_atr *atr;
> > > > > > > +	unsigned long bc_rate;
> > > > > > 
> > > > > > Not sure why we need this to be public except, probably, atr...
> > > > > 
> > > > > The port and atr are used by the serializers, for atr. The bc_rate is used
> > > > > by the serializers to figure out the clocking (they may use the FPD-Link's
> > > > > frequency internally).
> > > > 
> > > > The plain numbers can be passed as device properties. That's why the question
> > > > about platform data. Platform data in general is discouraged to be used in a
> > > > new code.
> > > 
> > > Device properties, as in, coming from DT?
> > 
> >  From anywhere.
> > 
> > > The port could be in the DT, but
> > > the others are not hardware properties.
> > 
> > Why do we need them? For example, bc_rate.
> 
> The atr pointer is needed so that the serializers (ub913, ub953) can add
> their i2c adapter to the deserializer's i2c-atr. The port is also needed for
> that.
> 
> The bc rate (back-channel rate) is the FPD-Link back-channel rate which the
> serializers use for various functionalities. At the moment only the ub953
> uses it for calculating an output clock rate.
> 
> The bc-rate could be implemented using the clock framework, even if it's not
> quite a plain clock. I had that code at some point, but it felt a bit off
> and as we needed the pdata for the ATR, I added the bc-rate there.

And I don't see why it is not a property of the device.

> > > Yes, I don't like using platform data. We need some way to pass information
> > > between the drivers.
> > 
> > Device properties allow that and targeting to remove the legacy platform data
> > in zillions of the drivers.
> 
> Do you have any pointers to guide me into the right direction? I couldn't
> find anything with some grepping and googling.
> 
> If you mean "device properties" as in ACPI, and so similar to DT properties,
> aren't those hardware properties? Only the port here is about the hardware.

About hardware, or PCB, or as quirks for missing DT/ACPI/any FW properties,
like clock rates.

The Linux kernel layer for that is called software nodes. The rough
approximation to see where and how it's being used can be achieved
by grepping for specific macros:

	git grep -lw PROPERTY_ENTRY_.*

E.g. arch/arm/mach-tegra/board-paz00.c tegra_paz00_wifikill_init()
implementation.

-- 
With Best Regards,
Andy Shevchenko