Re: [PATCH v4 1/4] dt-bindings: media: imx258: add bindings for IMX258 sensor

From: Krzysztof Kozlowski <krzk@kernel.org>
To: Sakari Ailus <sakari.ailus@linux.intel.com>
Cc: Mauro Carvalho Chehab <mchehab@kernel.org>,
	Rob Herring <robh+dt@kernel.org>, Shawn Guo <shawnguo@kernel.org>,
	Sascha Hauer <s.hauer@pengutronix.de>,
	Pengutronix Kernel Team <kernel@pengutronix.de>,
	Fabio Estevam <festevam@gmail.com>,
	NXP Linux Team <linux-imx@nxp.com>,
	linux-media@vger.kernel.org, devicetree@vger.kernel.org,
	linux-arm-kernel@lists.infradead.org,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH v4 1/4] dt-bindings: media: imx258: add bindings for IMX258 sensor
Date: Tue, 29 Sep 2020 11:46:36 +0200	[thread overview]
Message-ID: <20200929094636.GA11333@kozik-lap> (raw)
In-Reply-To: <20200929094046.GH26842@paasikivi.fi.intel.com>

On Tue, Sep 29, 2020 at 12:40:46PM +0300, Sakari Ailus wrote:
> On Tue, Sep 29, 2020 at 11:18:46AM +0200, Krzysztof Kozlowski wrote:
> > On Tue, 29 Sep 2020 at 11:15, Sakari Ailus <sakari.ailus@linux.intel.com> wrote:
> > >
> > > Hi Krzysztof,
> > >
> > > On Wed, Sep 23, 2020 at 05:21:26PM +0200, Krzysztof Kozlowski wrote:
> > > > Add bindings for the IMX258 camera sensor.  The bindings, just like the
> > > > driver, are quite limited, e.g. do not support regulator supplies.
> > > >
> > > > Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
> > > >
> > > > ---
> > > >
> > > > Changes since v3:
> > > > 1. Document also two lane setup.
> > > >
> > > > Changes since v2:
> > > > 1. Remove clock-frequency, add reset GPIOs, add supplies.
> > >
> > > Oops. I missed this one.
> > >
> > > How does the driver know the appropriate clock frequency for the platform
> > > if it's not in DT? The sensor supports a range of frequencies, not a single
> > > frequency.
> > >
> > > Could you add clock-frequency back?
> > 
> > Not really, it was removed on Rob's request. The bindings do not
> > describe driver's behavior so how the driver gets frequency should not
> > be part of the bindings. Also it's not a real problem - the driver
> > just calls clk_get_rate().
> 
> How is the rate determined? I mean, many ISPs or CSI-2 receivers that
> provide the clock are also capable of using a variety of frequencies. But
> only one can be used on the platform in general.

Having "clock-frequency" property in DTS did not solve that. It has no
effect on actual frequency.

> 
> Where does it come from if it's not in DT?

The frequency is either chosen by consumer (imx258) or pre-assigned from
DT, but not with "clock-frequency" property. There are generic
properties for this: assigned-clocks, assigned-clock-rates and
assigned-clock-parents.

These properties should be added to DTS if additionalProperties is
false, which is the case here... so I could add them. My DTS anyway does
not use them, as the clock is generated internally on a camera board so
I don't have a control over it.

Best regards,
Krzysztof

> 
> Using another frequency generally leads to failure later on as the desired
> link frequency likely is not available for a random external clock
> frequency.