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

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Message-ID: <20200929094046.GH26842@paasikivi.fi.intel.com>
Date:   Tue, 29 Sep 2020 12:40:46 +0300
From:   Sakari Ailus <sakari.ailus@...ux.intel.com>
To:     Krzysztof Kozlowski <krzk@...nel.org>
Cc:     Mauro Carvalho Chehab <mchehab@...nel.org>,
        Rob Herring <robh+dt@...nel.org>,
        Shawn Guo <shawnguo@...nel.org>,
        Sascha Hauer <s.hauer@...gutronix.de>,
        Pengutronix Kernel Team <kernel@...gutronix.de>,
        Fabio Estevam <festevam@...il.com>,
        NXP Linux Team <linux-imx@....com>,
        linux-media@...r.kernel.org, devicetree@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v4 1/4] dt-bindings: media: imx258: add bindings for
 IMX258 sensor

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@...ux.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@...nel.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.

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

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

-- 
Sakari Ailus