Re: [PATCH v4 1/4] dt-bindings: media: imx258: add bindings for IMX258 sensor
From: Krzysztof Kozlowski <krzk@kernel.org>
Date: 2020-10-02 12:15:18
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On Tue, Sep 29, 2020 at 02:02:55PM +0300, Sakari Ailus wrote:
On Tue, Sep 29, 2020 at 11:46:36AM +0200, Krzysztof Kozlowski wrote:quoted
On Tue, Sep 29, 2020 at 12:40:46PM +0300, Sakari Ailus wrote:quoted
On Tue, Sep 29, 2020 at 11:18:46AM +0200, Krzysztof Kozlowski wrote:quoted
On Tue, 29 Sep 2020 at 11:15, Sakari Ailus [off-list ref] wrote:quoted
Hi Krzysztof, On Wed, Sep 23, 2020 at 05:21:26PM +0200, Krzysztof Kozlowski wrote:quoted
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.It's up to the driver to do what's needed, yes. Please see examples in e.g. drivers/media/i2c/ov8856.c and Documentation/devicetree/bindings/media/i2c/ov8856.yaml .
It seems the ov8856 driver uses this property in different way than
imx258 driver. It is more appropriate and quite similar to clock
providers and buses - to set the desired frequency on input clock.
Therefore what is the point of using this DT property comparing to
assigned-clock-rates?
It's the same. So let's use generic (already documented)
assigned-clock-rates.
For your question (not related to the bindings but to driver
implementation): "How is the rate determined?", simple: clk_get_rate.
The driver then uses it like this:
if (clk_get_rate() != only_working_configuration_hz)
return -EINVAL;
From the bindings point of view, the clock can be anything within a
range of sensor's accepted values. The clock frequency is a property of
a clock, not of a sensor. Therefore for HW description it should be
described in the clock bindings, not in the sensor bindings.
To summarize, the "clock-frequency" property of sensor node:
1. As a way to configure the clock should be replaced with
assigned-clock properties,
2. As a way to describe the hardware is simply invalid. It is not a HW
description, because HW requires just a clock of frequency within
given range, not a fixed frequency clock.
Consider the example:
camera@1a {
compatible = "sony,imx258";
reg = <0x1a>;
clocks = <&imx258_clk>;
clock-names = "clk";
/* Oscillator on camera board */
imx258_clk: clk {
compatible = "fixed-clock";
#clock-cells = <0>;
clock-frequency = <19200000>;
};
port {
...
};
};
What is the point to add "clock-frequency" property to the camera
itself, since it is already clearly defined by the clock?
Or another example:
camera@1a {
compatible = "sony,imx258";
reg = <0x1a>;
clocks = <&iclk 0>;
clock-names = "clk";
assigned-clocks = <&clk 0>;
assigned-clock-rates = <19200000>;
port {
...
};
};
Again, no reason for artificial clock-frequency property. It is not part
of HW description of the sensor.
Best regards,
Krzysztof
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