linux-kernel - Re: [PATCH v3 1/4] media: dt-bindings: media: i2c: Switch to assigned-clock-rates

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Date:   Thu, 19 Mar 2020 15:03:48 +0200
From:   Laurent Pinchart <laurent.pinchart@...asonboard.com>
To:     Maxime Ripard <maxime@...no.tech>
Cc:     Lad Prabhakar <prabhakar.mahadev-lad.rj@...renesas.com>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Shawn Guo <shawnguo@...nel.org>,
        Sascha Hauer <s.hauer@...gutronix.de>,
        Pengutronix Kernel Team <kernel@...gutronix.de>,
        Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Sakari Ailus <sakari.ailus@...ux.intel.com>,
        NXP Linux Team <linux-imx@....com>,
        Magnus Damm <magnus.damm@...il.com>,
        Ezequiel Garcia <ezequiel@...labora.com>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-renesas-soc@...r.kernel.org,
        Fabio Estevam <festevam@...il.com>,
        linux-media@...r.kernel.org, linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v3 1/4] media: dt-bindings: media: i2c: Switch to
 assigned-clock-rates

Hi Maxime,

On Thu, Mar 19, 2020 at 01:44:52PM +0100, Maxime Ripard wrote:
> On Fri, Mar 13, 2020 at 09:12:31PM +0000, Lad Prabhakar wrote:
> > Use assigned-clock-rates to specify the clock rate. Also mark
> > clock-frequency property as deprecated.
> >
> > Signed-off-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@...renesas.com>
> > ---
> >  Documentation/devicetree/bindings/media/i2c/ov5645.txt | 5 +++--
> >  1 file changed, 3 insertions(+), 2 deletions(-)
> >
> > diff --git a/Documentation/devicetree/bindings/media/i2c/ov5645.txt b/Documentation/devicetree/bindings/media/i2c/ov5645.txt
> > index 72ad992..e62fe82 100644
> > --- a/Documentation/devicetree/bindings/media/i2c/ov5645.txt
> > +++ b/Documentation/devicetree/bindings/media/i2c/ov5645.txt
> > @@ -8,7 +8,7 @@ Required Properties:
> >  - compatible: Value should be "ovti,ov5645".
> >  - clocks: Reference to the xclk clock.
> >  - clock-names: Should be "xclk".
> > -- clock-frequency: Frequency of the xclk clock.
> > +- clock-frequency (deprecated): Frequency of the xclk clock.
> >  - enable-gpios: Chip enable GPIO. Polarity is GPIO_ACTIVE_HIGH. This corresponds
> >    to the hardware pin PWDNB which is physically active low.
> >  - reset-gpios: Chip reset GPIO. Polarity is GPIO_ACTIVE_LOW. This corresponds to
> > @@ -37,7 +37,8 @@ Example:
> >
> >  			clocks = <&clks 200>;
> >  			clock-names = "xclk";
> > -			clock-frequency = <24000000>;
> > +			assigned-clocks = <&clks 200>;
> > +			assigned-clock-rates = <24000000>;
> >
> >  			vdddo-supply = <&camera_dovdd_1v8>;
> >  			vdda-supply = <&camera_avdd_2v8>;
> 
> clock-frequency is quite different from assigned-clock-rates though,
> semantically speaking. clock-frequency is only about what the clock
> frequency is, while assigned-clock-rates will change the rate as well,
> and you have no idea how long it will last.

The driver currently reads the clock-frequency property and then calls
clk_set_rate(). I agree tht assigned-clock-rates isn't a panacea, but I
think it's less of a hack than what we currently have.

As discussed on IRC, maybe the best option in this specific case is to
drop clock-frequency and assigned-clok-rates, and call clk_set_rate()
with a hardcoded frequency of 24MHz in the driver, as that's the only
frequency the driver supports.

> If you want to retrieve that through the clock framework, then just
> making clock-frequency optional is enough and falling back to
> clk_get_rate on the clocks property already provided is enough.

-- 
Regards,

Laurent Pinchart