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

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Message-ID: <20200324154045.whiy6uvlg2mrjv5a@gilmour.lan>
Date:   Tue, 24 Mar 2020 16:40:45 +0100
From:   Maxime Ripard <maxime@...no.tech>
To:     "Lad, Prabhakar" <prabhakar.csengg@...il.com>
Cc:     Laurent Pinchart <laurent.pinchart@...asonboard.com>,
        Mark Rutland <mark.rutland@....com>,
        "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
        <devicetree@...r.kernel.org>, Fabio Estevam <festevam@...il.com>,
        Sakari Ailus <sakari.ailus@...ux.intel.com>,
        Ezequiel Garcia <ezequiel@...labora.com>,
        Sascha Hauer <s.hauer@...gutronix.de>,
        Magnus Damm <magnus.damm@...il.com>,
        Lad Prabhakar <prabhakar.mahadev-lad.rj@...renesas.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux-Renesas <linux-renesas-soc@...r.kernel.org>,
        Rob Herring <robh+dt@...nel.org>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        NXP Linux Team <linux-imx@....com>,
        Pengutronix Kernel Team <kernel@...gutronix.de>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Shawn Guo <shawnguo@...nel.org>,
        LAK <linux-arm-kernel@...ts.infradead.org>,
        linux-media <linux-media@...r.kernel.org>
Subject: Re: [PATCH v3 1/4] media: dt-bindings: media: i2c: Switch to
 assigned-clock-rates

On Thu, Mar 19, 2020 at 01:17:51PM +0000, Lad, Prabhakar wrote:
> Hi Laurent,
>
> On Thu, Mar 19, 2020 at 1:04 PM Laurent Pinchart
> <laurent.pinchart@...asonboard.com> wrote:
> >
> > 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.
> >
> Does this mean any driver which has a fixed clock requirement shouldn't be a
> DT property and should be just handled by the drivers internally ?

It's hard to give a generic policy, but here, the hardware is pretty
flexible since it can deal with anything between 6MHz to 50-something
MHz, it's the driver that chooses to enforce a 24MHz and be pedantic
about it, so it's up to the driver to enforce that policy, not to the
DT since it's essentially a software limitation, not a hardware one.

Maxime

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