linux-kernel - Re: [PATCH v6 1/3] media: dt-bindings: ov8856: Document YAML bindings

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Date:   Tue, 7 Apr 2020 14:32:32 +0200
From:   Maxime Ripard <maxime@...no.tech>
To:     Robert Foss <robert.foss@...aro.org>
Cc:     Sakari Ailus <sakari.ailus@....fi>,
        Dongchun Zhu <dongchun.zhu@...iatek.com>,
        Fabio Estevam <festevam@...il.com>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Tomasz Figa <tfiga@...omium.org>,
        linux-media <linux-media@...r.kernel.org>,
        "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
        <devicetree@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        "moderated list:ARM/FREESCALE IMX / MXC ARM ARCHITECTURE" 
        <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH v6 1/3] media: dt-bindings: ov8856: Document YAML bindings

Hi Robert,

On Tue, Apr 07, 2020 at 01:29:05PM +0200, Robert Foss wrote:
> On Tue, 7 Apr 2020 at 10:36, Maxime Ripard <maxime@...no.tech> wrote:
> > On Mon, Apr 06, 2020 at 11:35:07AM +0300, Sakari Ailus wrote:
> > > > But that 19.2MHz is not a limitation of the device itself, it's a
> > > > limitation of our implementation, so we can instead implement
> > > > something equivalent in Linux using a clk_set_rate to 19.2MHz (to make
> > > > sure that our parent clock is configured at the right rate) and the
> > > > clk_get_rate and compare that to 19.2MHz (to make sure that it's not
> > > > been rounded too far apart from the frequency we expect).
> > > >
> > > > This is doing exactly the same thing, except that we don't encode our
> > > > implementation limitations in the DT, but in the driver instead.
> > >
> > > What I really wanted to say that a driver that doesn't get the clock
> > > frequency from DT but still sets that frequency is broken.
> > >
> > > This frequency is highly system specific, and in many cases only a certain
> > > frequency is usable, for a few reasons: On many SoCs, not all common
> > > frequencies can be used (e.g. 9,6 MHz, 19,2 MHz and 24 MHz; while others
> > > are being used as well), and then that frequency affects the usable CSI-2
> > > bus frequencies directly --- and of those, only safe, known-good ones
> > > should be used. IOW, getting the external clock frequency wrong typically
> > > has an effect that that none of the known-good CSI-2 bus clock frequencies
> > > are available.
> >
> > So clock-frequency is not about the "Frequency of the xvclk clock in
> > Hertz", but the frequency at which that clock must run on this
> > particular SoC / board to be functional?
> >
> > If so, then yeah, we should definitely keep it, but the documentation
> > of the binding should be made clearer as well.
>
> Alright so, let me summarise the desired approach then.

There's a separate discussion on the same topic here:
https://lore.kernel.org/linux-media/20200407122106.GD4751@pendragon.ideasonboard.com/

> ACPI:
>   - Fetch the "clock-frequency" property
>   - Verify it to be 19.2Mhz
>
> DT:
>   - Fetch the "clock-frequency" property
>   - Verify it to be 19.2Mhz
>   - Get xvclk clock
>   - Get xvclk clock rate
>   - Verify xvclk clock rate to be 19.2Mhz

The current status is that you should
's/clock-frequency/link-frequencies/', and in order to replace
assigned-clock-rates, you'll want to have a clk_set_rate to 19.2MHz
between steps 3 and 4

Maxime

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