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Date:   Wed, 8 Apr 2020 18:28:58 +0300
From:   Sakari Ailus <sakari.ailus@iki.fi>
To:     Maxime Ripard <maxime@cerno.tech>
Cc:     Tomasz Figa <tfiga@chromium.org>,
        Robert Foss <robert.foss@linaro.org>,
        Dongchun Zhu <dongchun.zhu@mediatek.com>,
        Fabio Estevam <festevam@gmail.com>,
        Andy Shevchenko <andriy.shevchenko@linux.intel.com>,
        linux-media <linux-media@vger.kernel.org>,
        "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
        <devicetree@vger.kernel.org>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        "moderated list:ARM/FREESCALE IMX / MXC ARM ARCHITECTURE" 
        <linux-arm-kernel@lists.infradead.org>
Subject: Re: [PATCH v6 1/3] media: dt-bindings: ov8856: Document YAML bindings
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Hi Maxime,

On Wed, Apr 08, 2020 at 03:43:15PM +0200, Maxime Ripard wrote:
> On Wed, Apr 08, 2020 at 02:35:28PM +0200, Tomasz Figa wrote:
> > On Wed, Apr 8, 2020 at 2:21 PM Maxime Ripard <maxime@cerno.tech> wrote:
> > > On Tue, Apr 07, 2020 at 08:20:35PM +0300, Sakari Ailus wrote:
> > > > On Tue, Apr 07, 2020 at 06:46:06PM +0200, Tomasz Figa wrote:
> > > > > On Tue, Apr 7, 2020 at 6:40 PM Sakari Ailus <sakari.ailus@iki.fi> wrote:
> > > > > >
> > > > > > On Tue, Apr 07, 2020 at 05:47:41PM +0200, Robert Foss wrote:
> > > > > > > On Tue, 7 Apr 2020 at 14:32, Maxime Ripard <maxime@cerno.tech> wrote:
> > > > > > > >
> > > > > > > > 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@cerno.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/
> > > > > > >
> > > > > > > Thanks for the link.
> > > > > > >
> > > > > > > >
> > > > > > > > > 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
> > > > > > >
> > > > > > > Would we want to 's/clock-frequency/link-frequencies/' for ACPI too?
> > > > > > > I imagine that would cause some breakage.
> > > > > >
> > > > > > It would, yes, and it would be no more correct on DT either.
> > > > > >
> > > > > > There are basically two possibilities here; either use the clock-frequency
> > > > > > property and set the frequency, or rely on assigned-clock-rates, and get
> > > > > > the frequency instead.
> > > > > >
> > > > > > The latter, while I understand it is generally preferred, comes with having
> > > > > > to figure out the register list set that closest matches the frequency
> > > > > > obtained. The former generally gets around this silently by the clock
> > > > > > driver setting the closest frequency it can support.
> > > > >
> > > > > Wouldn't the former actually cause problems, because the closest
> > > > > frequency the clock driver can support could be pretty far from the
> > > > > one requested? (E.g. 19.2 MHz vs 24 MHz) The driver needs to check the
> > > > > resulting frequency anyway.
> > > >
> > > > That's possible, yes; in this case there wouldn't be a guarantee the
> > > > frequency wouldn't be far off.
> > >
> > > assigned-clock-rates is really fragile... There's zero guarantee on
> > > how far the actual rate is going to be from the asked one, but more
> > > importantly you have zero guarantee on the time frame that rate is
> > > going to be enforced for.
> >
> > Is there such a guarantee if clk_set_rate() is called?
> 
> with clk_set_rate itself, no, but...
> 
> > > It's simply going to change the rate as a one-off thing, and if
> > > there's the next millisecond someone else is going to change its rate
> > > one way or another, it's going to do so and you won't have any
> > > notification.
> 
> You can get notified, and you can use clk_set_rate_exclusive if you
> *really* want to enforce it.

Is the conclusion then we should go back to relying on the clock-frequency
property?

This has been discussed multiple times over the years, and I don't really
disagree with the above. The frequency is typically indeed hand-picked for
the hardware, and no other frequency should be used in any circumstances.

No sensor driver I've seen has used clk_set_rate_exclusive() but I guess
they should. The absence of practical problems has been probably because of
two factors; firstly, these are typically clocks dedicated to the sensors
and secondly, good luck.

-- 
Regards,

Sakari Ailus