Re: [PATCH v5 2/5] media: i2c: ov5645: Drop reading clock-frequency dt-property

[Maxime Ripard]

On Tue, Apr 14, 2020 at 11:55:52PM +0300, Laurent Pinchart wrote:
> Hi Sakari,
>
> On Tue, Apr 07, 2020 at 06:14:01PM +0300, Sakari Ailus wrote:
> > On Tue, Apr 07, 2020 at 03:21:06PM +0300, Laurent Pinchart wrote:
> > > On Tue, Apr 07, 2020 at 09:22:41AM +0300, Sakari Ailus wrote:
> > >> On Mon, Apr 06, 2020 at 08:32:34PM +0300, Laurent Pinchart wrote:
> > >>> On Mon, Apr 06, 2020 at 07:51:08PM +0300, Sakari Ailus wrote:
> > >>>> On Mon, Apr 06, 2020 at 05:42:38PM +0100, Lad Prabhakar wrote:
> > >>>>> Modes in the driver are based on xvclk frequency fixed to 24MHz, but where
> > >>>>> as the OV5645 sensor can support the xvclk frequency ranging from 6MHz to
> > >>>>> 24MHz. So instead making clock-frequency as dt-property just let the
> > >>>>> driver enforce the required clock frequency.
> > >>>>
> > >>>> Even if some current systems where the driver is used are using 24 MHz
> > >>>> clock, that doesn't mean there wouldn't be systems using another frequency
> > >>>> that the driver does not support right now.
> > >>>>
> > >>>> The driver really should not set the frequency unless it gets it from DT,
> > >>>> but I think the preferred means is to use assigned-clock-rates instead, and
> > >>>> not to involve the driver with setting the frequency.
> > >>>>
> > >>>> Otherwise we'll make it impossible to support other frequencies, at least
> > >>>> without more or less random defaults.
> > >>>
> > >>> We're running in circles here.
> > >>>
> > >>> As the driver only supports 24MHz at the moment, the frequency should be
> > >>> set by the driver, as it's a driver limitation. We can then work on
> > >>> supporting additional frequencies, which will require DT to provide a
> > >>> list of supported frequencies for the system, but that can be done on
> > >>> top.
> > >>
> > >> I guess it would be possible to use different external clock frequencies on
> > >> a sensor in a given system but that seems to be a bit far fetched, to the
> > >> extent I've never seen anyone doing that in practice.
> > >>
> > >> Originally, the driver set the frequency based on the clock-frequency
> > >> property. If we're removing that but use a fixed frequency instead, then
> > >> how is that going to work going forward when someone adds support for other
> > >> frequencies in the driver and has a system requiring that, while there are
> > >> some other platforms relying on the driver setting a particular frequency?
> > >
> > > The standard property for this is link-frequencies, not clock-frequency.
> > > Deprecating clock-frequency now paves the way to use the standard
> > > property later when/if someone implements support for additional
> > > frequencies.
> >
> > The external clock frequency and link frequency are different indeed, but
> > they are related. The link frequency has been selected in a way that it is
> > possible to generate that exact frequency using the chosen external clock
> > frequency. If you change the external clock frequency, chances are good
> > there is no PLL configuration to generate that link frequency.
>
> But aren't we supposed to pick the clock frequency based on the link
> frequency specified in DT ?
>
> In any case, this policy needs to be carefully documented.
>
> > >> Although, if you're saying that this driver only needs to work with DT that
> > >> comes with the kernel and you don't care about DT binary compatibility,
> > >> this would be fine.
> > >
> > > I believe this series to not break backward compatibility, as the driver
> > > only works with a 24MHz clock, so I expect all DTs to specify that.
> >
> > What you're still doing here is defining the DT bindings based on the
> > current driver implementation, not the device properties.
>
> Quite the contrary, the device doesn't require any particular input
> clock frequency, so we're removing that from DT :-) Specifying the clock
> frequency in DT is in my opinion a manual workaround for not computing
> it at runtime based on the desired link frequency, while the link
> frequency is a property of the system as it specifies the range of link
> frequencies that are safe to use from an EMC point of view.

I 100% agree with this.

Maxime

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