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

[Sakari Ailus]

Hi Laurent,

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 ?

No. In a general case there is no reliable way to come up with an external
clock frequency based on another, different if related, frequency.

> 
> In any case, this policy needs to be carefully documented.

I thought after ten or so years this would be already an established
practice. :-)

I agree it should be documented. We don't seem to have specific
documentation for camera sensor drivers at the moment. I can submit a
patch...

> 
> > >> 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.

The external clock frequency is significantly lower than the link frequency
(usually), but it still comes out of the SoC (or a PMIC chip). The clock
signal track on PCB as well as wiring may also be rather long, depending on
where the camera sensor is --- quite possibly tens of centimetres.
Therefore I wouldn't categorically rule out possible EMC issues with that
one either.

The bottom line is: use a known-good, safe frequency.

-- 
Regards,

Sakari Ailus