Hi Sakari, On Tue, Apr 14, 2020 at 11:55:54PM +0300, Laurent Pinchart wrote: > 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. And by this I mean in a central place, not leaving it to individual bindings. Maxime, we've previously discussed this issue privately on IRC, what's your opinion ? > >>> 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. -- Regards, Laurent Pinchart
