Re: [RFC PATCH 5/7] ov7670: add devicetree support

[Laurent Pinchart <laurent.pinchart@ideasonboard.com>]

Hello Sakari,

On Wednesday 21 Sep 2016 14:33:29 Sakari Ailus wrote:
> Hi Laurent,
> 
> On Wed, Aug 17, 2016 at 03:44:00PM +0300, Laurent Pinchart wrote:
> > > +			assigned-clock-rates = <24000000>;
> > 
> > You should compute and set the clock rate dynamically in the driver, not
> > hardcode it in DT.
> 
> This frequency is typically defined by hardware engineers and it's
> hand-picked from possible ranges. What counts is EMC so you don't disturb
> a GSM/3G/4G modem (if you have one) or GPS receiver, for instance. In order
> to freely choose this frequency, you'd also need to be aware of the limits
> of the sensor's internal clock tree, and make sure you still would be able
> to obtain the desired frame rates for there are often corner cases where the
> resulting maximum pixel clock may be substantially lower if your input
> frequency goes up.
> 
> I also haven't encountered a use case for more than a single, fixed
> frequency. In other words I'd keep this constant.

My review predates our recent discussion on this topic. I still believe that 
in the general case we could want to express allowable frequencies constraints 
in DT and let the driver compute the desired frequency based on timings. 
However, I also agree that there are very few cases for this, so it's not 
really worth tackling the issue at the moment. I'm thus fine with the usage of 
assigned-clock-rates.

-- 
Regards,

Laurent Pinchart