On Tue, Oct 20, 2020 at 02:26:21PM +0200, Krzysztof Kozlowski wrote: > On Tue, Oct 20, 2020 at 03:00:58PM +0300, Sakari Ailus wrote: > > Hi Krzysztof, > > > > On Tue, Oct 20, 2020 at 12:54:09PM +0200, Krzysztof Kozlowski wrote: > > > On Tue, 20 Oct 2020 at 12:38, Sakari Ailus <sakari.ailus@xxxxxxxxxxxxxxx> wrote: > > > > > > > > Hi Krzysztof, > > > > > > > > On Mon, Oct 19, 2020 at 07:02:44PM +0200, Krzysztof Kozlowski wrote: > > > > > Add bindings for the IMX258 camera sensor. The bindings, just like the > > > > > driver, are quite limited, e.g. do not support regulator supplies. > > > > > > > > > > Signed-off-by: Krzysztof Kozlowski <krzk@xxxxxxxxxx> > > > > > Reviewed-by: Rob Herring <robh@xxxxxxxxxx> > > > > > > > > > > --- > > > > > > > > > > Changes since v4: > > > > > 1. Add clock-lanes, > > > > > 2. Add Rob's review, > > > > > 3. Add one more example and extend existing one, > > > > > 4. Add common clock properties (assigned-*). > > > > > > > > Using the assigned-* clock properties may be workable for this driver at > > > > the moment. But using these properties does not guarantee the external > > > > clock frequency intended to be used on the hardware. > > > > > > It guarantees it. The clock frequency will be as expected (except if > > > someone misconfigures the DTS). > > > > Is that guaranteed? > > > > I'm not saying no to the approach, but if we change how camera sensor DT > > bindings are defined, I'd prefer an informed decision is made on the > > matter. > > > > > > > > > Using other > > > > frequencies *is not* expected to work. That applies to this driver as well. > > > > > > This is the binding which is HW description. According to HW datasheet > > > other frequencies from described range are accepted and expected to > > > work. > > > > As per datasheet, yes, different external clock frequencies can be used. > > But the link frequency is still not independent of the external clock > > frequency. > > > > The properties of the sensor's PLL tree determines what can be achieved > > given a certain external clock frequency. So picking a wrong external clock > > frequency quite possibly means that none of the designated link frequencies > > are available, rendering the sensor inoperable. > > The driver then controls the HW and knows exactly what is needed. If > link frequency (which has its own DT property) requires some clock > frequency, the driver will configure the clock to that value. The same Well it doesn't if it doesn't get that information from DT. The frequency is usually a range, and looking at these bindings, it's from 6 MHz to 27 MHz. That'd be a lot of frequencies for a driver to try. > going other direction. Driver has the knowledge about both its input > clock and link frequency, therefore it can make the best decision. Again you're assuming a particular driver implementation. Typically only a few frequencies are really available on platforms, so a in practice a driver would not be able to get any requested frequency. I wouldn't start hard-coding every possible frequency to camera sensor drivers. > > > > This, instead of the clock-frequency property, effectively removes the > > > > ability to set the correct frequency from the driver, at least with current > > > > set of the used APIs. > > > > > > It seems you confuse DT bindings with some specific driver > > > implementation. Bindings do not describe the driver behavior but the > > > HW. The ability to set the correct frequency from the driver is not > > > removed. It was never part of the bindings and never should. It is > > > part of the driver. > > > > > > > > > > > I suppose you could add a function to set the assigned clock frequency and > > > > keep it, just as clk_set_rate_exclusive does? > > I did not reply to this comment, so let me know. Of course, one could > add such functions. It's not a job for DT bindings, though. I'm not suggesting to add it to DT binding patch. What I'm saying that with this approach is looks like it may well be needed. -- Regards, Sakari Ailus
