On Mon, Sep 16, 2013 at 10:59:58AM -0600, Stephen Warren wrote: > On 09/16/2013 10:49 AM, Guenter Roeck wrote: > > On Mon, Sep 16, 2013 at 10:34:28AM -0600, Stephen Warren wrote: > >> On 09/12/2013 06:55 PM, Soren Brinkmann wrote: > >>> Add a driver for SILabs 570, 571, 598, 599 programmable oscillators. > >>> The devices generate low-jitter clock signals and are reprogrammable via > >>> an I2C interface. > >> > >>> diff --git a/Documentation/devicetree/bindings/clock/silabs,si570.txt b/Documentation/devicetree/bindings/clock/silabs,si570.txt > >> > >>> +Required properties: > >>> + - compatible: Shall be one of "silabs,si57x", "silabs,si59x". > >>> + - reg: I2C device address. > >>> + - #clock-cells: From common clock bindings: Shall be 0. > >>> + - factory-fout: Factory set default frequency > >> > >> So, there's no way to query this from the device at all? Looking at the > >> data-sheet, all the frequency generation parameters are in registers, > >> and the device supports I2C read commands. As such, I'm not convinced > >> this property is necessary. > > > > Unfortunately, the chip does not report the factory setting for fout, > > so the property is needed. The chip can not be programmed without it. > > So fout is not the default overall output frequency, but rather > something internal that that feeds into the frequency generation > process, and the registers in the device only describe that frequency > generation process, not the frequency that feeds into it? > Here is what the datasheet has to say: "The device's default output frequency is set at the factory and can be reprogrammed through the two-wire I2C serial port. Once the device is powered down, it will return to its factory-set default output frequency." So it is a default output frequency. However, the chip not report what it is, and it can be programmed to other frequencies (and commonly is). In reality it is a reference frequency which happens to match the output frequency if all registers are at their default (post-reset) setting. Correct, the registers only describe the frequency generation process relative to the reference frequency, so that frequency has to be known to calculate the correct register values for other frequencies. > If so, it might be worth enhancing the binding documentation to briefly > describe that. Presumably the details are all in the HW documentation, > but it'd be nice if the binding doc was obviously correct without having > to fully understand the entire HW documentation. > > >>> +Optional properties: > >>> + - initial-fout: Initial output frequency to set during probe > >> > >> "probe" is a Linux-specific concept. This property should be removed. If > >> the driver is asked to set a specific frequency, it should do so, but I > >> don't think it should program something pro-actively just because it > >> starts up. > >> > >> If this property is acceptable, it'd be better to describe it more along > >> the lines of the following: > >> > >> initial-fout: The frequency at which the system requires the clock to > >> operate. > > > > It should probably be something like "clock-frequency". In many use cases > > the programmed frequency is set to a constant frequency at system startup > > and never changed, similar to other clocks. > > I was going to suggest that too, but re-considered since I think > clock-frequency is more appropriate for fixed-frequency clocks, rather > than to specify the value at which a programmable clock generator should > operate? > > I don't think we have a good story yet for how to represent > how-we-want-the-clock-tree-configured, as opposed to representing the HW > itself (which is what DT should be more about). > In many cases the chip _is_ used to generate a fixed frequency, so we will have to have a means to describe it. That it _can_ be used differently is a different matter. After all, that is true for many clock generators. Guenter -- To unsubscribe from this list: send the line "unsubscribe linux-doc" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
