On Thursday 22 of August 2013 15:43:31 Mike Turquette wrote: > Quoting Sascha Hauer (2013-08-22 14:00:35) > > > On Thu, Aug 22, 2013 at 01:09:31PM +0100, Mark Rutland wrote: > > > On Thu, Aug 22, 2013 at 08:19:10AM +0100, Mike Turquette wrote: > > > > Quoting Tomasz Figa (2013-08-21 14:34:55) > > > > > > > > > On Wednesday 21 of August 2013 09:50:15 Mark Rutland wrote: > > > > > > On Tue, Aug 20, 2013 at 01:06:25AM +0100, Mike Turquette wrote: > > > > > > > Quoting Mark Rutland (2013-08-19 02:35:43) > > > > > > > > > > > > > > > On Sat, Aug 17, 2013 at 04:17:18PM +0100, Tomasz Figa wrote: > > > > > > > > > On Saturday 17 of August 2013 16:53:16 Sascha Hauer wrote: > > > > > > > > > > On Sat, Aug 17, 2013 at 02:28:04PM +0200, Tomasz Figa wrote: > > > > > > > > > > > > > > Also I would make this option required. Use a > > > > > > > > > > > > > > dummy > > > > > > > > > > > > > > clock for > > > > > > > > > > > > > > mux > > > > > > > > > > > > > > inputs that are grounded for a specific SoC. > > > > > > > > > > > > > > > > > > > > > > > > > > Some clocks are not from CCM and we haven't > > > > > > > > > > > > > defined in > > > > > > > > > > > > > imx6q-clk.txt, > > > > > > > > > > > > > so in most cases we can't provide a phandle for > > > > > > > > > > > > > them, eg: > > > > > > > > > > > > > spdif_ext. > > > > > > > > > > > > > I think it's a bit hard to force it to be > > > > > > > > > > > > > 'required'. An > > > > > > > > > > > > > 'optional' > > > > > > > > > > > > > looks more flexible to me and a default one is > > > > > > > > > > > > > ensured > > > > > > > > > > > > > even if > > > > > > > > > > > > > it's > > > > > > > > > > > > > missing. > > > > > > > > > > > > > > > > > > > > > > > > <&clks 0> is the dummy clock. This can be used for > > > > > > > > > > > > all input > > > > > > > > > > > > clocks > > > > > > > > > > > > not > > > > > > > > > > > > defined by the SoC. > > > > > > > > > > > > > > > > > > > > > > Where does this assumption come from? Is it > > > > > > > > > > > documented > > > > > > > > > > > anywhere? > > > > > > > > > > > > > > > > > > > > This is how all i.MX clock bindings currently are. See > > > > > > > > > > Documentation/devicetree/bindings/clock/imx*-clock.txt > > > > > > > > > > > > > > > > > > OK, thanks. > > > > > > > > > > > > > > > > > > I guess we need some discussion on dummy clocks vs > > > > > > > > > skipped clocks. > > > > > > > > > I think we want some consistency on this, don't we? > > > > > > > > > > > > > > > > > > If we really need a dummy clock, then we might also want > > > > > > > > > a generic > > > > > > > > > way to specify it. > > > > > > > > > > > > > > > > What do we actually mean by a "dummy clock"? We already > > > > > > > > have > > > > > > > > bindings > > > > > > > > for "fixed-clock" and co friends describe relatively > > > > > > > > simple > > > > > > > > preconfigured clocks. > > > > > > > > > > > > > > Some platforms have a fake clock which defines noops > > > > > > > callbacks and > > > > > > > basically doesn't do anything. This is analogous to the > > > > > > > dummy > > > > > > > regulator > > > > > > > implementation. A central one could be registered by the > > > > > > > clock core, > > > > > > > as > > > > > > > is done by the regulator core. > > > > > > > > > > > > When you say some platforms, you presumably mean the platform > > > > > > code in > > > > > > Linux? A dummy clock sounds like a completely Linux-specific > > > > > > abstraction rather than a description of the hardware, and I > > > > > > don't see why we need that in the DT: > > > > > > > > > > > > * If a clock is wired up and running (as presumably the dummy > > > > > > clock is), then surely it's a fixed-clock (it's running, we > > > > > > and we have no control over it, but we presumably know its > > > > > > rate) and can be described as such? > > > > > > > > > > > > * If no clock is wired up, then we should be able to describe > > > > > > that. If a driver believes that a clock is required when it > > > > > > isn't (for some level of functionality), then that driver > > > > > > should be fixed up to support the clock as being optional. > > > > > > > > > > > > Am I missing something? > > > > > > > > > > I second that. > > > > > > > > > > Moreover, I don't think that device tree should deal with dummy > > > > > anything. It should be able to describe hardware that is > > > > > available on given system, not list what hardware is not > > > > > available. > > > > > > > > I wasn't clear. The dummy clock IS a completely Linux-specific > > > > abstraction. > > > > > > > > I'm not advocating a dummy clock in DT. I am advocating > > > > consolidation of the implementation of a clock that does nothing > > > > into the clock core. This code could easily live in > > > > drivers/clk/clk.c instead of having everyone open-code it. > > > > > > > > As far as specifying a dummy clock in DT? I dunno. DT should > > > > describe > > > > real hardware so there isn't much use for a dummy clock. > > > > > > Sorry, I misunderstood. Good to hear we're on the same page :) > > > > > > > I'm guessing one of the reasons for such a clock are drivers do > > > > not > > > > honor the clk.h api and they freak out when clk_get gives them a > > > > NULL > > > > pointer? > > > > > > I'm not sure. Sascha, could you shed some light on the matter? > > > > The original reason introducing the dummy clocks in the i.MX dtbs > > was to provide devices a clock which the driver requests but is > > not software controllable. We often have the case where the same > > devices are on several SoCs, but not on all of them all clocks have > > a bit to en/disable them. > > > > Anyway, to accomplish this we don't need dummy clocks. We can just > > describe the real clocks. > > You could use a dummy clk for the Linux implementation, but the downside > is that a dummy clock has a rate of 0 always and a your clocks likely > have non-zero rates. > > It is probably better for you define a clock which only implements the > .recalc_rate callback. If the rate of this clock changes without Linux > having knowledge of it you can use the CLK_GET_RATE_NOCACHE flag. I doubt that rate of a dummy clock could ever change... unless it is a rather smart dummy. > > BTW with the S/PDIF core on which not all mux inputs are connected > > to actual clocks we could also describe the unconnected inputs as > > ground clocks with rate 0. This way we describe something which > > is really there instead of dummy clocks ;) > > Again you could use a dummy clock for this OR a fixed-rate clock with a > rate of zero from the perspective of the Linux implementation. > > Do you think it worthwhile to have a DT binding for a grounded clock? > That is not an entirely uncommon case. Well, how would that differ from skipping a clock from clocks list, i.e. not specifying it in clock-names and clocks properties? > > Background to why it might be a good idea to connect a ground clock > > to the unconnected input pins is that a driver has a chance to > > successfully grab all clocks. Otherwise how does the driver > > distinguish > > between an unconnected and an erroneous clock? > > Sorry, I don't follow this last question. Do you mean how to distinguish > based on the value returned from clk_get? Hmm, in theory, a driver could want to distinguish an error case (e.g. clock specified, but there is a problem with it) from no clock (e.g. clock not specified in DT, because it is not available on particular board). Best regards, Tomasz -- To unsubscribe from this list: send the line "unsubscribe devicetree" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
