From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pb0-f43.google.com (mail-pb0-f43.google.com [209.85.160.43]) (using TLSv1 with cipher ECDHE-RSA-RC4-SHA (128/128 bits)) (Client CN "smtp.gmail.com", Issuer "Google Internet Authority" (not verified)) by ozlabs.org (Postfix) with ESMTPS id D47972C009D for ; Sat, 24 Aug 2013 07:41:50 +1000 (EST) Received: by mail-pb0-f43.google.com with SMTP id md4so1149218pbc.16 for ; Fri, 23 Aug 2013 14:41:48 -0700 (PDT) Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 To: Sascha Hauer , Mark Rutland From: Mike Turquette In-Reply-To: <20130823140128.GI31036@pengutronix.de> References: <20130822210035.GB31036@pengutronix.de> <20130822224331.8231.75743@quantum> <34430785.ApIqfgatMc@flatron> <20130823063421.GF31036@pengutronix.de> <20130823125815.GG25856@e106331-lin.cambridge.arm.com> <20130823140128.GI31036@pengutronix.de> Message-ID: <20130823214144.8231.2039@quantum> Subject: Re: [alsa-devel] [PATCH v5 1/2] ASoC: fsl: Add S/PDIF CPU DAI driver Date: Fri, 23 Aug 2013 14:41:44 -0700 Cc: "devicetree@vger.kernel.org" , "alsa-devel@alsa-project.org" , "lars@metafoo.de" , "ian.campbell@citrix.com" , Pawel Moll , "swarren@wwwdotorg.org" , "linuxppc-dev@lists.ozlabs.org" , Nicolin Chen , Tomasz Figa , "rob.herring@calxeda.com" , "timur@tabi.org" , "broonie@kernel.org" , "p.zabel@pengutronix.de" , "galak@codeaurora.org" , "shawn.guo@linaro.org" , "festevam@gmail.com" List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Quoting Sascha Hauer (2013-08-23 07:01:28) > On Fri, Aug 23, 2013 at 01:58:15PM +0100, Mark Rutland wrote: > > On Fri, Aug 23, 2013 at 07:34:21AM +0100, Sascha Hauer wrote: > > > On Fri, Aug 23, 2013 at 12:49:19AM +0200, Tomasz Figa wrote: > > > > 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 wrot= e: > > > > > > > > Quoting Tomasz Figa (2013-08-21 14:34:55) > > > > > > > > = > > > > > > > > > On Wednesday 21 of August 2013 09:50:15 Mark Rutland wrot= e: > > > > > > > > > > On Tue, Aug 20, 2013 at 01:06:25AM +0100, Mike Turquett= e = > > > > wrote: > > > > > > > > > > > Quoting Mark Rutland (2013-08-19 02:35:43) > > > > > > > > > > > = > > > > > > > > > > > > On Sat, Aug 17, 2013 at 04:17:18PM +0100, Tomasz Fi= ga = > > > > wrote: > > > > > > > > > > > > > On Saturday 17 of August 2013 16:53:16 Sascha Hau= er = > > > > wrote: > > > > > > > > > > > > > > On Sat, Aug 17, 2013 at 02:28:04PM +0200, Tomas= z 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 phand= le 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 o= ne is > > > > > > > > > > > > > > > > > ensured > > > > > > > > > > > > > > > > > even if > > > > > > > > > > > > > > > > > it's > > > > > > > > > > > > > > > > > missing. > > > > > > > > > > > > > > > > = > > > > > > > > > > > > > > > > <&clks 0> is the dummy clock. This can be u= sed 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 a= re. See > > > > > > > > > > > > > > Documentation/devicetree/bindings/clock/imx*-cl= ock.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 w= e? > > > > > > > > > > > > > = > > > > > > > > > > > > > If we really need a dummy clock, then we might al= so want > > > > > > > > > > > > > a generic > > > > > > > > > > > > > way to specify it. > > > > > > > > > > > > = > > > > > > > > > > > > What do we actually mean by a "dummy clock"? We alr= eady > > > > > > > > > > > > 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 t= he > > > > > > > > > > > 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 pl= atform > > > > > > > > > > code in > > > > > > > > > > Linux? A dummy clock sounds like a completely Linux-spe= cific > > > > > > > > > > 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 de= scribe > > > > > > > > > > that. If a driver believes that a clock is required whe= n it > > > > > > > > > > isn't (for some level of functionality), then that driv= er > > > > > > > > > > should be fixed up to support the clock as being option= al. > > > > > > > > > > = > > > > > > > > > > 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-speci= fic > > > > > > > > abstraction. > > > > > > > > = > > > > > > > > I'm not advocating a dummy clock in DT. I am advocating > > > > > > > > consolidation of the implementation of a clock that does no= thing > > > > > > > > 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 driver= s do > > > > > > > > not > > > > > > > > honor the clk.h api and they freak out when clk_get gives t= hem 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 dt= bs > > > > > > 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 j= ust > > > > > > describe the real clocks. > > > > > = > > > > > You could use a dummy clk for the Linux implementation, but the d= ownside > > > > > is that a dummy clock has a rate of 0 always and a your clocks li= kely > > > > > have non-zero rates. > > > > > = > > > > > It is probably better for you define a clock which only implement= s 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 i= s a = > > > > rather smart dummy. > > > > = > > > > > > BTW with the S/PDIF core on which not all mux inputs are connec= ted > > > > > > 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 cl= ock? > > > > > 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? > > > = > > > The difference is that you can successfully grab it in your driver. > > = > > That's a driver-specific issue. The driver knows best which clocks it > > can live without (if it's poking only a subset of the hardware, it may > > not need some just yet, but could for extended functionality in future > > when support is extended), and could assign a dummy to those clocks it > > knows it doesn't need that aren't described. That doesn't need to be in > > the dt, and shouldn't be, because it's OS and driver specific. > > = > > > = > > > > = > > > > > > Background to why it might be a good idea to connect a ground c= lock > > > > > > 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 dist= inguish > > > > > 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 boar= d). > > > = > > > Yes, that's what I meant. To illustrate the problem for this driver: > > > = > > > for (i =3D 0; i < STC_TXCLK_SRC_MAX; i++) { > > > sprintf(tmp, "rxtx%d", i); > > > clk =3D devm_clk_get(&pdev->dev, tmp); > > > if (IS_ERR(clk)) { > > = > > [...] > > = > > /* > > * ERR_PTR(-ENOENT) returned when clock not > > * present in the dt (i.e. not wired up). We can > > * live without this clock, so assign a dummy > > * (NULL) to simplify the rest of the code. If > > * the clock is present but something else went > > * wrong, we'll get a different ERR_PTR value > > * and actually fail. > > */ > > if (clk =3D=3D ERR_PTR(-ENOENT) > > clk =3D NULL; > > = > > > } > > > } > > > = > > > This could be solved by always specifying all input clocks in the > > > devicetree. > > = > > As far as I can see, the above is sufficient, and leaves the knowledge > > of skippable clocks in the driver, where I believe it should be. > = > You miss my point. Once a clock is specified in the devicetree it is no > longer optional. The driver currently can't find out whether a clock > hasn't been specified (and it's ok that it's not there) or a clock has > been specified, but some error prevents actually grabbing the clock (in > which case the driver should bail out) Seems like the regulator framework is solving this with the new regulator_get_optional() call. This leaves the optional-versus-not-optional logic up to the driver. Regards, Mike > = > Sascha > = > = > -- = > Pengutronix e.K. | | > Industrial Linux Solutions | http://www.pengutronix.de/ | > Peiner Str. 6-8, 31137 Hildesheim, Germany | Phone: +49-5121-206917-0 | > Amtsgericht Hildesheim, HRA 2686 | Fax: +49-5121-206917-5555 |