From: Mike Turquette <mturquette@ti.com> To: linux-arm-kernel@lists.infradead.org Subject: Re: Device tree binding for DVFS table Date: Fri, 13 Jul 2012 17:25:32 +0000 [thread overview] Message-ID: <20120713172532.GN2772@gmail.com> (raw) In-Reply-To: <20120713103408.GF9437@tbergstrom-lnx.Nvidia.com> On 20120713-13:34, Peter De Schrijver wrote: > Hi Mike, > > > > > Peter, > > > > I agree with your observations in general, but I think some specificity > > is needed: > > > > > + frequency/voltage relationships > > > > We should be clear that the voltage does NOT belong to the clock, but to > > the device/module/IP block that consumes that clock. This is an > > important detail since it means that a clock does not have a > > corresponding table of voltages (e.g. one table per clock), but instead > > a device has a table of voltages corresponding to each clock. > > > > Or the other way around, a table of clock frequencies, 1 for each voltage. > ACK You said what I meant to say. Some previous iterations of an omap-specific dvfs api tried to use voltage as the unique key for accessing frequency/voltage tables, and that can get really ugly for a variety of reasons. Plus device drivers typically request performance in Hertz (e.g. clk_set_rate), so exposing a voltage-centric interface to them is generally useless. The OPP library does some of this right (drivers/base/power/opp.c). It keeps a list of tables on a per-device bases, which is a big step in the right direction. Unfortunately it is lacking in other areas, such as not specifying which clock corresponds to the to table. Thus a device can only have one table and that table does not encode any information about which clock to use (which is important for devices that use a mux to change rate). The dvfs DT bindings should not repeat those mistakes. > > This is very necessary when a single clock drives multiple devices which > > are driven by separate voltage rails. > > > > Ah ok. How does this work in practice? A device can only run at a given clock > rate if all the rails are at a certain voltage? > More like an adjustable rate clock higher up the tree affects the rate of multiple devices downstream, and those devices happen to be on separate rails. Either way I think that the design should account for this possibility. Regards, Mike > > > + power rail constraints (eg voltage difference limit between 2 rails) > > > > This should come from regulator DT data and not anything DVFS-specific, > > correct? > > > > That's true. I think it can even be open-coded as this is a SoC internal > thing. All boards using this SoC will have the same limits, so I see little > reason to move this info to DT. > > > > + clock constraints (eg. clock x frequency must be a fixed ratio of clock y > > > frequency) > > > > Yeah, after sending my email above yesterday I instantly regretted it. > > It is true that *functional* clock dependencies are really the purview > > of the device driver. E.g. for Device X to operate at FAST_SPEED, scale > > functional_clk up to 200MHz and l3_ddr_clk up to 100MHz. On OMAP our > > display subsystem block also has clock ratio rules that must be honored, > > but it just open-coded. > > > > It is possible to model those in DT if we really want, but shouldn't be > > a priority for these dvfs-specific bindings. > > > > Agreed. > > Cheers, > > Peter.
WARNING: multiple messages have this Message-ID (diff)
From: mturquette@ti.com (Mike Turquette) To: linux-arm-kernel@lists.infradead.org Subject: Device tree binding for DVFS table Date: Fri, 13 Jul 2012 10:25:32 -0700 [thread overview] Message-ID: <20120713172532.GN2772@gmail.com> (raw) In-Reply-To: <20120713103408.GF9437@tbergstrom-lnx.Nvidia.com> On 20120713-13:34, Peter De Schrijver wrote: > Hi Mike, > > > > > Peter, > > > > I agree with your observations in general, but I think some specificity > > is needed: > > > > > + frequency/voltage relationships > > > > We should be clear that the voltage does NOT belong to the clock, but to > > the device/module/IP block that consumes that clock. This is an > > important detail since it means that a clock does not have a > > corresponding table of voltages (e.g. one table per clock), but instead > > a device has a table of voltages corresponding to each clock. > > > > Or the other way around, a table of clock frequencies, 1 for each voltage. > ACK You said what I meant to say. Some previous iterations of an omap-specific dvfs api tried to use voltage as the unique key for accessing frequency/voltage tables, and that can get really ugly for a variety of reasons. Plus device drivers typically request performance in Hertz (e.g. clk_set_rate), so exposing a voltage-centric interface to them is generally useless. The OPP library does some of this right (drivers/base/power/opp.c). It keeps a list of tables on a per-device bases, which is a big step in the right direction. Unfortunately it is lacking in other areas, such as not specifying which clock corresponds to the to table. Thus a device can only have one table and that table does not encode any information about which clock to use (which is important for devices that use a mux to change rate). The dvfs DT bindings should not repeat those mistakes. > > This is very necessary when a single clock drives multiple devices which > > are driven by separate voltage rails. > > > > Ah ok. How does this work in practice? A device can only run at a given clock > rate if all the rails are at a certain voltage? > More like an adjustable rate clock higher up the tree affects the rate of multiple devices downstream, and those devices happen to be on separate rails. Either way I think that the design should account for this possibility. Regards, Mike > > > + power rail constraints (eg voltage difference limit between 2 rails) > > > > This should come from regulator DT data and not anything DVFS-specific, > > correct? > > > > That's true. I think it can even be open-coded as this is a SoC internal > thing. All boards using this SoC will have the same limits, so I see little > reason to move this info to DT. > > > > + clock constraints (eg. clock x frequency must be a fixed ratio of clock y > > > frequency) > > > > Yeah, after sending my email above yesterday I instantly regretted it. > > It is true that *functional* clock dependencies are really the purview > > of the device driver. E.g. for Device X to operate at FAST_SPEED, scale > > functional_clk up to 200MHz and l3_ddr_clk up to 100MHz. On OMAP our > > display subsystem block also has clock ratio rules that must be honored, > > but it just open-coded. > > > > It is possible to model those in DT if we really want, but shouldn't be > > a priority for these dvfs-specific bindings. > > > > Agreed. > > Cheers, > > Peter.
next prev parent reply other threads:[~2012-07-13 17:25 UTC|newest] Thread overview: 60+ messages / expand[flat|nested] mbox.gz Atom feed top 2012-07-11 12:56 Device tree binding for DVFS table Prashant Gaikwad 2012-07-11 13:08 ` Prashant Gaikwad 2012-07-11 14:03 ` Rob Herring 2012-07-11 14:03 ` Rob Herring 2012-07-11 14:44 ` Mark Brown 2012-07-11 14:44 ` Mark Brown 2012-07-11 20:04 ` Mike Turquette 2012-07-11 20:04 ` Mike Turquette 2012-07-12 4:14 ` Prashant Gaikwad 2012-07-12 4:26 ` Prashant Gaikwad 2012-07-12 14:10 ` Peter De Schrijver 2012-07-12 14:10 ` Peter De Schrijver 2012-07-12 17:10 ` Mike Turquette 2012-07-12 17:10 ` Mike Turquette 2012-07-12 17:15 ` Mark Brown 2012-07-12 17:15 ` Mark Brown 2012-07-13 10:34 ` Peter De Schrijver 2012-07-13 10:34 ` Peter De Schrijver 2012-07-13 17:25 ` Mike Turquette [this message] 2012-07-13 17:25 ` Mike Turquette 2012-07-12 4:17 ` Prashant Gaikwad 2012-07-12 4:29 ` Prashant Gaikwad 2012-07-12 15:23 ` Mark Brown 2012-07-12 15:23 ` Mark Brown 2012-07-12 17:01 ` Mike Turquette 2012-07-12 17:01 ` Mike Turquette 2012-07-12 8:19 ` Peter De Schrijver 2012-07-12 8:19 ` Peter De Schrijver 2012-07-12 4:08 ` Prashant Gaikwad 2012-07-12 4:20 ` Prashant Gaikwad 2012-07-13 18:30 ` Prashant Gaikwad 2012-07-13 18:42 ` Prashant Gaikwad 2012-07-15 21:40 ` Mark Brown 2012-07-15 21:40 ` Mark Brown 2012-07-15 23:42 ` Rob Herring 2012-07-15 23:42 ` Rob Herring 2012-07-16 18:36 ` Turquette, Mike 2012-07-16 18:36 ` Turquette, Mike 2012-07-17 12:37 ` Prashant Gaikwad 2012-07-17 12:49 ` Prashant Gaikwad 2012-07-17 13:20 ` Mark Brown 2012-07-17 13:20 ` Mark Brown 2012-07-17 14:22 ` Prashant Gaikwad 2012-07-17 14:34 ` Prashant Gaikwad 2012-07-17 14:37 ` Mark Brown 2012-07-17 14:37 ` Mark Brown 2012-07-18 12:46 ` Prashant Gaikwad 2012-07-18 12:58 ` Prashant Gaikwad 2012-07-18 21:19 ` Mark Brown 2012-07-18 21:19 ` Mark Brown 2012-07-18 17:08 ` Shawn Guo 2012-07-18 17:08 ` Shawn Guo 2012-07-11 13:08 함명주 2012-07-11 13:08 ` 함명주 2012-07-11 13:49 ` Peter De Schrijver 2012-07-11 13:49 ` Peter De Schrijver 2012-07-11 13:49 ` Peter De Schrijver 2012-07-11 13:49 ` Peter De Schrijver 2012-07-12 4:22 ` Prashant Gaikwad 2012-07-12 4:34 ` Prashant Gaikwad
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