| Message ID | 1385556285-19180-3-git-send-email-b.brezillon@overkiz.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
Boris, Thanks for posting this series. Bear with me as I'm attempting to give MikeT a hand. Don't be afraid to tell me a question is stupid :-) On Wed, Nov 27, 2013 at 01:44:45PM +0100, Boris BREZILLON wrote: > This patch adds support for accuracy retrieval on fixed clocks. > It also adds a new dt property called 'clock-accuracy' to define the clock > accuracy. > > This can be usefull for oscillator (RC, crystal, ...) definitions which are > always given an accuracy characteristic. I think we need to be more explicit in the binding and the API, especially when providing a method to recalculate the accuracy. I presume this recalculated value would be relative against the root clock? There really needs to be two attributes here: the rated accuracy from the manufacturer, and the calculated accuracy wrt another clock in the system. We only need a binding for the manufacturer rating since the calculated accuracy is determined at runtime. I would also prefer to see an unknown accuracy be -1. There are already clocks on the market (PPS reference clocks) with accuracies of 0.1ppb/day [1]. Obviously, these aren't system clocks. So the limit on accuracy may be a non-issue. However, it may be worth changing the binding property to express the units. > Signed-off-by: Boris BREZILLON <b.brezillon@overkiz.com> > --- > .../devicetree/bindings/clock/fixed-clock.txt | 3 ++ > drivers/clk/clk-fixed-rate.c | 43 +++++++++++++++++--- > include/linux/clk-provider.h | 4 ++ > 3 files changed, 44 insertions(+), 6 deletions(-) > > diff --git a/Documentation/devicetree/bindings/clock/fixed-clock.txt b/Documentation/devicetree/bindings/clock/fixed-clock.txt > index 0b1fe78..48ea0ad 100644 > --- a/Documentation/devicetree/bindings/clock/fixed-clock.txt > +++ b/Documentation/devicetree/bindings/clock/fixed-clock.txt > @@ -10,6 +10,8 @@ Required properties: > - clock-frequency : frequency of clock in Hz. Should be a single cell. > > Optional properties: > +- clock-accuracy : accuracy of clock in ppb (parts per billion). > + Should be a single cell. I would prefer to call this property 'clock-rated-ppb'. > - gpios : From common gpio binding; gpio connection to clock enable pin. > - clock-output-names : From common clock binding. > > @@ -18,4 +20,5 @@ Example: > compatible = "fixed-clock"; > #clock-cells = <0>; > clock-frequency = <1000000000>; > + clock-accuracy = <100>; > }; thx, Jason. [1] http://www.vectron.com/products/modules/md-010.htm
Hi Jason, On 27/11/2013 15:56, Jason Cooper wrote: > Boris, > > Thanks for posting this series. Bear with me as I'm attempting to give > MikeT a hand. Nice to hear. Mike already took a look at this series, but I'm happy to get more feedbacks. > Don't be afraid to tell me a question is stupid :-) Your questions are far from stupid ;-). > > On Wed, Nov 27, 2013 at 01:44:45PM +0100, Boris BREZILLON wrote: >> This patch adds support for accuracy retrieval on fixed clocks. >> It also adds a new dt property called 'clock-accuracy' to define the clock >> accuracy. >> >> This can be usefull for oscillator (RC, crystal, ...) definitions which are >> always given an accuracy characteristic. > I think we need to be more explicit in the binding and the API, > especially when providing a method to recalculate the accuracy. I > presume this recalculated value would be relative against the root > clock? Yes, indirectly. Actually the clk accuracy depends on the whole clock chain, and is calculated either by comparing the real clk rate to the theorical clk rate (accuracy = absolute_value((theorical_clk_rate - real_clk_rate)) / theorical_clk_rate), or by adding all the accuracies (expressed in ppm, ppb or ppt) of the clk chain (accuracy = current_clk_accuracy + parent_clk_accuracy). Say you have a root clk with a +-10000 ppb accuracy, then a pll multiplying this root clk by 40 and introducing a possible drift of +- 1000 ppb and eventually a system clk based on this pll with a perfect div by 2 prescaler (accuracy = 0 ppb). If I understand correctly how accuracy propagates accross the clk tree, you'll end up with a system clk with a +- 11000 ppb accuracy. e.g.: root clk = 12MHz +- 10000 ppb => 12 MHz * (1 - (10000 / 10^9)) < root clk < 12 MHz * (1 + (10000 / 10^9)) => 11,99988 MHz < root clk < 12,00012 MHz pll clk = ((root clk) * 40) +- 1000 ppb => (11,99988 MHz * 40) * (1 - (1000 / 10^9)) < pll clk < (12,00012 MHz * 40) * (1 + (1000 / 10^9)) => 479,994720005 MHz < pll clk < 480,005280005 MHz system clk = ((pll clk) / 2) +- XXX ppb => 479,994720005 MHz / 2 < system clk < 480,005280005 MHz / 2 => 239,997360002 MHz < system clk < 240,002640002 MHz => system clk accuracy = 0,002640002 / 240 = 11000 ppb Please tell me if my assumptions are false. > > There really needs to be two attributes here: the rated accuracy from > the manufacturer, and the calculated accuracy wrt another clock in the > system. We only need a binding for the manufacturer rating since the > calculated accuracy is determined at runtime. Actually when I proposed this new functionnality I only had the theorical (or manufacturer rated) accuracy in mind. But providing an estimated accuracy (based on another clk) sounds interresting if your reference clk is an extremly accurate one. > > I would also prefer to see an unknown accuracy be -1. I decided to keep 0 as a default value for unimplemented recalc_accuracy (or unspecified fixed accuracy) to keep existing implementation coherent. 0 means the clk is perfect, and I thought it would be easier to handle a perfect clk (even if this is not really the case) than handling an error case. Another aspect is the propagation of the clk accuracy accross the clk tree. Returning -1 in the middle of the clk chain will drop the previous clk accuracy calculation. Anyway, I can change this if you think this is more appropriate. > There are already > clocks on the market (PPS reference clocks) with accuracies of > 0.1ppb/day [1]. Obviously, these aren't system clocks. So the limit on > accuracy may be a non-issue. However, it may be worth changing the > binding property to express the units. Wow, 0.1 ppb, this is impressive :-). This needs more than changing the dt bindings: I currently store the accuracy value in an unsigned long field, and expressing this in ppt (parts per trillion) may implies storing this in an u64 field (or store a unit field). > >> Signed-off-by: Boris BREZILLON <b.brezillon@overkiz.com> >> --- >> .../devicetree/bindings/clock/fixed-clock.txt | 3 ++ >> drivers/clk/clk-fixed-rate.c | 43 +++++++++++++++++--- >> include/linux/clk-provider.h | 4 ++ >> 3 files changed, 44 insertions(+), 6 deletions(-) >> >> diff --git a/Documentation/devicetree/bindings/clock/fixed-clock.txt b/Documentation/devicetree/bindings/clock/fixed-clock.txt >> index 0b1fe78..48ea0ad 100644 >> --- a/Documentation/devicetree/bindings/clock/fixed-clock.txt >> +++ b/Documentation/devicetree/bindings/clock/fixed-clock.txt >> @@ -10,6 +10,8 @@ Required properties: >> - clock-frequency : frequency of clock in Hz. Should be a single cell. >> >> Optional properties: >> +- clock-accuracy : accuracy of clock in ppb (parts per billion). >> + Should be a single cell. > I would prefer to call this property 'clock-rated-ppb'. Depending on what we choose to do with the accuracy field, this might be an option. > >> - gpios : From common gpio binding; gpio connection to clock enable pin. >> - clock-output-names : From common clock binding. >> >> @@ -18,4 +20,5 @@ Example: >> compatible = "fixed-clock"; >> #clock-cells = <0>; >> clock-frequency = <1000000000>; >> + clock-accuracy = <100>; >> }; > thx, > > Jason. > > [1] http://www.vectron.com/products/modules/md-010.htm Thanks for your review, and don't hesitate to ask more questions, or to point out incoherencies in my approach (I'm not an expert in clk and clk accuracy calculation, and I might be wrong ;-)). Best Regards, Boris
Quoting boris brezillon (2013-11-27 09:19:08) > Hi Jason, > > On 27/11/2013 15:56, Jason Cooper wrote: > > Boris, > > > > Thanks for posting this series. Bear with me as I'm attempting to give > > MikeT a hand. > Nice to hear. > Mike already took a look at this series, but I'm happy to get more > feedbacks. > > > Don't be afraid to tell me a question is stupid :-) > Your questions are far from stupid ;-). > > > > > On Wed, Nov 27, 2013 at 01:44:45PM +0100, Boris BREZILLON wrote: > >> This patch adds support for accuracy retrieval on fixed clocks. > >> It also adds a new dt property called 'clock-accuracy' to define the clock > >> accuracy. > >> > >> This can be usefull for oscillator (RC, crystal, ...) definitions which are > >> always given an accuracy characteristic. > > I think we need to be more explicit in the binding and the API, > > especially when providing a method to recalculate the accuracy. I > > presume this recalculated value would be relative against the root > > clock? > > Yes, indirectly. > Actually the clk accuracy depends on the whole clock chain, and is > calculated > either by comparing the real clk rate to the theorical clk rate > (accuracy = absolute_value((theorical_clk_rate - real_clk_rate)) / > theorical_clk_rate), > or by adding all the accuracies (expressed in ppm, ppb or ppt) of the > clk chain > (accuracy = current_clk_accuracy + parent_clk_accuracy). > > Say you have a root clk with a +-10000 ppb accuracy, then a pll multiplying > this root clk by 40 and introducing a possible drift of +- 1000 ppb and > eventually a system clk based on this pll with a perfect div by 2 prescaler > (accuracy = 0 ppb). > > If I understand correctly how accuracy propagates accross the clk tree, > you'll end up with a system clk with a +- 11000 ppb accuracy. > > e.g.: > root clk = 12MHz +- 10000 ppb => 12 MHz * (1 - (10000 / 10^9)) < root > clk < 12 MHz * (1 + (10000 / 10^9)) > => 11,99988 MHz < > root clk < 12,00012 MHz > pll clk = ((root clk) * 40) +- 1000 ppb => (11,99988 MHz * 40) * (1 - > (1000 / 10^9)) < pll clk < (12,00012 MHz * 40) * (1 + (1000 / 10^9)) > => > 479,994720005 MHz < pll clk < 480,005280005 MHz > > system clk = ((pll clk) / 2) +- XXX ppb => 479,994720005 MHz / 2 < > system clk < 480,005280005 MHz / 2 > => > 239,997360002 MHz < system clk < 240,002640002 MHz > => system > clk accuracy = 0,002640002 / 240 = 11000 ppb > > Please tell me if my assumptions are false. > > > > There really needs to be two attributes here: the rated accuracy from > > the manufacturer, and the calculated accuracy wrt another clock in the > > system. We only need a binding for the manufacturer rating since the > > calculated accuracy is determined at runtime. > > Actually when I proposed this new functionnality I only had the theorical > (or manufacturer rated) accuracy in mind. > But providing an estimated accuracy (based on another clk) sounds > interresting if your reference clk is an extremly accurate one. Is there a need to model clock accuracy across the clock chain? I'm OK modeling it in DT, and the code to do it in the clk framework isn't very much ... but I also wonder if we're just adding complexity for no reason. > > > > > I would also prefer to see an unknown accuracy be -1. > I decided to keep 0 as a default value for unimplemented recalc_accuracy > (or unspecified fixed accuracy) to keep existing implementation coherent. > > 0 means the clk is perfect, and I thought it would be easier to handle a > perfect clk (even if this is not really the case) than handling an error > case. > > Another aspect is the propagation of the clk accuracy accross the clk tree. > Returning -1 in the middle of the clk chain will drop the previous clk > accuracy > calculation. > > Anyway, I can change this if you think this is more appropriate. What about the absence of the property? Instead of requiring a -1 value can we simply detect that the property does not exist? This is nicer for backwards compatibility with existing DTS. Regards, Mike > > > There are already > > clocks on the market (PPS reference clocks) with accuracies of > > 0.1ppb/day [1]. Obviously, these aren't system clocks. So the limit on > > accuracy may be a non-issue. However, it may be worth changing the > > binding property to express the units. > Wow, 0.1 ppb, this is impressive :-). > > > This needs more than changing the dt bindings: I currently store the > accuracy value in an unsigned long field, and expressing this in ppt > (parts per trillion) may implies storing this in an u64 field (or store a > unit field). > > > > > >> Signed-off-by: Boris BREZILLON <b.brezillon@overkiz.com> > >> --- > >> .../devicetree/bindings/clock/fixed-clock.txt | 3 ++ > >> drivers/clk/clk-fixed-rate.c | 43 +++++++++++++++++--- > >> include/linux/clk-provider.h | 4 ++ > >> 3 files changed, 44 insertions(+), 6 deletions(-) > >> > >> diff --git a/Documentation/devicetree/bindings/clock/fixed-clock.txt b/Documentation/devicetree/bindings/clock/fixed-clock.txt > >> index 0b1fe78..48ea0ad 100644 > >> --- a/Documentation/devicetree/bindings/clock/fixed-clock.txt > >> +++ b/Documentation/devicetree/bindings/clock/fixed-clock.txt > >> @@ -10,6 +10,8 @@ Required properties: > >> - clock-frequency : frequency of clock in Hz. Should be a single cell. > >> > >> Optional properties: > >> +- clock-accuracy : accuracy of clock in ppb (parts per billion). > >> + Should be a single cell. > > I would prefer to call this property 'clock-rated-ppb'. > > Depending on what we choose to do with the accuracy field, this might be > an option. > > > > >> - gpios : From common gpio binding; gpio connection to clock enable pin. > >> - clock-output-names : From common clock binding. > >> > >> @@ -18,4 +20,5 @@ Example: > >> compatible = "fixed-clock"; > >> #clock-cells = <0>; > >> clock-frequency = <1000000000>; > >> + clock-accuracy = <100>; > >> }; > > thx, > > > > Jason. > > > > [1] http://www.vectron.com/products/modules/md-010.htm > > Thanks for your review, and don't hesitate to ask more questions, or to > point out > incoherencies in my approach (I'm not an expert in clk and clk accuracy > calculation, > and I might be wrong ;-)). > > Best Regards, > > Boris
On 27/11/2013 19:10, Mike Turquette wrote: > Quoting boris brezillon (2013-11-27 09:19:08) >> Hi Jason, >> >> On 27/11/2013 15:56, Jason Cooper wrote: >>> Boris, >>> >>> Thanks for posting this series. Bear with me as I'm attempting to give >>> MikeT a hand. >> Nice to hear. >> Mike already took a look at this series, but I'm happy to get more >> feedbacks. >> >>> Don't be afraid to tell me a question is stupid :-) >> Your questions are far from stupid ;-). >> >>> On Wed, Nov 27, 2013 at 01:44:45PM +0100, Boris BREZILLON wrote: >>>> This patch adds support for accuracy retrieval on fixed clocks. >>>> It also adds a new dt property called 'clock-accuracy' to define the clock >>>> accuracy. >>>> >>>> This can be usefull for oscillator (RC, crystal, ...) definitions which are >>>> always given an accuracy characteristic. >>> I think we need to be more explicit in the binding and the API, >>> especially when providing a method to recalculate the accuracy. I >>> presume this recalculated value would be relative against the root >>> clock? >> Yes, indirectly. >> Actually the clk accuracy depends on the whole clock chain, and is >> calculated >> either by comparing the real clk rate to the theorical clk rate >> (accuracy = absolute_value((theorical_clk_rate - real_clk_rate)) / >> theorical_clk_rate), >> or by adding all the accuracies (expressed in ppm, ppb or ppt) of the >> clk chain >> (accuracy = current_clk_accuracy + parent_clk_accuracy). >> >> Say you have a root clk with a +-10000 ppb accuracy, then a pll multiplying >> this root clk by 40 and introducing a possible drift of +- 1000 ppb and >> eventually a system clk based on this pll with a perfect div by 2 prescaler >> (accuracy = 0 ppb). >> >> If I understand correctly how accuracy propagates accross the clk tree, >> you'll end up with a system clk with a +- 11000 ppb accuracy. >> >> e.g.: >> root clk = 12MHz +- 10000 ppb => 12 MHz * (1 - (10000 / 10^9)) < root >> clk < 12 MHz * (1 + (10000 / 10^9)) >> => 11,99988 MHz < >> root clk < 12,00012 MHz >> pll clk = ((root clk) * 40) +- 1000 ppb => (11,99988 MHz * 40) * (1 - >> (1000 / 10^9)) < pll clk < (12,00012 MHz * 40) * (1 + (1000 / 10^9)) >> => >> 479,994720005 MHz < pll clk < 480,005280005 MHz >> >> system clk = ((pll clk) / 2) +- XXX ppb => 479,994720005 MHz / 2 < >> system clk < 480,005280005 MHz / 2 >> => >> 239,997360002 MHz < system clk < 240,002640002 MHz >> => system >> clk accuracy = 0,002640002 / 240 = 11000 ppb >> >> Please tell me if my assumptions are false. >>> There really needs to be two attributes here: the rated accuracy from >>> the manufacturer, and the calculated accuracy wrt another clock in the >>> system. We only need a binding for the manufacturer rating since the >>> calculated accuracy is determined at runtime. >> Actually when I proposed this new functionnality I only had the theorical >> (or manufacturer rated) accuracy in mind. >> But providing an estimated accuracy (based on another clk) sounds >> interresting if your reference clk is an extremly accurate one. > Is there a need to model clock accuracy across the clock chain? > I'm OK > modeling it in DT, and the code to do it in the clk framework isn't very > much ... but I also wonder if we're just adding complexity for no > reason. AFAIK the most important node in the clock chain (regarding accuracy) is the root node. But some nodes (like PLLs) might introduce more innacuracy. This series propose a simple way (or at least tries to keep it simple :-)) to express accuracy over the whole clk chain by means of the recalc_accuracy. I'm not sure keeping the accuracy calculation (or retrieval) in the root clk node only will simplify the calculation (or retrieval) of a leaf clk node accuracy (you'd still have to walk over the clock chain to get the root clk accuracy). My primary goal with this series is to provide a simple way (for a clock user) to choose the most accurate clock among several available clocks. This is a real need on AT91 platforms which provides internal RC oscillators with a really poor accuracy (+- 5% <=> +- 50000 ppm). > >>> I would also prefer to see an unknown accuracy be -1. >> I decided to keep 0 as a default value for unimplemented recalc_accuracy >> (or unspecified fixed accuracy) to keep existing implementation coherent. >> >> 0 means the clk is perfect, and I thought it would be easier to handle a >> perfect clk (even if this is not really the case) than handling an error >> case. >> >> Another aspect is the propagation of the clk accuracy accross the clk tree. >> Returning -1 in the middle of the clk chain will drop the previous clk >> accuracy >> calculation. >> >> Anyway, I can change this if you think this is more appropriate. > What about the absence of the property? > Instead of requiring a -1 value > can we simply detect that the property does not exist? This is nicer for > backwards compatibility with existing DTS. I already handle the absence of the clock-accuracy property. In this case the clock is considered perfect (accuracy = 0 ppb). Mike, do you want me to return an error in the recalc_accuracy callback to notifiy the absence of the clock-accuracy property ? > > Regards, > Mike > >>> There are already >>> clocks on the market (PPS reference clocks) with accuracies of >>> 0.1ppb/day [1]. Obviously, these aren't system clocks. So the limit on >>> accuracy may be a non-issue. However, it may be worth changing the >>> binding property to express the units. >> Wow, 0.1 ppb, this is impressive :-). >> >> >> This needs more than changing the dt bindings: I currently store the >> accuracy value in an unsigned long field, and expressing this in ppt >> (parts per trillion) may implies storing this in an u64 field (or store a >> unit field). >> >> >>>> Signed-off-by: Boris BREZILLON <b.brezillon@overkiz.com> >>>> --- >>>> .../devicetree/bindings/clock/fixed-clock.txt | 3 ++ >>>> drivers/clk/clk-fixed-rate.c | 43 +++++++++++++++++--- >>>> include/linux/clk-provider.h | 4 ++ >>>> 3 files changed, 44 insertions(+), 6 deletions(-) >>>> >>>> diff --git a/Documentation/devicetree/bindings/clock/fixed-clock.txt b/Documentation/devicetree/bindings/clock/fixed-clock.txt >>>> index 0b1fe78..48ea0ad 100644 >>>> --- a/Documentation/devicetree/bindings/clock/fixed-clock.txt >>>> +++ b/Documentation/devicetree/bindings/clock/fixed-clock.txt >>>> @@ -10,6 +10,8 @@ Required properties: >>>> - clock-frequency : frequency of clock in Hz. Should be a single cell. >>>> >>>> Optional properties: >>>> +- clock-accuracy : accuracy of clock in ppb (parts per billion). >>>> + Should be a single cell. >>> I would prefer to call this property 'clock-rated-ppb'. >> Depending on what we choose to do with the accuracy field, this might be >> an option. >> >>>> - gpios : From common gpio binding; gpio connection to clock enable pin. >>>> - clock-output-names : From common clock binding. >>>> >>>> @@ -18,4 +20,5 @@ Example: >>>> compatible = "fixed-clock"; >>>> #clock-cells = <0>; >>>> clock-frequency = <1000000000>; >>>> + clock-accuracy = <100>; >>>> }; >>> thx, >>> >>> Jason. >>> >>> [1] http://www.vectron.com/products/modules/md-010.htm >> Thanks for your review, and don't hesitate to ask more questions, or to >> point out >> incoherencies in my approach (I'm not an expert in clk and clk accuracy >> calculation, >> and I might be wrong ;-)). >> >> Best Regards, >> >> Boris
Boris, Sorry for the delay. On Wed, Nov 27, 2013 at 06:19:08PM +0100, boris brezillon wrote: > On 27/11/2013 15:56, Jason Cooper wrote: > >On Wed, Nov 27, 2013 at 01:44:45PM +0100, Boris BREZILLON wrote: > >>This patch adds support for accuracy retrieval on fixed clocks. > >>It also adds a new dt property called 'clock-accuracy' to define the clock > >>accuracy. > >> > >>This can be usefull for oscillator (RC, crystal, ...) definitions which are > >>always given an accuracy characteristic. > > > >I think we need to be more explicit in the binding and the API, > >especially when providing a method to recalculate the accuracy. I > >presume this recalculated value would be relative against the root > >clock? > > Yes, indirectly. > Actually the clk accuracy depends on the whole clock chain, and is > calculated either by comparing the real clk rate to the theorical clk > rate > (accuracy = absolute_value((theorical_clk_rate - real_clk_rate)) / > theorical_clk_rate), > or by adding all the accuracies (expressed in ppm, ppb or ppt) of > the clk chain > (accuracy = current_clk_accuracy + parent_clk_accuracy). > > Say you have a root clk with a +-10000 ppb accuracy, then a pll multiplying > this root clk by 40 and introducing a possible drift of +- 1000 ppb and > eventually a system clk based on this pll with a perfect div by 2 prescaler > (accuracy = 0 ppb). > > If I understand correctly how accuracy propagates accross the clk tree, > you'll end up with a system clk with a +- 11000 ppb accuracy. > > e.g.: > root clk = 12MHz +- 10000 ppb => 12 MHz * (1 - (10000 / 10^9)) < > root clk < 12 MHz * (1 + (10000 / 10^9)) > => 11,99988 MHz > < root clk < 12,00012 MHz > pll clk = ((root clk) * 40) +- 1000 ppb => (11,99988 MHz * 40) * > (1 - (1000 / 10^9)) < pll clk < (12,00012 MHz * 40) * (1 + (1000 / > 10^9)) > => > 479,994720005 MHz < pll clk < 480,005280005 MHz > > system clk = ((pll clk) / 2) +- XXX ppb => 479,994720005 MHz / 2 < > system clk < 480,005280005 MHz / 2 > => > 239,997360002 MHz < system clk < 240,002640002 MHz > => > system clk accuracy = 0,002640002 / 240 = 11000 ppb > > Please tell me if my assumptions are false. Nope, it looks fine by me afaict. Thanks for the clear walk through. > >There really needs to be two attributes here: the rated accuracy from > >the manufacturer, and the calculated accuracy wrt another clock in the > >system. We only need a binding for the manufacturer rating since the > >calculated accuracy is determined at runtime. > > Actually when I proposed this new functionnality I only had the theorical > (or manufacturer rated) accuracy in mind. Yes, I see we are concerned about two different things. You need to get the theoretical accuracy to assist with clock selection. I was concerned that the recalc function was attempting to measure the real accuracy of a given clock from a tree. Since we're only talking theoretical accuracy, that makes things a lot simpler. :) > But providing an estimated accuracy (based on another clk) sounds > interresting if your reference clk is an extremly accurate one. Yes, I was thinking against a GPS PPS signal, but again, not relevant to this patch series. Also, it would be complicated by the fact that there is no high-speed counter on ARM. > >I would also prefer to see an unknown accuracy be -1. > I decided to keep 0 as a default value for unimplemented recalc_accuracy > (or unspecified fixed accuracy) to keep existing implementation coherent. > > 0 means the clk is perfect, and I thought it would be easier to handle a > perfect clk (even if this is not really the case) than handling an > error case. > > Another aspect is the propagation of the clk accuracy accross the clk tree. > Returning -1 in the middle of the clk chain will drop the previous > clk accuracy > calculation. > > Anyway, I can change this if you think this is more appropriate. No, in light of this being purely theoretical accuracy, I'm fine with it if Mike is. > >There are already > >clocks on the market (PPS reference clocks) with accuracies of > >0.1ppb/day [1]. Obviously, these aren't system clocks. So the limit on > >accuracy may be a non-issue. However, it may be worth changing the > >binding property to express the units. > Wow, 0.1 ppb, this is impressive :-). > > > This needs more than changing the dt bindings: I currently store the > accuracy value in an unsigned long field, and expressing this in ppt > (parts per trillion) may implies storing this in an u64 field (or store a > unit field). No, let's not derail this series. ;-) You've addressed my concerns. Thanks for taking the time to bring me up to speed. thx, Jason.
On Thu, Nov 28, 2013 at 09:02:58AM +0100, boris brezillon wrote: > On 27/11/2013 19:10, Mike Turquette wrote: > >Quoting boris brezillon (2013-11-27 09:19:08) > >>>On Wed, Nov 27, 2013 at 01:44:45PM +0100, Boris BREZILLON wrote: ... > >>>I would also prefer to see an unknown accuracy be -1. > >>I decided to keep 0 as a default value for unimplemented recalc_accuracy > >>(or unspecified fixed accuracy) to keep existing implementation coherent. > >> > >>0 means the clk is perfect, and I thought it would be easier to handle a > >>perfect clk (even if this is not really the case) than handling an error > >>case. > >> > >>Another aspect is the propagation of the clk accuracy accross the clk tree. > >>Returning -1 in the middle of the clk chain will drop the previous clk > >>accuracy > >>calculation. > >> > >>Anyway, I can change this if you think this is more appropriate. > >What about the absence of the property? > >Instead of requiring a -1 value > >can we simply detect that the property does not exist? This is nicer for > >backwards compatibility with existing DTS. > > I already handle the absence of the clock-accuracy property. > In this case the clock is considered perfect (accuracy = 0 ppb). Yeah, in order to calculate the theoretical accuracy of a leaf node, a missing accuracy in the middle of the chain really derails things. Since my previous concern (modelling the real accuracy of the clocks) is not an issue here, I think assuming the absent accuracy is 0 is fine. Especially since all Boris is trying to do is avoid the RC clocks. thx, Jason.
Quoting Jason Cooper (2013-12-01 19:15:58) > On Thu, Nov 28, 2013 at 09:02:58AM +0100, boris brezillon wrote: > > On 27/11/2013 19:10, Mike Turquette wrote: > > >Quoting boris brezillon (2013-11-27 09:19:08) > > >>>On Wed, Nov 27, 2013 at 01:44:45PM +0100, Boris BREZILLON wrote: > ... > > >>>I would also prefer to see an unknown accuracy be -1. > > >>I decided to keep 0 as a default value for unimplemented recalc_accuracy > > >>(or unspecified fixed accuracy) to keep existing implementation coherent. > > >> > > >>0 means the clk is perfect, and I thought it would be easier to handle a > > >>perfect clk (even if this is not really the case) than handling an error > > >>case. > > >> > > >>Another aspect is the propagation of the clk accuracy accross the clk tree. > > >>Returning -1 in the middle of the clk chain will drop the previous clk > > >>accuracy > > >>calculation. > > >> > > >>Anyway, I can change this if you think this is more appropriate. > > >What about the absence of the property? > > >Instead of requiring a -1 value > > >can we simply detect that the property does not exist? This is nicer for > > >backwards compatibility with existing DTS. > > > > I already handle the absence of the clock-accuracy property. > > In this case the clock is considered perfect (accuracy = 0 ppb). > > Yeah, in order to calculate the theoretical accuracy of a leaf node, a > missing accuracy in the middle of the chain really derails things. > Since my previous concern (modelling the real accuracy of the clocks) is > not an issue here, I think assuming the absent accuracy is 0 is fine. > Especially since all Boris is trying to do is avoid the RC clocks. Agreed. If accuracy data must exist along the entire chain for the calculation to be successful then the feature will be useless due to lack of adoption of the accuracy data. Regards, Mike > > thx, > > Jason.
diff --git a/Documentation/devicetree/bindings/clock/fixed-clock.txt b/Documentation/devicetree/bindings/clock/fixed-clock.txt index 0b1fe78..48ea0ad 100644 --- a/Documentation/devicetree/bindings/clock/fixed-clock.txt +++ b/Documentation/devicetree/bindings/clock/fixed-clock.txt @@ -10,6 +10,8 @@ Required properties: - clock-frequency : frequency of clock in Hz. Should be a single cell. Optional properties: +- clock-accuracy : accuracy of clock in ppb (parts per billion). + Should be a single cell. - gpios : From common gpio binding; gpio connection to clock enable pin. - clock-output-names : From common clock binding. @@ -18,4 +20,5 @@ Example: compatible = "fixed-clock"; #clock-cells = <0>; clock-frequency = <1000000000>; + clock-accuracy = <100>; }; diff --git a/drivers/clk/clk-fixed-rate.c b/drivers/clk/clk-fixed-rate.c index 1ed591a..0fc56ab 100644 --- a/drivers/clk/clk-fixed-rate.c +++ b/drivers/clk/clk-fixed-rate.c @@ -34,22 +34,31 @@ static unsigned long clk_fixed_rate_recalc_rate(struct clk_hw *hw, return to_clk_fixed_rate(hw)->fixed_rate; } +static unsigned long clk_fixed_rate_recalc_accuracy(struct clk_hw *hw, + unsigned long parent_accuracy) +{ + return to_clk_fixed_rate(hw)->fixed_accuracy; +} + const struct clk_ops clk_fixed_rate_ops = { .recalc_rate = clk_fixed_rate_recalc_rate, + .recalc_accuracy = clk_fixed_rate_recalc_accuracy, }; EXPORT_SYMBOL_GPL(clk_fixed_rate_ops); /** - * clk_register_fixed_rate - register fixed-rate clock with the clock framework + * clk_register_fixed_rate_with_accuracy - register fixed-rate clock with the + * clock framework * @dev: device that is registering this clock * @name: name of this clock * @parent_name: name of clock's parent * @flags: framework-specific flags * @fixed_rate: non-adjustable clock rate + * @fixed_accuracy: non-adjustable clock rate */ -struct clk *clk_register_fixed_rate(struct device *dev, const char *name, - const char *parent_name, unsigned long flags, - unsigned long fixed_rate) +struct clk *clk_register_fixed_rate_with_accuracy(struct device *dev, + const char *name, const char *parent_name, unsigned long flags, + unsigned long fixed_rate, unsigned long fixed_accuracy) { struct clk_fixed_rate *fixed; struct clk *clk; @@ -70,16 +79,33 @@ struct clk *clk_register_fixed_rate(struct device *dev, const char *name, /* struct clk_fixed_rate assignments */ fixed->fixed_rate = fixed_rate; + fixed->fixed_accuracy = fixed_accuracy; fixed->hw.init = &init; /* register the clock */ clk = clk_register(dev, &fixed->hw); - if (IS_ERR(clk)) kfree(fixed); return clk; } +EXPORT_SYMBOL_GPL(clk_register_fixed_rate_with_accuracy); + +/** + * clk_register_fixed_rate - register fixed-rate clock with the clock framework + * @dev: device that is registering this clock + * @name: name of this clock + * @parent_name: name of clock's parent + * @flags: framework-specific flags + * @fixed_rate: non-adjustable clock rate + */ +struct clk *clk_register_fixed_rate(struct device *dev, const char *name, + const char *parent_name, unsigned long flags, + unsigned long fixed_rate) +{ + return clk_register_fixed_rate_with_accuracy(dev, name, parent_name, + flags, fixed_rate, 0); +} EXPORT_SYMBOL_GPL(clk_register_fixed_rate); #ifdef CONFIG_OF @@ -91,13 +117,18 @@ void of_fixed_clk_setup(struct device_node *node) struct clk *clk; const char *clk_name = node->name; u32 rate; + u32 accuracy = 0; if (of_property_read_u32(node, "clock-frequency", &rate)) return; + of_property_read_u32(node, "clock-accuracy", &accuracy); + of_property_read_string(node, "clock-output-names", &clk_name); - clk = clk_register_fixed_rate(NULL, clk_name, NULL, CLK_IS_ROOT, rate); + clk = clk_register_fixed_rate_with_accuracy(NULL, clk_name, NULL, + CLK_IS_ROOT, rate, + accuracy); if (!IS_ERR(clk)) of_clk_add_provider(node, of_clk_src_simple_get, clk); } diff --git a/include/linux/clk-provider.h b/include/linux/clk-provider.h index 16d182c..5429f5d 100644 --- a/include/linux/clk-provider.h +++ b/include/linux/clk-provider.h @@ -204,6 +204,7 @@ struct clk_hw { struct clk_fixed_rate { struct clk_hw hw; unsigned long fixed_rate; + unsigned long fixed_accuracy; u8 flags; }; @@ -211,6 +212,9 @@ extern const struct clk_ops clk_fixed_rate_ops; struct clk *clk_register_fixed_rate(struct device *dev, const char *name, const char *parent_name, unsigned long flags, unsigned long fixed_rate); +struct clk *clk_register_fixed_rate_with_accuracy(struct device *dev, + const char *name, const char *parent_name, unsigned long flags, + unsigned long fixed_rate, unsigned long fixed_accuracy); void of_fixed_clk_setup(struct device_node *np);
This patch adds support for accuracy retrieval on fixed clocks. It also adds a new dt property called 'clock-accuracy' to define the clock accuracy. This can be usefull for oscillator (RC, crystal, ...) definitions which are always given an accuracy characteristic. Signed-off-by: Boris BREZILLON <b.brezillon@overkiz.com> --- .../devicetree/bindings/clock/fixed-clock.txt | 3 ++ drivers/clk/clk-fixed-rate.c | 43 +++++++++++++++++--- include/linux/clk-provider.h | 4 ++ 3 files changed, 44 insertions(+), 6 deletions(-)