dt-bindings: hwmon: pwm-fan: Document default-pwm property
Subsystems (2)
PATCH v4 1/2 dt-bindings: hwmon: pwm-fan: Document default-pwm property
The pwm-fan driver uses full PWM (255) duty cycle at startup, which may not always be desirable because of noise or power consumption peaks, so add an optional "default-pwm" property that can be used to specify a custom default PWM duty cycle (0..255).
This is somewhat similar to target-rpm from fan-common.yaml, but that cannot be used here as target-rpm specifies the target fan speed, whereas this is the default pwm to set when the device is instantiated - And the resulting fan RPM resulting from a given PWM duty cycle is fan dependent.
---
Changes since v3:
- Fix example syntax
- Extend description of why target-rpm cannot be used
Changes since v2:
- Recreated/resent
Changes since v1:
- Rename to default-pwm
Documentation/devicetree/bindings/hwmon/pwm-fan.yaml | 8 ++++++++ 1 file changed, 8 insertions(+)
Documentation/devicetree/bindings/hwmon/pwm-fan.yaml +8 -0
@@ -20,6 +20,13 @@ properties: items: maximum: 255 + default-pwm: + description: Default PWM duty cycle value to use at startup + $ref: /schemas/types.yaml#/definitions/uint32 + minimum: 0 + maximum: 255 + default: 255 + fan-supply: description: Phandle to the regulator that provides power to the fan.
@@ -100,6 +107,7 @@ examples: pwm-fan { compatible = "pwm-fan"; pwms = <&pwm 0 40000 0>; + default-pwm = <75>; fan-supply = <®_fan>; interrupt-parent = <&gpio5>; interrupts = <1 IRQ_TYPE_EDGE_FALLING>;
Signature · 1 line
2.39.5
PATCH v4 2/2 hwmon: (pwm-fan): Make default PWM duty cycle configurable
The pwm-fan driver uses full PWM (255) duty cycle at startup, which may not always be desirable because of noise or power consumption peaks, so support an optional "default-pwm" property (0..255) that can be used to specify the default PWM duty cycle.
---
Changes since v3:
- Readded validation code from v2, reworded commit message
Changes since v2:
- Recreated/resent
Changes since v1:
- Rename property to default-pwm
- Drop u32 cast
drivers/hwmon/pwm-fan.c | 11 +++++++++-- 1 file changed, 9 insertions(+), 2 deletions(-)
drivers/hwmon/pwm-fan.c +9 -2
@@ -499,6 +499,7 @@ static int pwm_fan_probe(struct platform_device *pdev)const struct hwmon_channel_info **channels;u32 pwm_min_from_stopped =0 ;u32 *fan_channel_config; +u32 default_pwm =MAX_PWM ;int channel_count =1 ;/* We always have a PWM channel. */ int i;
@@ -545,11 +546,17 @@ static int pwm_fan_probe(struct platform_device *pdev) ctx->enable_mode = pwm_disable_reg_enable; +device_property_read_u32 (dev,"default-pwm" , &default_pwm); +if (default_pwm >MAX_PWM ) { +dev_err (dev,"Invalid default-pwm: %u > %d\n" , default_pwm,MAX_PWM ); +return -EINVAL ; + } +/* -* Set duty cycle to maximum allowed and enable PWM output as well as +* Set duty cycle to default and enable PWM output as well as * the regulator. In case of error nothing is changed */ - ret =set_pwm (ctx,MAX_PWM ); + ret =set_pwm (ctx, default_pwm);if (ret) {dev_err (dev,"Failed to configure PWM: %d\n" , ret);return ret;
Signature · 1 line
2.39.5
Rob Herring
maintainerQuoted 9 lines · Peter Korsgaard
The pwm-fan driver uses full PWM (255) duty cycle at startup, which may not always be desirable because of noise or power consumption peaks, so add an optional "default-pwm" property that can be used to specify a custom default PWM duty cycle (0..255).
This is somewhat similar to target-rpm from
fan-common.yaml, but that cannot be used here as target-rpm specifies the target fan speed, whereas this is the default pwm to set when the device is instantiated - And the resulting fan RPM resulting from a given PWM duty cycle is fan dependent.
I still don't agree. Quoting Guenter:
Quoted 4 lines
The two values are also orthogonal. The fan rpm is fan dependent. Each fan will require a different pwm value to reach the target speed. Trying to use target-rpm to set a default pwm value would really not make much if any sense.
But RPM is ultimately what you care about and is the fan parameter that's universal yet independent of the underlying control. RPM is what determines noise and power consumption.
There's 2 cases to consider: you have a tach signal and know the fan RPM or you don't know the RPM. If you have a tach signal, we probably wouldn't be discussing this because target-rpm would be enough. So I'm assuming this is the case and you have no idea what RPM the fan runs at. The fan-common.yaml binding is a bit incomplete for this. What you need is some map of fan speed to PWM duty cycle as most likely it is not linear response. I think there are 2 options here:
Use the 'cooling-levels' property. Fan "speed" is the index of the array. So you just need a 'default-cooling-level' property that's the default index.
The other option is define an array of (fan RPM, PWM duty cycle) tuples. Then target-rpm can be used to select the entry. We already have something like this with 'gpio-fan,speed-map'.
There's also no definition of the minimum RPM or duty cycle in the pwm-fan binding. We have min-rpm in fan-common, but that doesn't work without a tach. A map would help here as well
This problem to me is similar to LEDs. Ultimately it's brightness that you care about, not the current or PWM duty cycle to get there.
Finally, whatever we end up with, it should go in fan-common.yaml. That supports PWMs too, so whatever we end up with is applicable to any PWM controlled fan.
Rob
Quoted 7 lines · Rob Herring
I still don't agree. Quoting Guenter:
Quoted 4 lines
The two values are also orthogonal. The fan rpm is fan dependent. Each fan will require a different pwm value to reach the target speed. Trying to use target-rpm to set a default pwm value would really not make much if any sense.
Quoted 8 lines
But RPM is ultimately what you care about and is the fan parameter that's universal yet independent of the underlying control. RPM is what determines noise and power consumption.
There's 2 cases to consider: you have a tach signal and know the fan RPM or you don't know the RPM. If you have a tach signal, we probably wouldn't be discussing this because target-rpm would be enough. So I'm assuming this is the case and you have no idea what RPM the fan runs at.
Correct, no tacho.
Quoted 7 lines
The
fan-common.yamlbinding is a bit incomplete for this. What you need is some map of fan speed to PWM duty cycle as most likely it is not linear response. I think there are 2 options here:Use the 'cooling-levels' property. Fan "speed" is the index of the array. So you just need a 'default-cooling-level' property that's the default index.
I am not sure I what you mean with the RPM reference here? The cooling-levels support in the fan-pwm.c driver is a mapping between cooling levels and PWM values, NOT RPM value.
Quoted 3 lines
The other option is define an array of (fan RPM, PWM duty cycle) tuples. Then target-rpm can be used to select the entry. We already have something like this with 'gpio-fan,speed-map'.
Where should these "invented" RPM values come from when there is no tacho signal? That sounds backwards / complicated for the very trivial "what should the default PWM value be at driver probe time" use case.
Quoted 3 lines
There's also no definition of the minimum RPM or duty cycle in the pwm-fan binding. We have min-rpm in fan-common, but that doesn't work without a tach. A map would help here as well
The minimum PWM is presumably 0, E.G. signal always low?
Quoted 2 lines
This problem to me is similar to LEDs. Ultimately it's brightness that you care about, not the current or PWM duty cycle to get there.
The use case (as described in the commit message) is to drive the fan less hard to limit noise and/or power consumption. The input to the fan drive control is a PWM setting, so it IMHO makes sense to specify that, as that is the interface provided by the fan-pwm driver - E.G. you boot up and tweak the pwm1 property in sysfs until you have a value that suits the noise/power consumption requirements and stick that value in the dts.
Quoted 3 lines
Finally, whatever we end up with, it should go in
fan-common.yaml. That supports PWMs too, so whatever we end up with is applicable to any PWM controlled fan.
What makes this "default-pwm" (or whatever it will be called) more generic than E.G. the recently added "fan-stop-to-start-percent" / "fan-stop-to-start-usec" properties added to pwm-fan.yaml by commit 80bc64201e78 ("dt-bindings: hwmon: pwm-fan: Document start from stopped state properties")?
Signature · 1 line
Bye, Peter Korsgaard
Rob Herring
maintainerQuoted 25 lines · Peter Korsgaard
Quoted 7 lines · Rob Herring
I still don't agree. Quoting Guenter:
Quoted 4 lines
The two values are also orthogonal. The fan rpm is fan dependent. Each fan will require a different pwm value to reach the target speed. Trying to use target-rpm to set a default pwm value would really not make much if any sense.
Quoted 8 lines
But RPM is ultimately what you care about and is the fan parameter that's universal yet independent of the underlying control. RPM is what determines noise and power consumption.
There's 2 cases to consider: you have a tach signal and know the fan RPM or you don't know the RPM. If you have a tach signal, we probably wouldn't be discussing this because target-rpm would be enough. So I'm assuming this is the case and you have no idea what RPM the fan runs at.
Correct, no tacho.
Quoted 4 lines
The
fan-common.yamlbinding is a bit incomplete for this. What you need is some map of fan speed to PWM duty cycle as most likely it is not linear response. I think there are 2 options here:
Quoted 7 lines
Quoted 3 lines
Use the 'cooling-levels' property. Fan "speed" is the index of the array. So you just need a 'default-cooling-level' property that's the default index.
I am not sure I what you mean with the RPM reference here? The cooling-levels support in the
fan-pwm.cdriver is a mapping between cooling levels and PWM values, NOT RPM value.
Did I say RPM anywhere for this option?
It is the index of the array that is meaningful to anything outside of the driver. The values are opaque. They are duty cycle in some cases and RPMs in other cases. The thermal subsystem knows nothing about PWM duty cycle nor RPMs.
Defining a default-cooling-level would be useful to anyone, not just your usecase.
IOW, you are proposing:
default-pwm = <123>;
I'm proposing doing this instead:
cooling-levels = <0 123 255>;
default-cooling-level = <1>;
Quoted 7 lines
Quoted 3 lines
The other option is define an array of (fan RPM, PWM duty cycle) tuples. Then target-rpm can be used to select the entry. We already have something like this with 'gpio-fan,speed-map'.
Where should these "invented" RPM values come from when there is no tacho signal? That sounds backwards / complicated for the very trivial "what should the default PWM value be at driver probe time" use case.
Every fan has at least a maximum RPM spec'ed. Probably a minium too. For anything in between, you're correct that you don't know. I guess you just assume a linear response.
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There's also no definition of the minimum RPM or duty cycle in the pwm-fan binding. We have min-rpm in fan-common, but that doesn't work without a tach. A map would help here as well
The minimum PWM is presumably 0, E.G. signal always low?
I'm talking about the duty cycle needed to start the fan spinning and to keep it spinning. I'm sure that value is not 1 for any fan except one in a physics textbook (the only place friction does not exist).
Maybe the minimum is index 0 of cooling-levels?
Quoted 9 lines
Quoted 2 lines
This problem to me is similar to LEDs. Ultimately it's brightness that you care about, not the current or PWM duty cycle to get there.
The use case (as described in the commit message) is to drive the fan less hard to limit noise and/or power consumption. The input to the fan drive control is a PWM setting, so it IMHO makes sense to specify that, as that is the interface provided by the fan-pwm driver - E.G. you boot up and tweak the pwm1 property in sysfs until you have a value that suits the noise/power consumption requirements and stick that value in the dts.
I understand what you want. I'm trying to think ahead about what's the next thing someone will want to add to the binding. Just adding 1 property at a time does not result in the best binding design. There's plenty of examples of that. Second, we have these fan bindings like pwm-fan which predate coming up with a common binding. Any further evolution of these bindings should not further diverge from the common binding.
If your process do this once for a given platform, then having this in DT is fine. If the process is every user of the platform does this, then I don't think it should be in DT. Having users tweak the DT is not a great experience compared to just putting the setting in a file on the rootfs.
Quoted 9 lines
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Finally, whatever we end up with, it should go in
fan-common.yaml. That supports PWMs too, so whatever we end up with is applicable to any PWM controlled fan.What makes this "default-pwm" (or whatever it will be called) more generic than E.G. the recently added "fan-stop-to-start-percent" / "fan-stop-to-start-usec" properties added to
pwm-fan.yamlby commit80bc64201e78("dt-bindings: hwmon: pwm-fan: Document start from stopped state properties")?
Nothing. Those should probably be moved.
Really, pwm-fan.yaml should reference fan-common.yaml and drop all the duplicate properties.
Rob
Guenter Roeck
maintainerQuoted 8 lines · Rob Herring
Quoted 1 line
The minimum PWM is presumably 0, E.G. signal always low?
I'm talking about the duty cycle needed to start the fan spinning and to keep it spinning. I'm sure that value is not 1 for any fan except one in a physics textbook (the only place friction does not exist).
Maybe the minimum is index 0 of cooling-levels?
Fan controllers often have a separate value for the lowest cooling level and for the PWM necessary to start a fan. The pwm associated with the lowest cooling level may be lower than the pwm necessary to start a fan.
The fan controller would start a fan by applying the "start fan" pwm value for a period of time, then reduce the pwm to the desired pwm value for the running fan.
I'll be happy to dig up datasheet references if needed.
Guenter
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"Rob" == Rob Herring <[email protected]> writes:
Quoted 14 lines · Rob Herring
Quoted 3 lines
I am not sure I what you mean with the RPM reference here? The cooling-levels support in the
fan-pwm.cdriver is a mapping between cooling levels and PWM values, NOT RPM value.Did I say RPM anywhere for this option?
It is the index of the array that is meaningful to anything outside of the driver. The values are opaque. They are duty cycle in some cases and RPMs in other cases. The thermal subsystem knows nothing about PWM duty cycle nor RPMs.
Defining a default-cooling-level would be useful to anyone, not just your usecase.
Quoted 8 lines
IOW, you are proposing:
default-pwm = <123>;
I'm proposing doing this instead:
cooling-levels = <0 123 255>;
default-cooling-level = <1>;
I don't have CONFIG_THERMAL enabled in my builds (and don't know the subsystem), but I see the pwm-fan driver has some logic to default to the highest cooling level, it just forgets to actually set the PWM to match it, so perhaps we can just fix that?
E.G. something like:
commit 02c8ba74eb7dddf210ceefa253385bc8e40f49ae
Author: Peter Korsgaard <[email protected]> Date: Thu Jan 2 18:26:45 2025 +0100
hwmon: (pwm-fan): Default to the Maximum cooling level if provided
The pwm-fan driver uses full PWM (255) duty cycle at startup, which may not always be desirable because of noise or power consumption peaks.
The driver optionally accept a list of "cooling-levels" for the thermal subsystem. If provided, use the PWM value corresponding to the maximum cooling level rather than the full level.
drivers/hwmon/pwm-fan.c +10 -6
@@ -499,6 +499,7 @@ static int pwm_fan_probe(struct platform_device *pdev)const struct hwmon_channel_info **channels;u32 pwm_min_from_stopped =0 ;u32 *fan_channel_config; +u32 default_pwm =MAX_PWM ;int channel_count =1 ;/* We always have a PWM channel. */ int i;
@@ -545,11 +546,18 @@ static int pwm_fan_probe(struct platform_device *pdev) ctx->enable_mode = pwm_disable_reg_enable; + ret =pwm_fan_get_cooling_data (dev, ctx); +if (ret) +return ret; + +if (ctx->pwm_fan_cooling_levels) + default_pwm = ctx->pwm_fan_cooling_levels[ctx->pwm_fan_max_state]; +/* -* Set duty cycle to maximum allowed and enable PWM output as well as +* Set duty cycle to default and enable PWM output as well as * the regulator. In case of error nothing is changed */ - ret =set_pwm (ctx,MAX_PWM ); + ret =set_pwm (ctx, default_pwm);if (ret) {dev_err (dev,"Failed to configure PWM: %d\n" , ret);return ret;
@@ -661,10 +669,6 @@ static int pwm_fan_probe(struct platform_device *pdev)return PTR_ERR (hwmon); } - ret =pwm_fan_get_cooling_data (dev, ctx); -if (ret) -return ret; - ctx->pwm_fan_state = ctx->pwm_fan_max_state;if (IS_ENABLED (CONFIG_THERMAL )) { cdev = devm_thermal_of_cooling_device_register(dev,
Guenter, what do you say? This way we don't need any new device tree properties. I personally find it less clear than a default-pwm property, but oh well.
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Bye, Peter Korsgaard
Guenter Roeck
maintainerQuoted 94 lines · Peter Korsgaard
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"Rob" == Rob Herring <[email protected]> writes:
Quoted 14 lines · Rob Herring
Quoted 3 lines
I am not sure I what you mean with the RPM reference here? The cooling-levels support in the
fan-pwm.cdriver is a mapping between cooling levels and PWM values, NOT RPM value.Did I say RPM anywhere for this option?
It is the index of the array that is meaningful to anything outside of the driver. The values are opaque. They are duty cycle in some cases and RPMs in other cases. The thermal subsystem knows nothing about PWM duty cycle nor RPMs.
Defining a default-cooling-level would be useful to anyone, not just your usecase.
Quoted 8 lines
IOW, you are proposing:
default-pwm = <123>;
I'm proposing doing this instead:
cooling-levels = <0 123 255>;
default-cooling-level = <1>;I don't have
CONFIG_THERMALenabled in my builds (and don't know the subsystem), but I see the pwm-fan driver has some logic to default to the highest cooling level, it just forgets to actually set the PWM to match it, so perhaps we can just fix that?E.G. something like:
commit
02c8ba74eb7dddf210ceefa253385bc8e40f49aeAuthor: Peter Korsgaard <[email protected]> Date: Thu Jan 2 18:26:45 2025 +0100hwmon: (pwm-fan): Default to the Maximum cooling level if provided
The pwm-fan driver uses full PWM (255) duty cycle at startup, which may not always be desirable because of noise or power consumption peaks.
The driver optionally accept a list of "cooling-levels" for the thermal subsystem. If provided, use the PWM value corresponding to the maximum cooling level rather than the full level.
Signed-off-by Peter Korsgaard <[email protected]>drivers/hwmon/pwm-fan.c +10
-6@@ -499,6 +499,7 @@ static int pwm_fan_probe(struct platform_device *pdev)const struct hwmon_channel_info **channels;u32 pwm_min_from_stopped =0 ;u32 *fan_channel_config; +u32 default_pwm =MAX_PWM ;int channel_count =1 ;/* We always have a PWM channel. */ int i;@@ -545,11 +546,18 @@ static int pwm_fan_probe(struct platform_device *pdev) ctx->enable_mode = pwm_disable_reg_enable; + ret =pwm_fan_get_cooling_data (dev, ctx); +if (ret) +return ret; + +if (ctx->pwm_fan_cooling_levels) + default_pwm = ctx->pwm_fan_cooling_levels[ctx->pwm_fan_max_state]; +/* -* Set duty cycle to maximum allowed and enable PWM output as well as +* Set duty cycle to default and enable PWM output as well as * the regulator. In case of error nothing is changed */ - ret =set_pwm (ctx,MAX_PWM ); + ret =set_pwm (ctx, default_pwm);if (ret) {dev_err (dev,"Failed to configure PWM: %d\n" , ret);return ret;@@ -661,10 +669,6 @@ static int pwm_fan_probe(struct platform_device *pdev)return PTR_ERR (hwmon); } - ret =pwm_fan_get_cooling_data (dev, ctx); -if (ret) -return ret; - ctx->pwm_fan_state = ctx->pwm_fan_max_state;if (IS_ENABLED (CONFIG_THERMAL )) { cdev = devm_thermal_of_cooling_device_register(dev,
Quoted 4 lines
Guenter, what do you say? This way we don't need any new device tree properties. I personally find it less clear than a default-pwm property, but oh well.
I would not call that "default". It is more along the line of "If available, use highest cooling level as maximum allowed".
Other than that, I don't like it, but since it looks like we won't get approval for the devicetree property, I'd say go for it.
Guenter
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"Guenter" == Guenter Roeck <[email protected]> writes:
Hi,
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Guenter, what do you say? This way we don't need any new device tree properties. I personally find it less clear than a default-pwm property, but oh well.
I would not call that "default". It is more along the line of "If available, use highest cooling level as maximum allowed".
Sorry, what are you referring to exactly? The commit message? The change is about the default/initial pwm setting, there is nothing disallowing user space to increase it afterwards?
Quoted 2 lines
Other than that, I don't like it, but since it looks like we won't get approval for the devicetree property, I'd say go for it.
Great, then we agree! I'll send a v5 then once it is clear what your comment above refers to.
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Bye, Peter Korsgaard
Guenter Roeck
maintainerQuoted 9 lines · Peter Korsgaard
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"Guenter" == Guenter Roeck <[email protected]> writes:
Hi,
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Guenter, what do you say? This way we don't need any new device tree properties. I personally find it less clear than a default-pwm property, but oh well.
Quoted 7 lines
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I would not call that "default". It is more along the line of "If available, use highest cooling level as maximum allowed".
Sorry, what are you referring to exactly? The commit message? The change is about the default/initial pwm setting, there is nothing disallowing user space to increase it afterwards?
From the current probe function:
"Set duty cycle to maximum allowed". If you dislike the term "maximum allowed", please consider something like "Set initial duty cycle to maximum or, if provided, to the highest cooling level".
Guenter
Guenter Roeck
maintainerQuoted 6 lines · Rob Herring
Quoted 5 lines · Peter Korsgaard
The pwm-fan driver uses full PWM (255) duty cycle at startup, which may not always be desirable because of noise or power consumption peaks, so add an optional "default-pwm" property that can be used to specify a custom default PWM duty cycle (0..255).
Quoted 12 lines
Quoted 4 lines
This is somewhat similar to target-rpm from
fan-common.yaml, but that cannot be used here as target-rpm specifies the target fan speed, whereas this is the default pwm to set when the device is instantiated - And the resulting fan RPM resulting from a given PWM duty cycle is fan dependent.I still don't agree. Quoting Guenter:
Quoted 4 lines
The two values are also orthogonal. The fan rpm is fan dependent. Each fan will require a different pwm value to reach the target speed. Trying to use target-rpm to set a default pwm value would really not make much if any sense.
That is a mis-quote, really. I was talking about target-rpm. The property to be added here is default-pwm, which is completely different.
Quoted 12 lines
But RPM is ultimately what you care about and is the fan parameter that's universal yet independent of the underlying control. RPM is what determines noise and power consumption.
There's 2 cases to consider: you have a tach signal and know the fan RPM or you don't know the RPM. If you have a tach signal, we probably wouldn't be discussing this because target-rpm would be enough. So I'm assuming this is the case and you have no idea what RPM the fan runs at. The
fan-common.yamlbinding is a bit incomplete for this. What you need is some map of fan speed to PWM duty cycle as most likely it is not linear response. I think there are 2 options here:
Quoted 8 lines
Use the 'cooling-levels' property. Fan "speed" is the index of the array. So you just need a 'default-cooling-level' property that's the default index.
The other option is define an array of (fan RPM, PWM duty cycle) tuples. Then target-rpm can be used to select the entry. We already have something like this with 'gpio-fan,speed-map'.
That won't work. Literally each individual fan (even the same model) runs at a different rpm for a given pwm value, and I am sure the rpm for a given pwm input changes depending on fan age and temperature.
Quoting from the NCT6796D-S datasheet, for an example:
"The default duty cycle is 7Fh, or 50% for SYSFANOUT, CPUFANOUT, AUXFANOUT0, AUXFANOUT1,
AUXFANOUT2, AUXFANOUT3 and AUXFANOUT4.
Note.The default speed of fan output is specified in registers CR[E0h] ~ CR[E4h] and CR[E7h] of Logical Device B, and the AUXFANOUT4 default speed of fan output is specified in register CR[E3h] of Logical Device D.
"
Note that the term "default speed" as used in the datasheet refers to pwm. Again, from the datasheet:
CR E3h. AUXFAN4 Duty Cycle Register
^^^^^^^^^^^^^^^^^^^
Setting the default value for such a register is what we are talking about here. No rpm involved, just default pwm values. This is completely independent of rpm.
Guenter
Guenter Roeck
maintainerQuoted 5 lines · Rob Herring
But RPM is ultimately what you care about and is the fan parameter that's universal yet independent of the underlying control. RPM is what determines noise and power consumption.
I forgot to add: I would argue that airflow restrictions play a substantial role for both power consumption and noise, especially when trying to achieve a specific fan speed. For power consumption, I don't even see how rpm would play a substantial role in the first place since power consumption depends on pwm, not on the resulting rpm (or airflow).
Guenter