The unit of dynamic-power-coefficient is described as mW/MHz/uV^2 whereas
its usage in the code assumes that unit is uW/MHz/V^2
In drivers/thermal/cpu_cooling.c, the code is :
power = (u64)capacitance * freq_mhz * voltage_mv * voltage_mv;
do_div(power, 1000000000);
which can be summarized as :
power (mW) = capacitance * freq_mhz/1000 * (voltage_mv/1000)^2
or
power (mW) = (capacitance * freq_mhz * (voltage_mv/1000)^2) / 1000
then
power (mW) = power (uW) / 1000
so
power (uW) = capacitance * freq_mhz * (voltage_mv/1000)^2
Furthermore, if we test basic values like :
voltage_mv = 1000mV = 1V
freq_mhz = 1000Mhz
The minimum possible power, when dynamic-power-coefficient equals 1, will
be with current unit:
min power = 1 * 1000 * (1000000)^2 = 10^15 mW
which is not realistic
With the unit used by the code, the min power is
min power = 1 * 1000 * 1^2 = 1000uW = 1mW which is far more realistic
Signed-off-by: Vincent Guittot <[email protected]>
---
Documentation/devicetree/bindings/arm/cpus.txt | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)
diff --git a/Documentation/devicetree/bindings/arm/cpus.txt b/Documentation/devicetree/bindings/arm/cpus.txt
index 29e1dc5..71d8cd0 100644
--- a/Documentation/devicetree/bindings/arm/cpus.txt
+++ b/Documentation/devicetree/bindings/arm/cpus.txt
@@ -274,7 +274,7 @@ described below.
Usage: optional
Value type: <prop-encoded-array>
Definition: A u32 value that represents the running time dynamic
- power coefficient in units of mW/MHz/uV^2. The
+ power coefficient in units of uW/MHz/V^2. The
coefficient can either be calculated from power
measurements or derived by analysis.
@@ -285,7 +285,7 @@ described below.
Pdyn = dynamic-power-coefficient * V^2 * f
- where voltage is in uV, frequency is in MHz.
+ where voltage is in V, frequency is in MHz.
Example 1 (dual-cluster big.LITTLE system 32-bit):
--
2.7.4
Hi Vincent,
Vincent Guittot <[email protected]> writes:
> The unit of dynamic-power-coefficient is described as mW/MHz/uV^2 whereas
> its usage in the code assumes that unit is uW/MHz/V^2
>
> In drivers/thermal/cpu_cooling.c, the code is :
>
> power = (u64)capacitance * freq_mhz * voltage_mv * voltage_mv;
> do_div(power, 1000000000);
>
> which can be summarized as :
> power (mW) = capacitance * freq_mhz/1000 * (voltage_mv/1000)^2
> or
> power (mW) = (capacitance * freq_mhz * (voltage_mv/1000)^2) / 1000
> then
> power (mW) = power (uW) / 1000
> so
> power (uW) = capacitance * freq_mhz * (voltage_mv/1000)^2
>
> Furthermore, if we test basic values like :
> voltage_mv = 1000mV = 1V
> freq_mhz = 1000Mhz
>
> The minimum possible power, when dynamic-power-coefficient equals 1, will
> be with current unit:
> min power = 1 * 1000 * (1000000)^2 = 10^15 mW
> which is not realistic
>
> With the unit used by the code, the min power is
> min power = 1 * 1000 * 1^2 = 1000uW = 1mW which is far more realistic
>
> Signed-off-by: Vincent Guittot <[email protected]>
Acked-by: Punit Agrawal <[email protected]>
Thanks for fixing the mismatch.
> ---
> Documentation/devicetree/bindings/arm/cpus.txt | 4 ++--
> 1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/Documentation/devicetree/bindings/arm/cpus.txt b/Documentation/devicetree/bindings/arm/cpus.txt
> index 29e1dc5..71d8cd0 100644
> --- a/Documentation/devicetree/bindings/arm/cpus.txt
> +++ b/Documentation/devicetree/bindings/arm/cpus.txt
> @@ -274,7 +274,7 @@ described below.
> Usage: optional
> Value type: <prop-encoded-array>
> Definition: A u32 value that represents the running time dynamic
> - power coefficient in units of mW/MHz/uV^2. The
> + power coefficient in units of uW/MHz/V^2. The
> coefficient can either be calculated from power
> measurements or derived by analysis.
>
> @@ -285,7 +285,7 @@ described below.
>
> Pdyn = dynamic-power-coefficient * V^2 * f
>
> - where voltage is in uV, frequency is in MHz.
> + where voltage is in V, frequency is in MHz.
>
> Example 1 (dual-cluster big.LITTLE system 32-bit):
On 29-08-18, 09:08, Vincent Guittot wrote:
> The unit of dynamic-power-coefficient is described as mW/MHz/uV^2 whereas
> its usage in the code assumes that unit is uW/MHz/V^2
>
> In drivers/thermal/cpu_cooling.c, the code is :
>
> power = (u64)capacitance * freq_mhz * voltage_mv * voltage_mv;
> do_div(power, 1000000000);
>
> which can be summarized as :
> power (mW) = capacitance * freq_mhz/1000 * (voltage_mv/1000)^2
> or
> power (mW) = (capacitance * freq_mhz * (voltage_mv/1000)^2) / 1000
> then
> power (mW) = power (uW) / 1000
> so
> power (uW) = capacitance * freq_mhz * (voltage_mv/1000)^2
>
> Furthermore, if we test basic values like :
> voltage_mv = 1000mV = 1V
> freq_mhz = 1000Mhz
>
> The minimum possible power, when dynamic-power-coefficient equals 1, will
> be with current unit:
> min power = 1 * 1000 * (1000000)^2 = 10^15 mW
> which is not realistic
>
> With the unit used by the code, the min power is
> min power = 1 * 1000 * 1^2 = 1000uW = 1mW which is far more realistic
>
> Signed-off-by: Vincent Guittot <[email protected]>
> ---
> Documentation/devicetree/bindings/arm/cpus.txt | 4 ++--
> 1 file changed, 2 insertions(+), 2 deletions(-)
Acked-by: Viresh Kumar <[email protected]>
--
viresh
On Wed, Aug 29, 2018 at 09:08:38AM +0200, Vincent Guittot wrote:
> The unit of dynamic-power-coefficient is described as mW/MHz/uV^2 whereas
> its usage in the code assumes that unit is uW/MHz/V^2
>
> In drivers/thermal/cpu_cooling.c, the code is :
>
> power = (u64)capacitance * freq_mhz * voltage_mv * voltage_mv;
> do_div(power, 1000000000);
>
> which can be summarized as :
> power (mW) = capacitance * freq_mhz/1000 * (voltage_mv/1000)^2
> or
> power (mW) = (capacitance * freq_mhz * (voltage_mv/1000)^2) / 1000
> then
> power (mW) = power (uW) / 1000
> so
> power (uW) = capacitance * freq_mhz * (voltage_mv/1000)^2
>
> Furthermore, if we test basic values like :
> voltage_mv = 1000mV = 1V
> freq_mhz = 1000Mhz
>
> The minimum possible power, when dynamic-power-coefficient equals 1, will
> be with current unit:
> min power = 1 * 1000 * (1000000)^2 = 10^15 mW
> which is not realistic
>
> With the unit used by the code, the min power is
> min power = 1 * 1000 * 1^2 = 1000uW = 1mW which is far more realistic
>
> Signed-off-by: Vincent Guittot <[email protected]>
> ---
> Documentation/devicetree/bindings/arm/cpus.txt | 4 ++--
> 1 file changed, 2 insertions(+), 2 deletions(-)
Applied, thanks.
Rob