Re: [PATCH] cpufreq: qcom-cpufreq-hw: Revert adding cpufreq qos

[Konrad Dybcio <konrad.dybcio@linaro.org>]

On 23.03.2023 23:33, Bjorn Andersson wrote:
> The OSM/EPSS hardware controls the frequency of each CPU cluster based
> on requests from the OS and various throttling events in the system.
> While throttling is in effect the related dcvs interrupt will be kept
> high. The purpose of the code handling this interrupt is to
> continuously report the thermal pressure based on the throttled
> frequency.
> 
> The reasoning for adding QoS control to this mechanism is not entirely
> clear, but the introduction of commit 'c4c0efb06f17 ("cpufreq:
> qcom-cpufreq-hw: Add cpufreq qos for LMh")' causes the
> scaling_max_frequncy to be set to the throttled frequency. On the next
> iteration of polling, the throttled frequency is above or equal to the
> newly requested frequency, so the polling is stopped.
Oh wow.. That must have been fun to debug..

Reviewed-by: Konrad Dybcio <konrad.dybcio@linaro.org>

Konrad
> 
> With cpufreq limiting the max frequency, the hardware no longer report a
> throttling state and no further updates to thermal pressure or qos
> state are made.
> 
> The result of this is that scaling_max_frequency can only go down, and
> the system becomes slower and slower every time a thermal throttling
> event is reported by the hardware.
> 
> Even if the logic could be improved, there is no reason for software to
> limit the max freqency in response to the hardware limiting the max
> frequency. At best software will follow the reported hardware state, but
> typically it will cause slower backoff of the throttling.
> 
> This reverts commit c4c0efb06f17fa4a37ad99e7752b18a5405c76dc.
> 
> Fixes: c4c0efb06f17 ("cpufreq: qcom-cpufreq-hw: Add cpufreq qos for LMh")
> Cc: stable@vger.kernel.org
> Reported-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
> Signed-off-by: Bjorn Andersson <quic_bjorande@quicinc.com>
> ---
>  drivers/cpufreq/qcom-cpufreq-hw.c | 14 --------------
>  1 file changed, 14 deletions(-)
> 
> diff --git a/drivers/cpufreq/qcom-cpufreq-hw.c b/drivers/cpufreq/qcom-cpufreq-hw.c
> index 575a4461c25a..1503d315fa7e 100644
> --- a/drivers/cpufreq/qcom-cpufreq-hw.c
> +++ b/drivers/cpufreq/qcom-cpufreq-hw.c
> @@ -14,7 +14,6 @@
>  #include <linux/of_address.h>
>  #include <linux/of_platform.h>
>  #include <linux/pm_opp.h>
> -#include <linux/pm_qos.h>
>  #include <linux/slab.h>
>  #include <linux/spinlock.h>
>  #include <linux/units.h>
> @@ -60,8 +59,6 @@ struct qcom_cpufreq_data {
>  	struct clk_hw cpu_clk;
>  
>  	bool per_core_dcvs;
> -
> -	struct freq_qos_request throttle_freq_req;
>  };
>  
>  static struct {
> @@ -351,8 +348,6 @@ static void qcom_lmh_dcvs_notify(struct qcom_cpufreq_data *data)
>  
>  	throttled_freq = freq_hz / HZ_PER_KHZ;
>  
> -	freq_qos_update_request(&data->throttle_freq_req, throttled_freq);
> -
>  	/* Update thermal pressure (the boost frequencies are accepted) */
>  	arch_update_thermal_pressure(policy->related_cpus, throttled_freq);
>  
> @@ -445,14 +440,6 @@ static int qcom_cpufreq_hw_lmh_init(struct cpufreq_policy *policy, int index)
>  	if (data->throttle_irq < 0)
>  		return data->throttle_irq;
>  
> -	ret = freq_qos_add_request(&policy->constraints,
> -				   &data->throttle_freq_req, FREQ_QOS_MAX,
> -				   FREQ_QOS_MAX_DEFAULT_VALUE);
> -	if (ret < 0) {
> -		dev_err(&pdev->dev, "Failed to add freq constraint (%d)\n", ret);
> -		return ret;
> -	}
> -
>  	data->cancel_throttle = false;
>  	data->policy = policy;
>  
> @@ -519,7 +506,6 @@ static void qcom_cpufreq_hw_lmh_exit(struct qcom_cpufreq_data *data)
>  	if (data->throttle_irq <= 0)
>  		return;
>  
> -	freq_qos_remove_request(&data->throttle_freq_req);
>  	free_irq(data->throttle_irq, data);
>  }
>