Re: [PATCH RFC 2/2] PM / devfreq: Use PM QoS for sysfs min/max_freq

From: Leonard Crestez <leonard.crestez@nxp.com>
To: Matthias Kaehlcke <mka@chromium.org>,
	"Rafael J. Wysocki" <rjw@rjwysocki.net>,
	Chanwoo Choi <cw00.choi@samsung.com>
Cc: "Kyungmin Park" <kyungmin.park@samsung.com>,
	"MyungJoo Ham" <myungjoo.ham@samsung.com>,
	"Artur Świgoń" <a.swigon@partner.samsung.com>,
	"Saravana Kannan" <saravanak@google.com>,
	"Krzysztof Kozlowski" <krzk@kernel.org>,
	"Alexandre Bailon" <abailon@baylibre.com>,
	"Georgi Djakov" <georgi.djakov@linaro.org>,
	"Jacky Bai" <ping.bai@nxp.com>,
	"Viresh Kumar" <viresh.kumar@linaro.org>,
	dl-linux-imx <linux-imx@nxp.com>,
	"linux-pm@vger.kernel.org" <linux-pm@vger.kernel.org>,
	"linux-arm-kernel@lists.infradead.org"
	<linux-arm-kernel@lists.infradead.org>
Subject: Re: [PATCH RFC 2/2] PM / devfreq: Use PM QoS for sysfs min/max_freq
Date: Mon, 25 Nov 2019 16:46:12 +0000	[thread overview]
Message-ID: <VI1PR04MB70237E859BBD7048B0DD0675EE4A0@VI1PR04MB7023.eurprd04.prod.outlook.com> (raw)
In-Reply-To: 20191121231622.GK27773@google.com

On 22.11.2019 01:16, Matthias Kaehlcke wrote:
> Hi Leonard,
> 
> this seems to work ok as long as there are no other ('external') PM QoS
> requests, however the aggregate max_freq can be incorrect when there are
> multiple requests.
> 
> I intended to test with devfreq_cooling using PM QoS, but this didn't work
> out due to limitations on my development platform. Instead I added another
> set of requests and sysfs attributes to devfreq (see patch below).

Thanks a lot for looking at this.

> With this we get:
> 
> cat available_frequencies
>    180000000 267000000 355000000 430000000 565000000 650000000 800000000
> 
> # ok
> cat max_freq
>    800000000
> 
> # wtf???
> echo 650000000 > max_freq
> cat max_freq
>    800000000
> 
> # this looks better
> echo 800000000 > test_max_freq
> cat max_freq
>    650000000
> 
> 
> The problem appears to be:
> 
> #define PM_QOS_MAX_FREQUENCY_DEFAULT_VALUE      (-1)
> 
> in include/linux/pm_qos.h (added by "PM / QoS: Restore
> DEV_PM_QOS_MIN/MAX_FREQUENCY")
> 
> The aggregate value returned by
> dev_pm_qos_read_value(dev, DEV_PM_QOS_MAX_FREQUENCY) is the
> smallest of all requests, which is PM_QOS_MAX_FREQUENCY_DEFAULT_VALUE
> unless all requests have set an actual constraint.
> 
> You probably want to change the constant to resolve to S32_MAX or some
> other big value.

Since dev_pm_qos is now layered on top of freq_qos that should be:

#define PM_QOS_MAX_FREQUENCY_DEFAULT_VALUE FREQ_QOS_MAX_DEFAULT_VALUE

I wrote a synthetic test and it seems your issue can be reproduced using 
kunit on top of raw freq_qos. Looking a bit deeper into pm_qos 
implementation it seems that there isn't much special treatment for 
pm_qos_constraints. A reasonable solution would be:

#define FREQ_QOS_MAX_DEFAULT_VALUE	S32_MAX

Cpufreq is not affected because it always sets "real" values but I 
believe that creating requests that should be ignored is a valid approach.

Update on pm/qos here: https://patchwork.kernel.org/cover/11260627/

--
Regards,
Leonard