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Wysocki" To: Leonard Crestez Subject: Re: [PATCH RFC 0/2] PM / devfreq: Add dev_pm_qos support with minimal changes Date: Wed, 04 Dec 2019 11:46:17 +0100 Message-ID: <5794906.l6Fuony6qs@kreacher> In-Reply-To: References: MIME-Version: 1.0 X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20191204_024630_164803_721727BA X-CRM114-Status: GOOD ( 19.44 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Artur =?utf-8?B?xZp3aWdvxYQ=?= , Jacky Bai , Saravana Kannan , linux-pm@vger.kernel.org, Viresh Kumar , NXP Linux Team , Krzysztof Kozlowski , Chanwoo Choi , Kyungmin Park , Matthias Kaehlcke , MyungJoo Ham , Georgi Djakov , linux-arm-kernel@lists.infradead.org, Alexandre Bailon Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+infradead-linux-arm-kernel=archiver.kernel.org@lists.infradead.org On Tuesday, November 19, 2019 5:12:12 PM CET Leonard Crestez wrote: > Add dev_pm_qos notifiers to devfreq core in order to support frequency > limits via dev_pm_qos_add_request. > > Unlike the rest of devfreq the dev_pm_qos frequency is measured in kHz, > this is consistent with current dev_pm_qos usage for cpufreq and > allows frequencies above 2Ghz (pm_qos expresses limits as s32). > > Like with cpufreq the handling of min_freq/max_freq is moved to the > dev_pm_qos mechanism. Constraints from userspace are no longer clamped on > store, instead all values can be written and we only check against OPPs in a > new devfreq_get_freq_range function. This is consistent with the design of > dev_pm_qos. > > Notifiers from pm_qos are executed under a single global dev_pm_qos_mtx and > need to take devfreq->lock, this means that calls into dev_pm_qos while holding > devfreq->lock are not allowed (lockdep warns about possible deadlocks). > > Fix this by only adding the qos request and notifiers after devfreq->lock is > released inside devfreq_add_device. In theory this means sysfs writes > are possible before the min/max requests are initialized so we guard > against that explictly. The dev_pm_qos_update_request function would > otherwise print a big WARN splat. > > Alternatively devfreq initialization could be refactored to avoid taking > devfreq->lock but that requires several intricate changes: > > https://patchwork.kernel.org/cover/11242865/ > > I considered making dev_pm_qos call notifiers outside the lock but > that's another complex refactoring and it's difficult to ensure > correctness. If two identical qos requests are made in parallel then the > second shouldn't return until all notifiers are completely executed for > the first and QOS is enforced; otherwise it mostly defeats the purpose > of making proactive requests. > > This series implements the minimal changes in order to implement dev_pm_qos > support for devfreq. It only costs a little defensive programming. > > This series is also marked as [RFC] because it depends on restoring > DEV_PM_QOS_MIN/MAX_FREQUENCY inside the pm core: > > https://patchwork.kernel.org/cover/11250413/ > > --- > Changes since "big version" v10: > * Drop accepted cleanups > * Work with current locking approach (split cleanups into other series) > * Drop acks and deliberately relabel as a new series. It still incorporates > most previous discussion but takes a different approach to locking. > * Don't print errors if devfreq_dev_release is called on error cleanup from > devfreq_add_device, just accept that requests and notifiers might not be > registered yet. I wish dev_pm_qos cleanups behaved like standard "kfree" and > silently did nothing when there's nothing to be done. > Link to v10: https://patchwork.kernel.org/project/linux-arm-kernel/list/?series=196443 > > Leonard Crestez (2): > PM / devfreq: Add PM QoS support > PM / devfreq: Use PM QoS for sysfs min/max_freq > > drivers/devfreq/devfreq.c | 151 ++++++++++++++++++++++++++++++++++---- > include/linux/devfreq.h | 14 +++- > 2 files changed, 145 insertions(+), 20 deletions(-) Please resend this series as non-RFC with the ACKs from Chanwoo included. It may still be viable to push it for 5.5 during the -rc period. Thanks! _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel