Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752532AbcKIBdV (ORCPT ); Tue, 8 Nov 2016 20:33:21 -0500 Received: from mailout1.samsung.com ([203.254.224.24]:44823 "EHLO mailout1.samsung.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750724AbcKIBdS (ORCPT ); Tue, 8 Nov 2016 20:33:18 -0500 MIME-version: 1.0 Content-type: text/plain; charset=utf-8 X-AuditID: cbfee61a-f79916d0000062de-03-58227cdb0030 Content-transfer-encoding: 8BIT Message-id: <58227CDB.6050907@samsung.com> Date: Wed, 09 Nov 2016 10:33:15 +0900 From: Chanwoo Choi Organization: Samsung Electronics User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.6.0 To: Saravana Kannan Cc: "Rafael J. Wysocki" , MyungJoo Ham , Kyungmin Park , linux-pm@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH v3] PM / devfreq: Restart previous governor if new governor fails to start References: <1477509425-16936-1-git-send-email-skannan@codeaurora.org> <5820CE7E.40802@codeaurora.org> <5821948B.2010907@samsung.com> <58223AF1.2030605@codeaurora.org> In-reply-to: <58223AF1.2030605@codeaurora.org> X-Brightmail-Tracker: H4sIAAAAAAAAA+NgFtrMIsWRmVeSWpSXmKPExsVy+t9jAd3bNUoRBp0HLSzONr1ht7i8aw6b xefeI4wWtxtXsFmcOX2J1eLAxYlsDmwel/t6mTy2XG1n8ejbsorR4/MmuQCWKDebjNTElNQi hdS85PyUzLx0W6XQEDddCyWFvMTcVFulCF3fkCAlhbLEnFIgz8gADTg4B7gHK+nbJbhlzF1+ galgtXTFqxln2RoY34p2MXJySAiYSPw+/58JwhaTuHBvPVsXIxeHkMBSRokXh3YzgyR4BQQl fky+x9LFyMHBLCAvceRSNoSpLjFlSi5E+QNGiX+vutggyrUkTh7ZBFbOIqAqseynAUiYDSi8 /8UNsBJ+AUWJqz8eM4KUiApESHSfqAQJiwjoSPz9c50FZCSzwF5GiaVvFrGDJIQFEiQa2o4x Q+zaxiQxZ/sDsEGcAnoSnT/PMk5gFJyF5NJZCJfOQrh0ASPzKkaJ1ILkguKk9FzDvNRyveLE 3OLSvHS95PzcTYzgCHsmtYPx4C73Q4wCHIxKPLwHHitGCLEmlhVX5h5ilOBgVhLhXV+tFCHE m5JYWZValB9fVJqTWnyI0RTo1YnMUqLJ+cDozyuJNzQxNzE3NrAwt7Q0MVIS522c/SxcSCA9 sSQ1OzW1ILUIpo+Jg1OqgXF9V9sNzqN6SkKKdvtvCf/bfsRht/A07QJWr4ptzBt0vA/FZJbE 6HllJhkvu62cMn8Vg5hR0+POqbNmP43RVDPx2u6ivzzeQk/BOl7nytbF/nPaFG2L5d7zinOb nev9eUd7U/QRFc7m46o/Z8RvnmFZeKfzkN+Z4GARtYnTNWKu/tyqvv9TlBJLcUaioRZzUXEi AOcNo+bGAgAA X-MTR: 20000000000000000@CPGS Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 3421 Lines: 78 On 2016년 11월 09일 05:52, Saravana Kannan wrote: > On 11/08/2016 01:02 AM, Chanwoo Choi wrote: >> Hi, >> >> On 2016년 11월 08일 03:57, Saravana Kannan wrote: >>> On 10/26/2016 05:06 PM, Chanwoo Choi wrote: >>>> Hi, >>>> >>>> On 2016년 10월 27일 04:17, Saravana Kannan wrote: >>>>> If the new governor fails to start, switch back to old governor so that the >>>>> devfreq state is not left in some weird limbo. >>>>> >>>>> Signed-off-by: Saravana Kannan >>>>> --- >>>>> * Fix NULL deref for real this time. >>>>> * Addressed some style preferences. >>>>> >>>>> drivers/devfreq/devfreq.c | 13 +++++++++++-- >>>>> 1 file changed, 11 insertions(+), 2 deletions(-) >>>>> >>>>> diff --git a/drivers/devfreq/devfreq.c b/drivers/devfreq/devfreq.c >>>>> index bf3ea76..77c41a5 100644 >>>>> --- a/drivers/devfreq/devfreq.c >>>>> +++ b/drivers/devfreq/devfreq.c >>>>> @@ -919,7 +919,7 @@ static ssize_t governor_store(struct device *dev, struct device_attribute *attr, >>>>> struct devfreq *df = to_devfreq(dev); >>>>> int ret; >>>>> char str_governor[DEVFREQ_NAME_LEN + 1]; >>>>> - struct devfreq_governor *governor; >>>>> + const struct devfreq_governor *governor, *prev_governor; >>>>> >>>>> ret = sscanf(buf, "%" __stringify(DEVFREQ_NAME_LEN) "s", str_governor); >>>>> if (ret != 1) >>>>> @@ -944,12 +944,21 @@ static ssize_t governor_store(struct device *dev, struct device_attribute *attr, >>>>> goto out; >>>>> } >>>>> } >>>>> + prev_governor = df->governor; >>>>> df->governor = governor; >>>>> strncpy(df->governor_name, governor->name, DEVFREQ_NAME_LEN); >>>>> ret = df->governor->event_handler(df, DEVFREQ_GOV_START, NULL); >>>>> - if (ret) >>>>> + if (ret) { >>>>> dev_warn(dev, "%s: Governor %s not started(%d)\n", >>>>> __func__, df->governor->name, ret); >>>>> + if (prev_governor) { >>>> >>>> I think that prev_governor is always set. You don't need to check it. >>>> Why do check it? >>> >>> Not true. Even higher up in this function, we check if df->governor != NULL. Simple example would be that the default governor passed in while adding the device isn't compiled in. >> >> I don't understand. If device is not registered by devfreq_add_device(), >> you don't change the governor by using governor_store(). >> >> If you can change the governor through governor_store(), >> it means that the devfreq instance was added without any problem. >> The added devfreq instance must have the sepecific governor >> on df->governor variable. >> >> So, I think that df->governor is always set and then prev_governor is always set. > > Read the code more closely. add_device doesn't (and shouldn't) fail if the governor isn't present. After that, the governor could be changed from user space. If governor is not present during devfreq_add_device(), the devfreq instance is not able to register the devfreq framework. So, the user never touch the sysfs[1] to change the governor because the sysfs[1] is not created by devfreq framework. [1] /sys/class/devfreq/[device name]/governor In summary, if governor is not present during devfreq_add_device(), the devfreq framework doesn't create tye sysfs[1] for governor. The user-space never change the governor through sysfs[1] because there is no any sysfs entry[1]. Best Regards, Chanwoo Choi