我用的是Dell XPS 13 9350. ArchLinux已经工作了一个月了，一直都不错。昨天晚上正常升级，因为过去升级一直都很好，所以也就十分放心，结果今天早上发现wifi不能使用。下面是总结的解决问题的方法。
1.由于是网络问题，首先用
ip link 命令查询一下
发现wlp58s0 状态(state)是 DOWN
2.那么尝试者启动它
ip link set wlp58s0 up
没有报错，但是状态仍然是DOWN
3.用lspci -k 查看，发现模块正常加载，没有是那么问题
4.现在查看一下pac的日志
/var/log/pacman.log, 找到昨天的历史记录
[2016-04-30 22:01] [PACMAN] Running 'pacman -Syu'
[2016-04-30 22:01] [PACMAN] synchronizing package lists
[2016-04-30 22:02] [PACMAN] starting full system upgrade
[2016-04-30 22:34] [ALPM] running '70-dkms-remove.hook'...
[2016-04-30 22:34] [ALPM-SCRIPTLET] ==> No kernel 4.5.1-1-ARCH headers. You must install them to use DKMS!
[2016-04-30 22:34] [ALPM] transaction started
[2016-04-30 22:34] [ALPM] upgraded glib2 (2.48.0-1 -> 2.48.0-2)
[2016-04-30 22:34] [ALPM] upgraded texinfo (6.1-1 -> 6.1-3)
[2016-04-30 22:34] [ALPM] upgraded libcups (2.1.3-1 -> 2.1.3-2)
[2016-04-30 22:34] [ALPM] upgraded cups (2.1.3-1 -> 2.1.3-2)
[2016-04-30 22:34] [ALPM] upgraded qt5-base (5.6.0-3 -> 5.6.0-4)
[2016-04-30 22:34] [ALPM] upgraded desktop-file-utils (0.22-1 -> 0.22-2)
[2016-04-30 22:34] [ALPM] upgraded gdk-pixbuf2 (2.34.0-1 -> 2.34.0-2)
[2016-04-30 22:34] [ALPM] upgraded gtk-update-icon-cache (3.20.3-1 -> 3.20.3-4)
[2016-04-30 22:34] [ALPM] upgraded shared-mime-info (1.6-1 -> 1.6-2)
[2016-04-30 22:34] [ALPM] upgraded librsvg (2:2.40.15-1 -> 2:2.40.15-2)
[2016-04-30 22:34] [ALPM] upgraded gtk2 (2.24.30-1 -> 2.24.30-2)
[2016-04-30 22:34] [ALPM] upgraded deepin-screenshot (3.1.7-1 -> 3.1.7-2)
[2016-04-30 22:34] [ALPM] upgraded dhclient (4.3.4-1 -> 4.3.4-2)
[2016-04-30 22:34] [ALPM] upgraded fcitx (4.2.9.1-1 -> 4.2.9.1-2)
[2016-04-30 22:34] [ALPM] upgraded fcitx-googlepinyin (0.1.6-4 -> 0.1.6-5)
[2016-04-30 22:34] [ALPM] upgraded fcitx-gtk2 (4.2.9.1-1 -> 4.2.9.1-2)
[2016-04-30 22:34] [ALPM] upgraded libsoup (2.54.0.1-1 -> 2.54.1-1)
[2016-04-30 22:34] [ALPM] upgraded gtk3 (3.20.3-1 -> 3.20.3-4)
[2016-04-30 22:34] [ALPM] upgraded fcitx-gtk3 (4.2.9.1-1 -> 4.2.9.1-2)
[2016-04-30 22:34] [ALPM] upgraded qt4 (4.8.7-8 -> 4.8.7-9)
[2016-04-30 22:34] [ALPM] upgraded fcitx-qt4 (4.2.9.1-1 -> 4.2.9.1-2)
[2016-04-30 22:34] [ALPM] upgraded ffmpeg (1:3.0.1-1 -> 1:3.0.2-1)
[2016-04-30 22:34] [ALPM] upgraded firefox (45.0.2-1 -> 46.0-2)
[2016-04-30 22:34] [ALPM] upgraded jre8-openjdk-headless (8.u77-1 -> 8.u92-1)
[2016-04-30 22:34] [ALPM] upgraded jre8-openjdk (8.u77-1 -> 8.u92-1)
[2016-04-30 22:34] [ALPM] upgraded freemind (1.0.1-1 -> 1.0.1-2)
[2016-04-30 22:34] [ALPM] upgraded gconf (3.2.6-3 -> 3.2.6-4)
[2016-04-30 22:34] [ALPM] upgraded grep (2.24-1 -> 2.25-1)
[2016-04-30 22:34] [ALPM] upgraded jdk8-openjdk (8.u77-1 -> 8.u92-1)
[2016-04-30 22:34] [ALPM] upgraded kcoreaddons (5.21.0-1 -> 5.21.0-3)
[2016-04-30 22:34] [ALPM] upgraded kio (5.21.0-1 -> 5.21.0-2)
[2016-04-30 22:34] [ALPM] upgraded lib32-elfutils (0.165-1 -> 0.166-1)
[2016-04-30 22:34] [ALPM] upgraded libbsd (0.8.2-2 -> 0.8.3-1)
[2016-04-30 22:34] [ALPM] upgraded libnm-glib (1.0.12-1 -> 1.2.0-3)
[2016-04-30 22:34] [ALPM] upgraded libwmf (0.2.8.4-12 -> 0.2.8.4-13)
[2016-04-30 22:34] [ALPM] upgraded nm-connection-editor (1.0.10-1 -> 1.2.0-1)
[2016-04-30 22:34] [ALPM] upgraded network-manager-applet (1.0.10-1 -> 1.2.0-1)
[2016-04-30 22:34] [ALPM] upgraded networkmanager (1.0.12-1 -> 1.2.0-3)
[2016-04-30 22:34] [ALPM] upgraded networkmanager-pptp (1.0.8-1 -> 1.2.0-1)
[2016-04-30 22:34] [ALPM] warning: /etc/pacman.d/mirrorlist installed as /etc/pacman.d/mirrorlist.pacnew
[2016-04-30 22:34] [ALPM] upgraded pacman-mirrorlist (20160422-1 -> 20160424-1)
[2016-04-30 22:34] [ALPM] upgraded perl-dbi (1.634-1 -> 1.636-1)
[2016-04-30 22:34] [ALPM] upgraded smplayer (16.4.0-1 -> 16.4.0-2)
[2016-04-30 22:34] [ALPM] upgraded virtualbox-guest-dkms (5.0.16-4 -> 5.0.18-1)
[2016-04-30 22:34] [ALPM] upgraded virtualbox-guest-utils (5.0.16-4 -> 5.0.18-1)
[2016-04-30 22:34] [ALPM] upgraded wine (1.9.8-1 -> 1.9.9-2)
[2016-04-30 22:34] [ALPM] upgraded xapian-core (1:1.2.22-1 -> 1:1.2.23-1)
[2016-04-30 22:34] [ALPM] transaction completed
[2016-04-30 22:34] [ALPM] running '70-dkms-install.hook'...
[2016-04-30 22:34] [ALPM-SCRIPTLET] ==> No kernel 4.5.1-1-ARCH headers. You must install them to use DKMS!
[2016-04-30 22:34] [ALPM] running 'gdk-pixbuf-query-loaders.hook'...
[2016-04-30 22:34] [ALPM] running 'gio-querymodules.hook'...
[2016-04-30 22:34] [ALPM] running 'glib-compile-schemes.hook'...
[2016-04-30 22:34] [ALPM] running 'gtk-query-immodules-2.0.hook'...
[2016-04-30 22:34] [ALPM] running 'gtk-query-immodules-3.0.hook'...
[2016-04-30 22:34] [ALPM] running 'gtk-update-icon-cache.hook'...
[2016-04-30 22:34] [ALPM] running 'texinfo-install.hook'...
[2016-04-30 22:34] [ALPM] running 'update-desktop-database.hook'...
[2016-04-30 22:34] [ALPM] running 'update-mime-database.hook'...
[2016-04-30 22:39] [PACMAN] Running 'pacman -Sy boost-libs'
[2016-04-30 22:39] [PACMAN] synchronizing package lists
[2016-04-30 22:40] [ALPM] transaction started
[2016-04-30 22:40] [ALPM] reinstalled boost-libs (1.60.0-4)
[2016-04-30 22:40] [ALPM] transaction completed
和网卡关系最大的应该是networkmanger, 从1.0.12-1 升级到了 1.2.0-1
5.现在开始做降级安装
主要参考wiki: https://wiki.archlinux.org/index.php/downgrading_packages
进入目录：
cd /var/cache/pacman/pkg/
运行命令：
pacman -U networkmanager-1.0.12-1-x86_64.pkg.tar.xz
现在可以看到wifi正常工作了。重启后再试，一切正常。