Ubuntu14.04 Nvidia Graphics Driver and CUDA Toolkit Installation

Hardware: Thinkpad W550s with NVIDIA Quadro K620M 2GB

% a simple method

Ctrl+Alt+F1;  sudo stop lightdm
sudo dpkg -i cuda-repo-<distro>_<version>_<architecture>.deb
sudo apt-get update
sudo apt-get install cuda
sudo reboot

download deb file from: https://developer.nvidia.com/cuda-downloads

maybe need:
sudo add-apt-repository ppa:bumblebee/stable
sudo apt-get update
sudo apt-get install bumblebee bumblebee-nvidia

% original method

driver: NVIDIA-Linux-x86_64-346.59.run
CUDA Toolkit: cuda_7.0.28_linux.run

Steps:

(1) Ctrl+Alt+F1; sudo stop lightdm; sudo sh NVIDIA-Linux-x86_64-346.59.run;

(2) Create a file at /etc/modprobe.d/blacklist-nouveau.conf with the following contents:
blacklist nouveau
options nouveau modeset=0

sudo vi /etc/modprobe.d/blacklist.conf

Regenerate the kernel initrd: $ sudo /sbin/mkinitrd

(3) sudo apt-get install bumblebee-nvidia

(4) prime-select query
sudo update-alternatives –config i386-linux-gnu_gl_conf
sudo update-alternatives –config x86_64-linux-gnu_gl_conf

(5) sudo reboot

(6) sudo sh cuda_7.0.28_linux.run
However, this will again cause the problem that ubuntu desktop does not appear. How to address?

Others:

Clean nvidia drivers:

sudo apt-get remove --purge nvidia-*
sudo apt-get install ubuntu-desktop
sudo rm /etc/X11/xorg.conf
echo 'nouveau' | sudo tee -a /etc/modules