GLSL Fragment Linear Depth

I have been trying for a while to get Linear Depth without using a texture. I am trying to compute it in the vertex shader and use it in the fragment shader. I have tried three different approaches below. I also tried to linearlize based on far and near clip planes. So far 3 game me the most accurate result but it still not perfect. In my technique I have: depthtest="always" depthwrite="false"

// 1
// VS
vWorldPos = vec4(worldPos, GetDepth(gl_Position));
// PS
float depth = vWorldPos.z/vWorldPos.w;

// 2
// PS
float depth = gl_FragCoord.z;

// 3
varying float v_depth;
// VS
v_depth = -(inverse(cViewInv * cViewProj) * gl_Position).z;
// PS
depth = v_depth;