I have done a lot of work to resolve the issue, and have even searched for the solution, but no result.
Suddenly I struck with this problem, probably due to some unknown short key combination. Brush in Adobe Photoshop, has become rough edged. Especially when its Hardness is 0, it looks like made up of particles only.
Luckily the culprit is right there in your screenshot. Your brush mode is set to
Dissolve. You should set that to
Note that a layer can also have a blend mode “dissolve”.
Source : Link , Question Author : Ahmed Naveed , Answer Author : Joonas