How Qt update framebuffer memory

hi,

I have few queries reading frame buffer,

1) Whether Qt update whole part of the frame buffer memory OR only a part of the frame buffer memory changes when window add or remove, hide widgets.

2) How Qt intimate the Linux that frame buffer content has changed? Or Display using frame buffer memory is managed by kernel and it doesn't have any relation with Qt(Qt only write to framebuffer memory)?

3) According me as per the qt code, qt has maped (mmap()) memory with /dev/fb0. so any write to qt memory will impact on /dev/fb0.

Your suggestions would be highly appriciable.