RAID-5 Parity calculation

Hi, I am a new guy for a Linux programming. I got a question for a
software RAID-5 parity calculation. 

If we suppose for data block D1, D2, D3, D4 with a parity block P, When
some data block is changed, P will be changed. I am not so clear how
does Linux md handle the parity re calculating, when data changed . 

Suppose there will be three cases, 

1. sequential write, is that get new P' directly with new data D1', D2',
D3' and D4' or need read D1-4 out from disk doing the normal operation. 

2. if we changed there blocks, D1',D2' and D3', does md read D1, D2, D3
out to rebuild P or just read D4 out from disk and rebuild P with D1',
D2', D3' and D4. 

3. is the simple case, just change one block. 

Thanks a lot, if some can help me get understand it.