[yocto] SRCREV spec not working for kernel git recipes

[Elvis Dowson]

Hi,
        I found out that if I specify SRCREV_pn-${PN}, it works and picks up the correct commit id, as follows:

SRCREV_pn-${PN} = "a5556fb251253ccb674bd22d9703c37f6f8807a8"

However, if you simply specifying SRCREV =  "a5556fb251253ccb674bd22d9703c37f6f8807a8", it has no effect and it always picks up the latest commit from the git repository.

It would be great if someone could please explain why this is so, because I search through the Yocto documentation and it isn't mentioned anywhere. The OE classic way of doing things, it used to pick up the correct commit by just specifying SRCREV. Now I find that you have to specify SRCREV_pn-${PN} to get it to work. 

Is this documented somewhere?

Best regards,

Elvis Dowson