Re: __pa() versus virt_to_phys()

[Lukas Jelinek <info@kernel-api.org>]

> Could anybody tell me the difference between these two symbols ?
> 
> I know that one is a macro and the other one is an inline function,
> so the latter performs type checkings but I don't see anything else.
> 
> thanks
> 
> Francis
> 

Here is the answer (by Dave Hansen, 2004):

__pa() is simply supposed to be the addr-PAGE_OFFSET calculation.
virt_to_phys() will be guaranteed to take care of any layout changes if
kernel addresses ever fail to be mapped flat, and 1:1 with the physical
address layout.

So, let's say that someone is working on ... say ... memory hotplug.
They will be modifying the virt_to_phys() function to make up for any
weird mappings that are going on.  But, they'll leave __{v,p}a alone,
because those are used for stuff that occurs very early, even at compile
time.

More virt_to_phys() and less __pa() will save me lots of auditing later
on :)  If you're not in early boot, or really know what you're doing,
use virt_to_phys() and cousins.

Plus, it's more type safe.



Lukas Jelinek