linux-kernel - Question concerning function illegal

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Date:	Tue, 8 Mar 2016 11:32:26 +0100
From:	Lino Sanfilippo <lsanfil@...vell.com>
To:	<netdev@...r.kernel.org>
CC:	Lino Sanfilippo <LinoSanfilippo@....de>,
	<linux-kernel@...r.kernel.org>
Subject: Question concerning function illegal_highdma

Hi all,

while trying to understand the meaning of the net_device flag NETIF_F_HIGHDMA
I came across the function illegal_highdma() which seems to perform two tasks:

1. Check if a network interface is able to handle high memory pages
2. Check if a network interface is able to handle the resulting DMA addresses

What I dont understand is why both conditions are only checked if CONFIG_HIGHMEM is set.
For 1. this makes sense, of course. But why is 2. also only checked with CONFIG_HIGHMEM?
Should not 2. always be checked, regardless of whether high memory exists in the system or not?

Thanks for any enlightenment in advance!

Regards,
Lino