From mboxrd@z Thu Jan 1 00:00:00 1970 From: Lino Sanfilippo Subject: Question concerning function illegal_highdma Date: Tue, 8 Mar 2016 11:32:26 +0100 Message-ID: <56DEAA3A.4080207@marvell.com> Mime-Version: 1.0 Content-Type: text/plain; charset="utf-8"; format=flowed Content-Transfer-Encoding: 7bit Cc: Lino Sanfilippo , To: Return-path: Received: from mx0b-0016f401.pphosted.com ([67.231.156.173]:22271 "EHLO mx0b-0016f401.pphosted.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932828AbcCHKcf (ORCPT ); Tue, 8 Mar 2016 05:32:35 -0500 Sender: netdev-owner@vger.kernel.org List-ID: 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