From mboxrd@z Thu Jan 1 00:00:00 1970 From: Sergei Shtylyov Subject: NETIF_F_HIGHDMA? Date: Thu, 13 Feb 2014 02:00:19 +0300 Message-ID: <52FBFD03.6000703@cogentembedded.com> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit To: David Miller , netdev@vger.kernel.org Return-path: Received: from mail-lb0-f178.google.com ([209.85.217.178]:47139 "EHLO mail-lb0-f178.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751218AbaBLWAS (ORCPT ); Wed, 12 Feb 2014 17:00:18 -0500 Received: by mail-lb0-f178.google.com with SMTP id u14so7723772lbd.9 for ; Wed, 12 Feb 2014 14:00:17 -0800 (PST) Sender: netdev-owner@vger.kernel.org List-ID: Hello. I fail to understand why this flag is needed at all. netdev-features.txt says: * Transmit DMA from high memory On platforms where this is relevant, NETIF_F_HIGHDMA signals that ndo_start_xmit can handle skbs with frags in high memory. TTBOMK, highmem is an area of the kernel *virtual* memory that maps physical memory that can't be directly mapped unto the linear kernel space, i.e. in case when there's more memory in the system than the kernel memory can directly map. But what this has to do with DMA which deals with the physical memory? High memory doesn't have to be beyond 4 GiB limit -- it can exist even on 32-bit system (consider 4 GiB memory and 2/2 or 3/1 GiB user/kernel VM split), contrary to the belief of the many Ethernet PCI drivers which only set NETIF_F_HIGHDMA when the PCI dual address cycle is supported (i.e. the address on PCI can be 64-bit)... So, in my opinion, each and every driver should set this flag (except maybe those arch-specific drivers where CONFIG_HIGHMEM is not supported but even it this case the flag wouldn't hurt), and hence it's not needed at all. What am I missing in my picture? WBR, Sergei