From mboxrd@z Thu Jan 1 00:00:00 1970 From: Ben Hutchings Subject: Re: invalid requirement from ethtool? Date: Mon, 15 Aug 2011 18:24:00 +0100 Message-ID: <1313429040.2731.17.camel@bwh-desktop> References: <20110726124222.GA4842@mtldesk30> Mime-Version: 1.0 Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: 7bit Cc: netdev@vger.kernel.org To: Eli Cohen , David Miller Return-path: Received: from mail.solarflare.com ([216.237.3.220]:13644 "EHLO exchange.solarflare.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751302Ab1HORYK (ORCPT ); Mon, 15 Aug 2011 13:24:10 -0400 In-Reply-To: <20110726124222.GA4842@mtldesk30> Sender: netdev-owner@vger.kernel.org List-ID: On Tue, 2011-07-26 at 15:42 +0300, Eli Cohen wrote: > Hi, > I see the following text in include/linux/ethtool.h and wonder what is > the reasoning for requiring that both params cannot be zero. I could > not track when and who inserted this text as it dates before git was > used to track kernel code, but my feeling is that is related to a > specific hardware limitation. > > /* How many packets to delay an RX interrupt after > * a packet arrives. If 0, only rx_coalesce_usecs is > * used. It is illegal to set both usecs and max frames > * to zero as this would cause RX interrupts to never be > * generated. > */ > __u32 rx_max_coalesced_frames; > > /* How many packets to delay a TX interrupt after > * a packet is sent. If 0, only tx_coalesce_usecs is > * used. It is illegal to set both usecs and max frames > * to zero as this would cause TX interrupts to never be > * generated. > */ > __u32 tx_max_coalesced_frames; > > I found this in tg3 driver: > /* No rx interrupts will be generated if both are zero */ > if ((ec->rx_coalesce_usecs == 0) && > (ec->rx_max_coalesced_frames == 0)) > return -EINVAL; > > However, bnx2 for example allows setting both to zero. > > I think both params zero should be allowed and mean coalescing is not > operational, thus we can remove these comments from ethtool.h If coalescing is not operational, the maximum number of completions before an interrupt is 1. So logically {rx,tx}_max_coalesced_frames should be 1, right? Although the comment does say 'How many packets ... after ...' which implies that the value of the field must be 1 less than the wanted maximum, i.e. 0, which is supposedly invalid. The first implementation of ethtool coalescing control was in tg3, so it should be a useful reference. David, I know you maintained tg3 for some time so I assume you have a hardware reference. Can you confirm whether a value of 1 in HOSTCC_{RX,TX}MAX_FRAMES results in an interrupt after 1 completion or after 2 completions? Ben. -- Ben Hutchings, Staff Engineer, Solarflare Not speaking for my employer; that's the marketing department's job. They asked us to note that Solarflare product names are trademarked.