From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756647AbaEEVKD (ORCPT ); Mon, 5 May 2014 17:10:03 -0400 Received: from shards.monkeyblade.net ([149.20.54.216]:33761 "EHLO shards.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755066AbaEEVKB convert rfc822-to-8bit (ORCPT ); Mon, 5 May 2014 17:10:01 -0400 Date: Mon, 05 May 2014 17:09:59 -0400 (EDT) Message-Id: <20140505.170959.2172440110209805460.davem@davemloft.net> To: soren.brinkmann@xilinx.com Cc: michal.simek@xilinx.com, nicolas.ferre@atmel.com, git@xilinx.com, linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org, netdev@vger.kernel.org Subject: Re: [PATCH 5/5] net: macb: Fix race between HW and driver From: David Miller In-Reply-To: <7c62baab-5f60-4f04-87f7-6b53fecc4746@CH1EHSMHS038.ehs.local> References: <1399243382-12528-6-git-send-email-soren.brinkmann@xilinx.com> <20140505.165653.1184059074645793091.davem@davemloft.net> <7c62baab-5f60-4f04-87f7-6b53fecc4746@CH1EHSMHS038.ehs.local> X-Mailer: Mew version 6.5 on Emacs 24.1 / Mule 6.0 (HANACHIRUSATO) Mime-Version: 1.0 Content-Type: Text/Plain; charset=iso-8859-1 Content-Transfer-Encoding: 8BIT X-Greylist: Sender succeeded SMTP AUTH, not delayed by milter-greylist-4.5.7 (shards.monkeyblade.net [149.20.54.216]); Mon, 05 May 2014 14:10:01 -0700 (PDT) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org From: Sören Brinkmann Date: Mon, 5 May 2014 14:05:19 -0700 > On Mon, 2014-05-05 at 04:56PM -0400, David Miller wrote: >> From: Soren Brinkmann >> Date: Sun, 4 May 2014 15:43:02 -0700 >> >> > Under "heavy" RX load, the driver cannot handle the descriptors fast >> > enough. In detail, when a descriptor is consumed, its used flag is >> > cleared and once the RX budget is consumed all descriptors with a >> > cleared used flag are prepared to receive more data. Under load though, >> > the HW may constantly receive more data and use those descriptors with a >> > cleared used flag before they are actually prepared for next usage. >> > >> > The head and tail pointers into the RX-ring should always be valid and >> > we can omit clearing and checking of the used flag. >> > >> > Signed-off-by: Soren Brinkmann >> >> Isn't the RX_USED bit the only thing that controls what RX entries >> the chip will try to use? >> >> I can't see how you can just remove the RX_USED bit handling >> altogether. >> >> The problem actually seems to be that in the current code we clear the >> RX_USED bit before we actually reallocate the buffer and set it up. >> >> It should be a bug to see the RX_USED bit set in gem_rx_refill(), and >> the only reason why it can happen is exactly because you're clearing it >> too early in gem_rx(). > > I don't follow. The HW uses the descriptor and the driver handles the > received data. So, in gem_rx_refill we should actually only replace > descriptor which have the RX_USED _set_, not? Currently the test tests > for the opposite, since SW clears RX_USED in gem_rx. This patch just > removes those two parts. The RX_USED is left as is (HW should have set > it). And in gem_rx_refill we simply rely on the head and tail pointers > to refill the used descriptors. I didn't see a reason to do the additional > checking of the RX_USED bit. > After the refill the RX_USED flags are of course cleared for all > refilled descriptors. Aha, that makes sense, I didn't notice that RX_USED is cleared as a side effect of gem_rx_refill()'s work. I'll apply this series, thanks for explaining.