From mboxrd@z Thu Jan 1 00:00:00 1970 From: Simon Thelen Subject: Low USB polling frequency with RT-PREEMPT Date: Tue, 5 Sep 2017 17:05:39 +0200 Message-ID: <20170905150539.GF2607@anonymous> Mime-Version: 1.0 Content-Type: text/plain; charset="utf-8" To: Return-path: Received: from scc-mailout-kit-02.scc.kit.edu ([129.13.231.82]:50874 "EHLO scc-mailout-kit-02.scc.kit.edu" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751386AbdIEPYZ (ORCPT ); Tue, 5 Sep 2017 11:24:25 -0400 Received: from kit-msx-25.kit.edu ([2a00:1398:9:f612::25]) by scc-mailout-kit-02.scc.kit.edu with esmtps (TLS1.2:ECDHE_RSA_AES_256_CBC_SHA384:256) (envelope-from ) id 1dpFQ4-0001We-7c for linux-rt-users@vger.kernel.org; Tue, 05 Sep 2017 17:05:41 +0200 Content-Disposition: inline Sender: linux-rt-users-owner@vger.kernel.org List-ID: Hello, We have some Dynamixel robitic actuators hooked up via USB to an Ubuntu 14.04 system running the 4.4.79-rt92 kernel. When using a kernel built without the realtime patches (CONFIG_PREEMPT not set) we can query the bus at 1000Hz (2 devices at 500Hz each) whereas with the realtime kernel (CONFIG_PREEMPT=y, CONFIG_PREEMPT_RT_FULL=y) we only get 62.5Hz per device. The USB device requests Ivl=0ms in both cases and the issue isn't related to load etc. The polling rate is always 62.5Hz (It takes ~15ms for every read()). Is this a known limitation of USB with the CONFIG_PREEMPT_RT patches and is there any method to improve the polling rate? -- Simon Thelen