Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753105AbbB1QBW (ORCPT ); Sat, 28 Feb 2015 11:01:22 -0500 Received: from mail-gw2-out.broadcom.com ([216.31.210.63]:14636 "EHLO mail-gw2-out.broadcom.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753058AbbB1QBP (ORCPT ); Sat, 28 Feb 2015 11:01:15 -0500 X-IronPort-AV: E=Sophos;i="5.09,667,1418112000"; d="scan'208";a="58250459" Message-ID: <54F1E647.10106@broadcom.com> Date: Sat, 28 Feb 2015 08:01:11 -0800 From: Scott Branden User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.4.0 MIME-Version: 1.0 To: Arnd Bergmann CC: , Rob Herring , Pawel Moll , Mark Rutland , "Ian Campbell" , Kumar Gala , Matt Mackall , Herbert Xu , Grant Likely , Ray Jui , Jonathan Richardson , Dmitry Torokhov , Anatol Pomazao , , , Subject: Re: [PATCH v2 2/2] hwrng: iproc-rng200 - Add Broadcom IPROC RNG driver References: <1424888184-22180-3-git-send-email-sbranden@broadcom.com> <4488852.TuVxKbAYde@wuerfel> <54EF9D7A.3090707@broadcom.com> <2093191.gujNGSl66k@wuerfel> In-Reply-To: <2093191.gujNGSl66k@wuerfel> Content-Type: text/plain; charset="windows-1252"; format=flowed Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1895 Lines: 44 On 15-02-27 01:14 AM, Arnd Bergmann wrote: > On Thursday 26 February 2015 14:26:02 Scott Branden wrote: >> On 15-02-26 12:15 PM, Arnd Bergmann wrote: >>>> On 15-02-25 11:17 AM, Arnd Bergmann wrote: >>>>> On Wednesday 25 February 2015 10:16:24 Scott Branden wrote: >>>> This code was following examples of other open source drivers - bcm2835 >>>> and exynos both use cpu_relax. I'll have to look into this more to >>>> understand. >>>> >>> >>> The majority of the driver apparently use udelay(10) to wait, which is >>> not much better but at least consistent. The cpu_relax() call probably >>> gives better throughput. >>> >>> I don't understand why none of the drivers actually attempts to >>> msleep(), but that may be because the delay is much too long. >>> >>> Can you find out what the expected latency is for new data to >>> become available on your hardware? >> RNG generates at a nominal 1 Mbps. So to generate 32 bits of data takes >> approximately 32 us. > > The udelay(10) that the other drivers have seems about appropriate then, > and we can independently think of a way to refine the interface. > Please add a comment that explains the rate. Also, is there some kind > of FIFO present in the hwrng device? If it can store close to 1ms work > of data (1000 bits), you can just use an msleep(1) to wait for the > pool to recover. FIFO is 512 bits. I will look as to whether we can live with 1/2 throughput. > > Another option would be to use usleep_range() with the exact amount > of time to wait for, the lower bound being the minimum number of > bytes asked for and the fifo size, the upper bound being the fifo > size. > > Arnd > -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/