Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932324AbbBZW0d (ORCPT ); Thu, 26 Feb 2015 17:26:33 -0500 Received: from mail-gw3-out.broadcom.com ([216.31.210.64]:52234 "EHLO mail-gw3-out.broadcom.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932263AbbBZW0b (ORCPT ); Thu, 26 Feb 2015 17:26:31 -0500 X-IronPort-AV: E=Sophos;i="5.09,655,1418112000"; d="scan'208";a="57987868" Message-ID: <54EF9D7A.3090707@broadcom.com> Date: Thu, 26 Feb 2015 14:26:02 -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> <3062825.7b1oYP4yuL@wuerfel> <54EF75F0.7000901@broadcom.com> <4488852.TuVxKbAYde@wuerfel> In-Reply-To: <4488852.TuVxKbAYde@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: 2438 Lines: 68 Hi Arnd, Latency is 32 us for 32bits of data - commented inline. What delay call do you recommend in this case? On 15-02-26 12:15 PM, Arnd Bergmann wrote: > On Thursday 26 February 2015 11:37:20 Scott Branden wrote: >> Response inline. >> >> On 15-02-25 11:17 AM, Arnd Bergmann wrote: >>> On Wednesday 25 February 2015 10:16:24 Scott Branden wrote: >>>> This adds a driver for random number generator present on Broadcom >>>> IPROC devices. >>>> >>>> Reviewed-by: Ray Jui >>>> Signed-off-by: Scott Branden >>> >>> The driver looks reasonable overall, I have just one question about >>> something that sticks out: >>> >>>> + while ((num_remaining > 0) && time_before(jiffies, idle_endtime)) { >>> ... >>>> + >>>> + /* Are there any random numbers available? */ >>>> + if ((ioread32(rng_base + RNG_FIFO_COUNT_OFFSET) & >>>> + RNG_FIFO_COUNT_RNG_FIFO_COUNT_MASK) > 0) { >>> ... >>>> + } else { >>>> + if (!wait) >>>> + /* Cannot wait, return immediately */ >>>> + return max - num_remaining; >>>> + >>>> + /* Can wait, give others chance to run */ >>>> + cpu_relax(); >>>> + } >>>> + } >>>> + >>> >>> It looks like you do a busy-loop around cpu_relax here if asked to wait. >>> Is this intentional? I would normally expect either cond_resched() or >>> some msleep() instead. >> >> 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. > > 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/