From mboxrd@z Thu Jan 1 00:00:00 1970 From: Scott Branden Subject: Re: [PATCH v2 2/2] hwrng: iproc-rng200 - Add Broadcom IPROC RNG driver Date: Sat, 28 Feb 2015 08:01:11 -0800 Message-ID: <54F1E647.10106@broadcom.com> References: <1424888184-22180-3-git-send-email-sbranden@broadcom.com> <4488852.TuVxKbAYde@wuerfel> <54EF9D7A.3090707@broadcom.com> <2093191.gujNGSl66k@wuerfel> Mime-Version: 1.0 Content-Type: text/plain; charset="windows-1252"; format=flowed Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <2093191.gujNGSl66k@wuerfel> Sender: devicetree-owner-u79uwXL29TY76Z2rM5mHXA@public.gmane.org To: Arnd Bergmann Cc: bcm-kernel-feedback-list-dY08KVG/lbpWk0Htik3J/w@public.gmane.org, Rob Herring , Pawel Moll , Mark Rutland , Ian Campbell , Kumar Gala , Matt Mackall , Herbert Xu , Grant Likely , Ray Jui , Jonathan Richardson , Dmitry Torokhov , Anatol Pomazao , devicetree-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, linux-arm-kernel-IAPFreCvJWM7uuMidbF8XUB+6BGkLq7r@public.gmane.org, linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org List-Id: devicetree@vger.kernel.org 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 devicetree" in the body of a message to majordomo-u79uwXL29TY76Z2rM5mHXA@public.gmane.org More majordomo info at http://vger.kernel.org/majordomo-info.html