From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753105AbbJFUve (ORCPT ); Tue, 6 Oct 2015 16:51:34 -0400 Received: from mail-wi0-f170.google.com ([209.85.212.170]:35752 "EHLO mail-wi0-f170.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753018AbbJFUvc (ORCPT ); Tue, 6 Oct 2015 16:51:32 -0400 Date: Tue, 6 Oct 2015 21:51:22 +0100 From: Lee Jones To: Russell King - ARM Linux Cc: linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, peter@korsgaard.com, kernel@stlinux.com, daniel.thompson@linaro.org, pankaj.dev@st.com, festevam@gmail.com, herbert@gondor.apana.org.au Subject: Re: [PATCH 3/3] hwrng: st: Use real-world device timings for timeout Message-ID: <20151006205122.GF17172@x1> References: <1444142640-14721-1-git-send-email-lee.jones@linaro.org> <1444142640-14721-3-git-send-email-lee.jones@linaro.org> <20151006193757.GY21513@n2100.arm.linux.org.uk> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <20151006193757.GY21513@n2100.arm.linux.org.uk> User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, 06 Oct 2015, Russell King - ARM Linux wrote: > On Tue, Oct 06, 2015 at 03:44:00PM +0100, Lee Jones wrote: > > Samples are documented to be available every 0.667us, so in theory > > the 8 sample deep FIFO should take 5.336us to fill. However, during > > thorough testing, it became apparent that filling the FIFO actually > > takes closer to 12us. > > Is that measured? I measured it using ktime. Hopefully that was adequate. > > +/* > > + * Samples are documented to be available every 0.667us, so in theory > > + * the 8 sample deep FIFO should take 5.336us to fill. However, during > > + * thorough testing, it became apparent that filling the FIFO actually > > + * takes closer to 12us. > > + */ > > +#define ST_RNG_FILL_FIFO_TIMEOUT 12 > > I hope you're not using such a precise figure with udelay(). udelay() > is not guaranteed to give exactly (or even at least) the delay you > request. It's defined to give an approximate delay. > > Many people have a problem understanding that, so I won't explain why > it is that way, just accept that it is and move on... it's not going > to magically get "fixed" because someone has just learnt about this. :) Thanks for the info. I did do testing, again using ktime, to make sure and on our platform (is it platform specific?) I measured udelay(1) to be ~1100ns. After moving to a 12us timeout and reading many MBs of randomness I am yet to receive any more timeouts. -- Lee Jones Linaro STMicroelectronics Landing Team Lead Linaro.org │ Open source software for ARM SoCs Follow Linaro: Facebook | Twitter | Blog