From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752502Ab1KYSKm (ORCPT ); Fri, 25 Nov 2011 13:10:42 -0500 Received: from bipbip.grupopie.com ([195.23.16.24]:40252 "EHLO bipbip.grupopie.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751604Ab1KYSKl (ORCPT ); Fri, 25 Nov 2011 13:10:41 -0500 Message-ID: <4ECFDA1D.3090303@grupopie.com> Date: Fri, 25 Nov 2011 18:10:37 +0000 From: Paulo Marques Organization: Grupo PIE User-Agent: Thunderbird 2.0.0.23 (X11/20090817) MIME-Version: 1.0 To: Nuno Santos CC: Jiri Slaby , linux-kernel@vger.kernel.org Subject: Re: Floating point usage inside kernel References: <4ECF789F.3040001@edigma.com> <4ECF8528.9080800@gmail.com> <4ECFBF56.4000002@edigma.com> <4ECFCAD5.5040606@grupopie.com> <4ECFD488.4060805@edigma.com> In-Reply-To: <4ECFD488.4060805@edigma.com> Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Nuno Santos wrote: >>[...] >> Unless you have overflow or need more than 16 bits of fractional >> precision, you'll have no problem with this approach. >> >> I hope this helps, > Sorry, i'm not sure if I have completely understand your suggestion. Are > you telling me to apply this transform only to my input data, or to all > the operations that are applied in the function used in kernel? Imagine that your matrix coefficients are: 2.5, 1, 4.7 45.3, 0.765, 10 0, 0, 1 and your input is: 3420.56, 5410.76, 1 You start by converting the matrix coefficients: 2.5 * 65536.0 = 163840 .... so the matrix becomes: 163840 65536 308019 2968781 50135 655360 0 0 65536 This can be done in userspace and the coefficients can be sent to the kernel as fixed point numbers. You do the same (this time on the kernel) with your input, so it becomes: 224169820 354599567 65536 Now you can do: q[0] = fixed_mul(p[0], a[0][0]) + fixed_mul(p[1], a[1][0]) + fixed_mul(p[2], a[2][0]); .... where "fixed_mul" is a function that does the multiplication as I explained earlier. To convert the result back to an integer, just shift down by 16. -- Paulo Marques - www.grupopie.com "Feed the hungry, save the whales, free the mallocs!"