From owner-freebsd-current@FreeBSD.ORG Tue Jan 10 20:18:33 2012 Return-Path: Delivered-To: freebsd-current@FreeBSD.org Received: from mx1.freebsd.org (mx1.freebsd.org [IPv6:2001:4f8:fff6::34]) by hub.freebsd.org (Postfix) with ESMTP id 5A547106564A for ; Tue, 10 Jan 2012 20:18:33 +0000 (UTC) (envelope-from avg@FreeBSD.org) Received: from citadel.icyb.net.ua (citadel.icyb.net.ua [212.40.38.140]) by mx1.freebsd.org (Postfix) with ESMTP id AAD858FC12 for ; Tue, 10 Jan 2012 20:18:32 +0000 (UTC) Received: from porto.starpoint.kiev.ua (porto-e.starpoint.kiev.ua [212.40.38.100]) by citadel.icyb.net.ua (8.8.8p3/ICyb-2.3exp) with ESMTP id WAA06151 for ; Tue, 10 Jan 2012 22:18:31 +0200 (EET) (envelope-from avg@FreeBSD.org) Received: from localhost ([127.0.0.1]) by porto.starpoint.kiev.ua with esmtp (Exim 4.34 (FreeBSD)) id 1Rki9O-0007wo-UO for freebsd-current@freebsd.org; Tue, 10 Jan 2012 22:18:30 +0200 Message-ID: <4F0C9D14.60705@FreeBSD.org> Date: Tue, 10 Jan 2012 22:18:28 +0200 From: Andriy Gapon User-Agent: Mozilla/5.0 (X11; FreeBSD amd64; rv:9.0) Gecko/20111222 Thunderbird/9.0 MIME-Version: 1.0 To: FreeBSD current X-Enigmail-Version: undefined Content-Type: text/plain; charset=X-VIET-VPS Content-Transfer-Encoding: 7bit Cc: Subject: bus dma: a flag/quirk for page zero X-BeenThere: freebsd-current@freebsd.org X-Mailman-Version: 2.1.5 Precedence: list List-Id: Discussions about the use of FreeBSD-current List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Tue, 10 Jan 2012 20:18:33 -0000 Some hardware interfaces may reserve a special meaning for a (physical) memory address value of zero. One example is the OHCI specification where a zero value in CurrentBufferPointer doesn't mean a physical address, but has a reserved meaning. To be honest I don't have another example :) but don't preclude its existence. To deal with this peculiarity we could use a special flag/quirk that would instruct the bus dma code to never use the page zero for communication with the hardware. Here's a proof of concept patch that implements the idea: http://people.freebsd.org/~avg/usb-dma-pagezero.diff Some concerns: - not sure if BUS_DMA_NO_PAGEZERO is the best name for the flag - the patch implements the flag only for x86 at the moment - usb code uses the flag regardless of the actual controller type What do you think? -- Andriy Gapon