Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753158AbdF2VAN (ORCPT ); Thu, 29 Jun 2017 17:00:13 -0400 Received: from smtp.codeaurora.org ([198.145.29.96]:54948 "EHLO smtp.codeaurora.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751776AbdF2VAL (ORCPT ); Thu, 29 Jun 2017 17:00:11 -0400 DMARC-Filter: OpenDMARC Filter v1.3.2 smtp.codeaurora.org 4335760D91 Authentication-Results: pdx-caf-mail.web.codeaurora.org; dmarc=none (p=none dis=none) header.from=codeaurora.org Authentication-Results: pdx-caf-mail.web.codeaurora.org; spf=none smtp.mailfrom=sboyd@codeaurora.org Date: Thu, 29 Jun 2017 14:00:02 -0700 From: Stephen Boyd To: Russell King - ARM Linux Cc: Viresh Kumar , Greg Kroah-Hartman , Rafael Wysocki , Vincent Guittot , Rob Herring , linux-kernel@vger.kernel.org, Mark Brown , rnayak@codeaurora.org, Shiraz Hashim , linux-arm-kernel@lists.infradead.org Subject: Re: [RFC 0/5] drivers: Add boot constraints core Message-ID: <20170629210002.GB22780@codeaurora.org> References: <20170629124908.GV4902@n2100.armlinux.org.uk> <20170629145808.GQ29665@vireshk-i7> <20170629154353.GX4902@n2100.armlinux.org.uk> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20170629154353.GX4902@n2100.armlinux.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 Content-Length: 3019 Lines: 60 On 06/29, Russell King - ARM Linux wrote: > On Thu, Jun 29, 2017 at 08:28:08PM +0530, Viresh Kumar wrote: > > On 29-06-17, 13:49, Russell King - ARM Linux wrote: > > > The thing that concerns me most about this is that typically the LCD > > > controller will be performing DMA to system RAM. > > > > > > The location of the frame buffer is unknown to the decompressor - and > > > as the decompressor self-relocates itself (using purely assembly code), > > > it could relocate itself on top of the frame buffer, causing the "nice" > > > image to become very colourful. > > > > > > The decompressor doesn't have the information from DT at that point to > > > know what are safe locations, so it's up to the boot loader to place > > > the frame buffer somewhere out of the way. (If people want to write > > > a DT parser in position independent ARM assembly code that may change.) > > > > > > As long as people realise this, then it's not a problem, but given the > > > number of problems that we've already encountered with boot loaders and > > > memory space layout, I don't trust them to get this right. > > > > > > Right now, the ARM kernel booting document requires: > > > > > > - Quiesce all DMA capable devices so that memory does not get > > > corrupted by bogus network packets or disk data. This will save > > > you many hours of debug. > > > > > > so we would need to modify that to make an exception for LCD controllers. > > > However, we definitely can't have devices left enabled which are capable > > > of writing to system memory, or which changing system memory is likely > > > to cause bad effects (eg, packet ring buffers, USB buffers etc, which is > > > really what the above requirement is about.) > > > > Well, LCD was just an example here. But yeah, it is one of the most > > probable case we have. > > > > So, this thing is already working for sure, of course with some out of > > tree hacks. Every smart phone shows their company's logo (some kind of > > flash) while the phone boots. How do they get around such issues? > > As far as the memory being used goes, they probably locate the frame > buffer well away from the kernel or any area that the kernel is likely > to use during decompression. > > It's probably also marked as a reserved memory region in DT to avoid > the kernel touching it during boot, or _maybe_ they just locate it > somewhere in memory that they've tested that the kernel doesn't touch > until after their kernel has initialised the LCD controller (thereby > avoiding the memory being permanently consumed.) > Yes. The display controller is typically pointed to a memory carveout that we treat as reserved in the kernel. I'm fairly certain that we avoid the "permanently consumed" problem by making it a carveout for the display controller, so that when the display controller probes it can take ownership of the memory from the bootloader. -- Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, a Linux Foundation Collaborative Project