Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752087AbbEKRK5 (ORCPT ); Mon, 11 May 2015 13:10:57 -0400 Received: from foss.arm.com ([217.140.101.70]:58810 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750876AbbEKRKu (ORCPT ); Mon, 11 May 2015 13:10:50 -0400 Date: Mon, 11 May 2015 18:10:46 +0100 From: Catalin Marinas To: Arnd Bergmann Cc: "linaro-acpi@lists.linaro.org" , "rjw@rjwysocki.net" , "will.deacon@arm.com" , "linux-kernel@vger.kernel.org" , "linux-acpi@vger.kernel.org" , "Suthikulpanit, Suravee" , "lenb@kernel.org" , "linux-arm-kernel@lists.infradead.org" , Charles Garcia-Tobin Subject: Re: [Linaro-acpi] [PATCH 2/2] ACPI / scan: Parse _CCA and setup device coherency Message-ID: <20150511171045.GJ18655@e104818-lin.cambridge.arm.com> References: <2817500.sgztgK2NzB@wuerfel> <20150501110644.GF27755@e104818-lin.cambridge.arm.com> <2927907.rePQ55aASC@wuerfel> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <2927907.rePQ55aASC@wuerfel> User-Agent: Mutt/1.5.23 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1869 Lines: 38 On Fri, May 08, 2015 at 04:08:53PM +0200, Arnd Bergmann wrote: > On Friday 01 May 2015 12:06:44 Catalin Marinas wrote: > > > If we just disallow DMA to devices that are marked with _CCA=0 > > > in ACPI, we can avoid this case, or discuss it by the time someone has hardware > > > that wants it, and then make a more informed decision about it. > > > > I don't think we should disallow DMA to devices with _CCA == 0 (only to > > those that don't have a _CCA property at all) as long as _CCA == 0 has > > clear semantics like only architected cache maintenance required (and > > that's what the ARMv8 ARM requires from compliant system caches). > > Even if we exclude all cases in which the behavior may be unexpected, > there is still the other point I raised initially: > > what would that be good for? > > Can you think of a case where a server system has a reason to use > a device in noncoherent mode? I think it's more likely to be a case > where a device got misconfigured accidentally by the firmware, and > we're better off warning about that in the kernel than trying to prepare > for an unknown hardware that might use an obscure feature of the spec. Maybe some of the people involved in arm64 servers can give a better answer, I'm not familiar with their hardware (plans). I would expect most DMA-capable devices to be cache coherent. However, for (system) performance reasons, some of them could be configured as non-coherent. An example, though unlikely on servers, is a display device continuously accessing a framebuffer. You may not want to overload the coherent interconnect. -- Catalin -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/