Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753837AbaKEHUP (ORCPT ); Wed, 5 Nov 2014 02:20:15 -0500 Received: from mail-ig0-f181.google.com ([209.85.213.181]:42753 "EHLO mail-ig0-f181.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753766AbaKEHUI (ORCPT ); Wed, 5 Nov 2014 02:20:08 -0500 MIME-Version: 1.0 In-Reply-To: References: Date: Wed, 5 Nov 2014 15:20:07 +0800 Message-ID: Subject: Re: CMA alignment question From: Weijie Yang To: Gregory Fong Cc: Michal Nazarewicz , linux-mm@kvack.org, Laura Abbott , iamjoonsoo.kim@lge.com, Marek Szyprowski , "linux-kernel@vger.kernel.org" , Florian Fainelli , Brian Norris Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Nov 5, 2014 at 12:18 PM, Gregory Fong wrote: > On Tue, Nov 4, 2014 at 2:27 PM, Michal Nazarewicz wrote: >> On Tue, Nov 04 2014, Gregory Fong wrote: >>> The alignment in cma_alloc() is done w.r.t. the bitmap. This is a >>> problem when, for example: >>> >>> - a device requires 16M (order 12) alignment >>> - the CMA region is not 16 M aligned I think the device driver should ensure that situation could not occur, by assign suitable alignment parameter in cma_declare_contiguous(). >>> In such a case, can result with the CMA region starting at, say, >>> 0x2f800000 but any allocation you make from there will be aligned from >>> there. Requesting an allocation of 32 M with 16 M alignment, will >>> result in an allocation from 0x2f800000 to 0x31800000, which doesn't >>> work very well if your strange device requires 16M alignment. >>> >>> This doesn't have the behavior I would expect, which would be for the >>> allocation to be aligned w.r.t. the start of memory. I realize that >>> aligning the CMA region is an option, but don't see why cma_alloc() >>> aligns to the start of the CMA region. Is there a good reason for >>> having cma_alloc() alignment work this way? >> >> No, it's a bug. The alignment should indicate alignment of physical >> address not position in CMA region. >> > > Ah, now I see that Marek submitted this patch from you back in 2011 > that would have allowed the bitmap lib to support an alignment offset: > http://thread.gmane.org/gmane.linux.kernel/1121103/focus=1121100 > > Any idea why this didn't make it into the later changesets? If not, > I'll resubmit it and to use it to fix this bug. > > Thanks, > Gregory > -- > 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/ -- 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/