Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753305AbaKEETa (ORCPT ); Tue, 4 Nov 2014 23:19:30 -0500 Received: from mail-qg0-f52.google.com ([209.85.192.52]:62891 "EHLO mail-qg0-f52.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752388AbaKEET3 (ORCPT ); Tue, 4 Nov 2014 23:19:29 -0500 MIME-Version: 1.0 In-Reply-To: References: From: Gregory Fong Date: Tue, 4 Nov 2014 20:18:58 -0800 Message-ID: Subject: Re: CMA alignment question To: Michal Nazarewicz Cc: 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 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 >> >> 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/