From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-qg0-f48.google.com (mail-qg0-f48.google.com [209.85.192.48]) by kanga.kvack.org (Postfix) with ESMTP id D4A686B00AF for ; Tue, 4 Nov 2014 23:19:29 -0500 (EST) Received: by mail-qg0-f48.google.com with SMTP id q108so11724681qgd.35 for ; Tue, 04 Nov 2014 20:19:29 -0800 (PST) Received: from mail-qc0-x22d.google.com (mail-qc0-x22d.google.com. [2607:f8b0:400d:c01::22d]) by mx.google.com with ESMTPS id l9si4252572qae.53.2014.11.04.20.19.28 for (version=TLSv1 cipher=ECDHE-RSA-RC4-SHA bits=128/128); Tue, 04 Nov 2014 20:19:29 -0800 (PST) Received: by mail-qc0-f173.google.com with SMTP id x3so12109031qcv.18 for ; Tue, 04 Nov 2014 20:19:28 -0800 (PST) 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 Content-Type: text/plain; charset=UTF-8 Sender: owner-linux-mm@kvack.org List-ID: 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 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, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org