Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S937945AbdCJO1V (ORCPT ); Fri, 10 Mar 2017 09:27:21 -0500 Received: from foss.arm.com ([217.140.101.70]:58766 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S933114AbdCJO1Q (ORCPT ); Fri, 10 Mar 2017 09:27:16 -0500 Date: Fri, 10 Mar 2017 14:27:09 +0000 From: Brian Starkey To: Robin Murphy Cc: Laura Abbott , devel@driverdev.osuosl.org, Rom Lemarchand , Linux Kernel Mailing List , Riley Andrews , "dri-devel@lists.freedesktop.org" , Michal Hocko , "linaro-mm-sig@lists.linaro.org" , linux-mm@kvack.org, Arve =?iso-8859-1?B?SGr4bm5lduVn?= , Mark Brown , Benjamin Gaignard , Greg Kroah-Hartman , Daniel Vetter , Sumit Semwal , linux-arm-kernel@lists.infradead.org, "linux-media@vger.kernel.org" Subject: Re: [RFC PATCH 00/12] Ion cleanup in preparation for moving out of staging Message-ID: <20170310142709.GB15945@e106950-lin.cambridge.arm.com> References: <20170303132949.GC31582@dhcp22.suse.cz> <20170306074258.GA27953@dhcp22.suse.cz> <20170306104041.zghsicrnadoap7lp@phenom.ffwll.local> <20170306105805.jsq44kfxhsvazkm6@sirena.org.uk> <20170306160437.sf7bksorlnw7u372@phenom.ffwll.local> <26bc57ae-d88f-4ea0-d666-2c1a02bf866f@redhat.com> <20170310103112.GA15945@e106950-lin.cambridge.arm.com> <2e295c28-04d6-a071-cf16-164d02d679c5@arm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii; format=flowed Content-Disposition: inline In-Reply-To: <2e295c28-04d6-a071-cf16-164d02d679c5@arm.com> 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: 2695 Lines: 69 On Fri, Mar 10, 2017 at 11:46:42AM +0000, Robin Murphy wrote: >On 10/03/17 10:31, Brian Starkey wrote: >> Hi, >> >> On Thu, Mar 09, 2017 at 09:38:49AM -0800, Laura Abbott wrote: >>> On 03/09/2017 02:00 AM, Benjamin Gaignard wrote: >> >> [snip] >> >>>> >>>> For me those patches are going in the right direction. >>>> >>>> I still have few questions: >>>> - since alignment management has been remove from ion-core, should it >>>> be also removed from ioctl structure ? >>> >>> Yes, I think I'm going to go with the suggestion to fixup the ABI >>> so we don't need the compat layer and as part of that I'm also >>> dropping the align argument. >>> >> >> Is the only motivation for removing the alignment parameter that >> no-one got around to using it for something useful yet? >> The original comment was true - different devices do have different >> alignment requirements. >> >> Better alignment can help SMMUs use larger blocks when mapping, >> reducing TLB pressure and the chance of a page table walk causing >> display underruns. > >For that use-case, though, alignment alone doesn't necessarily help - >you need the whole allocation granularity to match your block size (i.e. >given a 1MB block size, asking for 17KB and getting back 17KB starting >at a 1MB boundary doesn't help much - that whole 1MB needs to be >allocated and everyone needs to know it to ensure that the whole lot can >be mapped safely). Now, whether it's down to the callers or the heap >implementations to decide and enforce that granularity is another >question, but provided allocations are at least naturally aligned to >whatever the granularity is (which is a reasonable assumption to bake >in) then it's all good. > >Robin. Agreed, alignment alone isn't enough. But lets assume that an app knows what a "good" granularity is, and always asks for allocation sizes which are suitably rounded to allow blocks to be used. Currently it looks like a "standard" ION_HEAP_TYPE_CARVEOUT heap would give me back just a PAGE_SIZE aligned buffer. So even *if* the caller knows its desired block size, there's no way for it to get guaranteed better alignment, which wouldn't be a bad feature to have. Anyway as Daniel and Rob say, if the interface is designed properly this kind of extension would be possible later, or you can have a special heap with a larger granule. I suppose it makes sense to remove it while there's no-one actually implementing it, in case an alternate method proves more usable. -Brian > >> >> -Brian >> >> _______________________________________________ >> linux-arm-kernel mailing list >> linux-arm-kernel@lists.infradead.org >> http://lists.infradead.org/mailman/listinfo/linux-arm-kernel >