From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755104AbaKUL1C (ORCPT ); Fri, 21 Nov 2014 06:27:02 -0500 Received: from mout.kundenserver.de ([212.227.17.24]:57950 "EHLO mout.kundenserver.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750856AbaKUL1A (ORCPT ); Fri, 21 Nov 2014 06:27:00 -0500 From: Arnd Bergmann To: Catalin Marinas Cc: "linux-arm-kernel@lists.infradead.org" , Will Deacon , "linux-kernel@vger.kernel.org" , Ding Tianhong Subject: Re: For the problem when using swiotlb Date: Fri, 21 Nov 2014 12:26:45 +0100 Message-ID: <2310899.ejrSzj6BIr@wuerfel> User-Agent: KMail/4.11.5 (Linux/3.16.0-10-generic; KDE/4.11.5; x86_64; ; ) In-Reply-To: <20141121110609.GC19783@e104818-lin.cambridge.arm.com> References: <5469E26B.2010905@huawei.com> <3918100.vD6471iH9k@wuerfel> <20141121110609.GC19783@e104818-lin.cambridge.arm.com> MIME-Version: 1.0 Content-Transfer-Encoding: 7Bit Content-Type: text/plain; charset="us-ascii" X-Provags-ID: V02:K0:LF0qIIwSd6uI6+Yc7gJA1U6iyc5cCXy0opzFR7AQ9gh LVhoytpO6yDJvkrJI/Jwv+WD1Uwsnr7z4e+gCT7XWovUJ62IQt 7Rl0Av1PSWpmgZXr1CcYCpvvedUwQGtXWmWFOBqXRMqHL1t4jw 787kZYJjwTvHlEHN47ODQ6krObdgZ0wuha72hSoR2uVQpFGcew CP9Uwk79FXgd5Oxbcq/Q46RM9dBhjynK9xuk2PKDtgQItB1ehi RjDptVaV4kVF658ifFmj5kjm77LhA297mTHNy7MjOCsEGuFhp2 TB8oBPW2KVARF2ZGX+veybsyToDE1Mj/HHu29nP+QcKidUoKjI +ZIwriJsKo+ffML2DNQg= X-UI-Out-Filterresults: notjunk:1; Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Friday 21 November 2014 11:06:10 Catalin Marinas wrote: > On Wed, Nov 19, 2014 at 03:56:42PM +0000, Arnd Bergmann wrote: > > On Wednesday 19 November 2014 15:46:35 Catalin Marinas wrote: > > > Going back to original topic, the dma_supported() function on arm64 > > > calls swiotlb_dma_supported() which actually checks whether the swiotlb > > > bounce buffer is within the dma mask. This transparent bouncing (unlike > > > arm32 where it needs to be explicit) is not always optimal, though > > > required for 32-bit only devices on a 64-bit system. The problem is when > > > the driver is 64-bit capable but forgets to call > > > dma_set_mask_and_coherent() (that's not the only question I got about > > > running out of swiotlb buffers). > > > > I think it would be nice to warn once per device that starts using the > > swiotlb. Really all 32-bit DMA masters should have a proper IOMMU > > attached. > > It would be nice to have a dev_warn_once(). > > I think it makes sense on arm64 to avoid swiotlb bounce buffers for > coherent allocations altogether. The __dma_alloc_coherent() function > already checks coherent_dma_mask and sets GFP_DMA accordingly. If we > have a device that cannot even cope with a 32-bit ZONE_DMA, we should > just not support DMA at all on it (without an IOMMU). The arm32 > __dma_supported() has a similar check. If we ever encounter this case, we may have to add a smaller ZONE_DMA and use ZONE_DMA32 for the normal dma allocations. > Swiotlb is still required for the streaming DMA since we get bouncing > for pages allocated outside the driver control (e.g. VFS layer which > doesn't care about GFP_DMA), hoping a 16M bounce buffer would be enough. > > Ding seems to imply that CMA fixes the problem, which means that the > issue is indeed coherent allocations. I wonder what's going on here, since swiotlb_alloc_coherent() actually tries a regular __get_free_pages(flags, order) first, and when ZONE_DMA is set here, it just work without using the pool. Arnd