From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1758169AbaKULGc (ORCPT ); Fri, 21 Nov 2014 06:06:32 -0500 Received: from foss-mx-na.foss.arm.com ([217.140.108.86]:36379 "EHLO foss-mx-na.foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755473AbaKULGa (ORCPT ); Fri, 21 Nov 2014 06:06:30 -0500 Date: Fri, 21 Nov 2014 11:06:10 +0000 From: Catalin Marinas To: Arnd Bergmann Cc: "linux-arm-kernel@lists.infradead.org" , Will Deacon , "linux-kernel@vger.kernel.org" , Ding Tianhong Subject: Re: For the problem when using swiotlb Message-ID: <20141121110609.GC19783@e104818-lin.cambridge.arm.com> References: <5469E26B.2010905@huawei.com> <1497289.TPKf3qDiKk@wuerfel> <20141119154634.GE7156@e104818-lin.cambridge.arm.com> <3918100.vD6471iH9k@wuerfel> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <3918100.vD6471iH9k@wuerfel> Thread-Topic: For the problem when using swiotlb Accept-Language: en-GB, en-US Content-Language: en-US User-Agent: Mutt/1.5.23 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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. 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. -- Catalin