From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-wr0-f200.google.com (mail-wr0-f200.google.com [209.85.128.200]) by kanga.kvack.org (Postfix) with ESMTP id 0D3ED6B0005 for ; Tue, 24 Apr 2018 02:54:23 -0400 (EDT) Received: by mail-wr0-f200.google.com with SMTP id g7-v6so11879531wrb.19 for ; Mon, 23 Apr 2018 23:54:22 -0700 (PDT) Received: from newverein.lst.de (verein.lst.de. [213.95.11.211]) by mx.google.com with ESMTPS id b5-v6si10843345wrf.362.2018.04.23.23.54.21 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Mon, 23 Apr 2018 23:54:21 -0700 (PDT) Date: Tue, 24 Apr 2018 08:55:49 +0200 From: Christoph Hellwig Subject: Re: [PATCH 11/12] swiotlb: move the SWIOTLB config symbol to lib/Kconfig Message-ID: <20180424065549.GA18468@lst.de> References: <20180423170419.20330-1-hch@lst.de> <20180423170419.20330-12-hch@lst.de> <20180423235205.GH16141@n2100.armlinux.org.uk> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20180423235205.GH16141@n2100.armlinux.org.uk> Sender: owner-linux-mm@kvack.org List-ID: To: Russell King - ARM Linux Cc: Christoph Hellwig , Konrad Rzeszutek Wilk , iommu@lists.linux-foundation.org, linux-mips@linux-mips.org, linux-pci@vger.kernel.org, x86@kernel.org, linux-block@vger.kernel.org, linux-mm@kvack.org, sparclinux@vger.kernel.org, linux-arm-kernel@lists.infradead.org On Tue, Apr 24, 2018 at 12:52:05AM +0100, Russell King - ARM Linux wrote: > On Mon, Apr 23, 2018 at 07:04:18PM +0200, Christoph Hellwig wrote: > > This way we have one central definition of it, and user can select it as > > needed. Note that we also add a second ARCH_HAS_SWIOTLB symbol to > > indicate the architecture supports swiotlb at all, so that we can still > > make the usage optional for a few architectures that want this feature > > to be user selectable. > > > > Signed-off-by: Christoph Hellwig > > Hmm, this looks like we end up with NEED_SG_DMA_LENGTH=y on ARM by > default, which probably isn't a good idea - ARM pre-dates the dma_length > parameter in scatterlists, and I don't think all code is guaranteed to > do the right thing if this is enabled. We shouldn't end up with NEED_SG_DMA_LENGTH=y on ARM by default. It is only select by ARM_DMA_USE_IOMMU before the patch, and it will now also be selected by SWIOTLB, which for arm is never used or seleted directly by anything but xen-swiotlb. Then again looking at the series there shouldn't be any need to even select NEED_SG_DMA_LENGTH for swiotlb, as we'll never merge segments, so I'll fix that up.