From mboxrd@z Thu Jan 1 00:00:00 1970 From: Christoph Hellwig Subject: Re: [PATCH 16/19] dma-iommu: don't depend on CONFIG_DMA_DIRECT_REMAP Date: Mon, 11 Feb 2019 17:39:36 +0100 Message-ID: <20190211163936.GA29978@lst.de> References: <20190114094159.27326-1-hch@lst.de> <20190114094159.27326-17-hch@lst.de> <4698cdca-0e5b-c82f-bb80-3ba0f986c544@arm.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Content-Disposition: inline In-Reply-To: <4698cdca-0e5b-c82f-bb80-3ba0f986c544@arm.com> Sender: linux-kernel-owner@vger.kernel.org To: Robin Murphy Cc: Christoph Hellwig , Joerg Roedel , Catalin Marinas , Will Deacon , Tom Lendacky , iommu@lists.linux-foundation.org, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org List-Id: iommu@lists.linux-foundation.org On Wed, Feb 06, 2019 at 11:55:49AM +0000, Robin Murphy wrote: > On 14/01/2019 09:41, Christoph Hellwig wrote: >> For entirely dma coherent architectures there is no good reason to ever >> remap dma coherent allocation. > > Yes there is, namely assembling large buffers without the need for massive > CMA areas and compaction overhead under memory fragmentation. That has > always been a distinct concern from the DMA_DIRECT_REMAP cases; they've > just been able to share a fair few code paths. Well, I guess I need to reword this - there is no _requirement_ to remap. And x86 has been happy to not remap so far and I see absolutely no reason to force anyone to remap. >> Move all the remap and pool code under >> CONFIG_DMA_DIRECT_REMAP ifdefs, and drop the Kconfig dependency. > > As far as I'm concerned that splits things the wrong way. Logically, > iommu_dma_alloc() should always have done its own vmap() instead of just > returning the bare pages array, but that was tricky to resolve with the > design of having the caller handle everything to do with coherency (forcing > the caller to unpick that mapping just to remap it yet again in the > noncoherent case didn't seem sensible). I don't parse this. In the old code base before this series iommu_dma_alloc is a relatively low-level helper allocating and mapping pages. And that one should have done the remapping, and in fact does so since ("dma-iommu: refactor page array remap helpers"). It just happens that the function is now called iommu_dma_alloc_remap. The new iommu_dma_alloc is the high level entry point that handles every possible case of different allocations, including those where we do not have a virtual mapping.