From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail.parisc-linux.org (palinux.external.hp.com [192.25.206.14]) (using TLSv1 with cipher DHE-RSA-AES256-SHA (256/256 bits)) (Client CN "mail.parisc-linux.org", Issuer "CAcert Class 3 Root" (not verified)) by ozlabs.org (Postfix) with ESMTPS id 286BBB71C4 for ; Sat, 24 Dec 2011 03:40:55 +1100 (EST) Date: Fri, 23 Dec 2011 09:35:16 -0700 From: Matthew Wilcox To: Marek Szyprowski Subject: Re: [PATCH 00/14] DMA-mapping framework redesign preparation Message-ID: <20111223163516.GO20129@parisc-linux.org> References: <1324643253-3024-1-git-send-email-m.szyprowski@samsung.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii In-Reply-To: <1324643253-3024-1-git-send-email-m.szyprowski@samsung.com> Cc: linux-mips@linux-mips.org, linux-ia64@vger.kernel.org, linux-sh@vger.kernel.org, linux-mm@kvack.org, sparclinux@vger.kernel.org, linux-arch@vger.kernel.org, Stephen Rothwell , Jonathan Corbet , x86@kernel.org, Arnd Bergmann , microblaze-uclinux@itee.uq.edu.au, linaro-mm-sig@lists.linaro.org, Andrzej Pietrasiewicz , Thomas Gleixner , linux-arm-kernel@lists.infradead.org, discuss@x86-64.org, linux-kernel@vger.kernel.org, Kyungmin Park , linux-alpha@vger.kernel.org, Andrew Morton , linuxppc-dev@lists.ozlabs.org List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , On Fri, Dec 23, 2011 at 01:27:19PM +0100, Marek Szyprowski wrote: > The first issue we identified is the fact that on some platform (again, > mainly ARM) there are several functions for allocating DMA buffers: > dma_alloc_coherent, dma_alloc_writecombine and dma_alloc_noncoherent Is this write-combining from the point of view of the device (ie iommu), or from the point of view of the CPU, or both? > The next step in dma mapping framework update is the introduction of > dma_mmap/dma_mmap_attrs() function. There are a number of drivers > (mainly V4L2 and ALSA) that only exports the DMA buffers to user space. > Creating a userspace mapping with correct page attributes is not an easy > task for the driver. Also the DMA-mapping framework is the only place > where the complete information about the allocated pages is available, > especially if the implementation uses IOMMU controller to provide a > contiguous buffer in DMA address space which is scattered in physical > memory space. Surely we only need a helper which drivrs can call from their mmap routine to solve this? > Usually these drivers don't touch the buffer data at all, so the mapping > in kernel virtual address space is not needed. We can introduce > DMA_ATTRIB_NO_KERNEL_MAPPING attribute which lets kernel to skip/ignore > creation of kernel virtual mapping. This way we can save previous > vmalloc area and simply some mapping operation on a few architectures. I really think this wants to be a separate function. dma_alloc_coherent is for allocating memory to be shared between the kernel and a driver; we already have dma_map_sg for mapping userspace I/O as an alternative interface. This feels like it's something different again rather than an option to dma_alloc_coherent. -- Matthew Wilcox Intel Open Source Technology Centre "Bill, look, we understand that you're interested in selling us this operating system, but compare it to ours. We can't possibly take such a retrograde step."