From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-7.5 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,MENTIONS_GIT_HOSTING,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED, USER_AGENT_SANE_1 autolearn=unavailable autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 8360EC06511 for ; Mon, 1 Jul 2019 14:09:51 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 66D112175B for ; Mon, 1 Jul 2019 14:09:51 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1729399AbfGAOJt (ORCPT ); Mon, 1 Jul 2019 10:09:49 -0400 Received: from foss.arm.com ([217.140.110.172]:35704 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1728016AbfGAOJt (ORCPT ); Mon, 1 Jul 2019 10:09:49 -0400 Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 75D6E344; Mon, 1 Jul 2019 07:09:48 -0700 (PDT) Received: from [10.1.197.57] (e110467-lin.cambridge.arm.com [10.1.197.57]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 82FB63F246; Mon, 1 Jul 2019 07:09:47 -0700 (PDT) Subject: Re: DMA-API attr - DMA_ATTR_NO_KERNEL_MAPPING To: Pankaj Suryawanshi , Christoph Hellwig Cc: linux-mm@kvack.org, iommu@lists.linux-foundation.org, linux-kernel@vger.kernel.org, Vlastimil Babka , Michal Hocko References: <20190626175131.GA17250@infradead.org> From: Robin Murphy Message-ID: Date: Mon, 1 Jul 2019 15:09:46 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.6.1 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-GB Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 28/06/2019 17:29, Pankaj Suryawanshi wrote: > On Wed, Jun 26, 2019 at 11:21 PM Christoph Hellwig wrote: >> >> On Wed, Jun 26, 2019 at 10:12:45PM +0530, Pankaj Suryawanshi wrote: >>> [CC: linux kernel and Vlastimil Babka] >> >> The right list is the list for the DMA mapping subsystem, which is >> iommu@lists.linux-foundation.org. I've also added that. >> >>>> I am writing driver in which I used DMA_ATTR_NO_KERNEL_MAPPING attribute >>>> for cma allocation using dma_alloc_attr(), as per kernel docs >>>> https://www.kernel.org/doc/Documentation/DMA-attributes.txt buffers >>>> allocated with this attribute can be only passed to user space by calling >>>> dma_mmap_attrs(). >>>> >>>> how can I mapped in kernel space (after dma_alloc_attr with >>>> DMA_ATTR_NO_KERNEL_MAPPING ) ? >> >> You can't. And that is the whole point of that API. > > 1. We can again mapped in kernel space using dma_remap() api , because > when we are using DMA_ATTR_NO_KERNEL_MAPPING for dma_alloc_attr it > returns the page as virtual address(in case of CMA) so we can mapped > it again using dma_remap(). No, you really can't. A caller of dma_alloc_attrs(..., DMA_ATTR_NO_KERNEL_MAPPING) cannot make any assumptions about the void* it returns, other than that it must be handed back to dma_free_attrs() later. The implementation is free to ignore the flag and give back a virtual mapping anyway. Any driver which depends on how one particular implementation on one particular platform happens to behave today is, essentially, wrong. > 2. We can mapped in kernel space using vmap() as used for ion-cma > https://github.com/torvalds/linux/tree/master/drivers/staging/android/ion > as used in function ion_heap_map_kernel(). > > Please let me know if i am missing anything. If you want a kernel mapping, *don't* explicitly request not to have a kernel mapping in the first place. It's that simple. Robin.