From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754727Ab3AWJrT (ORCPT ); Wed, 23 Jan 2013 04:47:19 -0500 Received: from mailout3.w1.samsung.com ([210.118.77.13]:36124 "EHLO mailout3.w1.samsung.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754642Ab3AWJrO (ORCPT ); Wed, 23 Jan 2013 04:47:14 -0500 MIME-version: 1.0 Content-transfer-encoding: 8BIT Content-type: text/plain; charset=UTF-8; format=flowed X-AuditID: cbfec7f5-b7fd76d000007247-a7-50ffb1a0d336 Message-id: <50FFB19E.3020901@samsung.com> Date: Wed, 23 Jan 2013 10:47:10 +0100 From: Marek Szyprowski User-Agent: Mozilla/5.0 (Windows NT 5.1; rv:17.0) Gecko/20130107 Thunderbird/17.0.2 To: James Bottomley Cc: Geert Uytterhoeven , Mark Salter , Vineet Gupta , linux-arch@vger.kernel.org, linux-c6x-dev@linux-c6x.org, linux-kernel@vger.kernel.org Subject: Re: [Linux-c6x-dev] [PATCH 3/9] c6x: Provide dma_mmap_coherent() and dma_get_sgtable() References: <1358073890-3610-1-git-send-email-geert@linux-m68k.org> <1358073890-3610-3-git-send-email-geert@linux-m68k.org> <1358177872.4357.53.camel@t520.localdomain> <50F4D83A.7020803@synopsys.com> <50F56286.8070200@samsung.com> <1358269008.10591.11.camel@dabdike.int.hansenpartnership.com> <1358809159.3975.63.camel@dabdike.int.hansenpartnership.com> <1358849633.2387.11.camel@dabdike.int.hansenpartnership.com> In-reply-to: <1358849633.2387.11.camel@dabdike.int.hansenpartnership.com> X-Brightmail-Tracker: H4sIAAAAAAAAA+NgFupnluLIzCtJLcpLzFFi42I5/e/4Vd0FG/8HGNxdzWnRsesri8XlXXPY HJg8Pm+SC2CM4rJJSc3JLEst0rdL4MpY/mAve8EFvYrjvzawNDCeUeli5OSQEDCR+HTkOTOE LSZx4d56ti5GLg4hgaWMErM/zmIHSfAKCEr8mHyPpYuRg4NZQF7iyKVskDCzgJnEo5Z1zBD1 zUwS2289ZgSp4RXQkjh7NgrEZBFQlTg7Lx+knE3AUKLrbRcbiC0qECqxdt8asLUiAtYSX1Yt YAQZwyxwnVFi+5HJTCAJYYFEiZMX9rJAzN/BItG6bzJYN6eAp8Snzz+YJjAKzEJy3iyE82Yh OW8BI/MqRtHU0uSC4qT0XCO94sTc4tK8dL3k/NxNjJBA/LqDcekxq0OMAhyMSjy8jqv/Bwix JpYVV+YeYpTgYFYS4VVZCxTiTUmsrEotyo8vKs1JLT7EyMTBKdXAeNbwQ/2SpK8fzU0fyjx1 3b+/5UygetSrhbHKIX3JqZts2YKuZXnsqW3fVy7ef+Zk8Xvjwr7SzdWZm2bksQv/eOXTL14a 5sO7Q4vDkFtM9sOrBHuGjN+XrbsZItoFD3049eh2wD7hI/WVUQpXmr2TmZj2dYi32UxoX+/R db58/w+mr/vO736gxFKckWioxVxUnAgA/PP1ryICAAA= Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 1/22/2013 11:13 AM, James Bottomley wrote: > On Mon, 2013-01-21 at 22:59 +0000, James Bottomley wrote: > > On Mon, 2013-01-21 at 21:00 +0100, Geert Uytterhoeven wrote: > > > On Tue, Jan 15, 2013 at 5:56 PM, James Bottomley > > > wrote: > > > > On Tue, 2013-01-15 at 15:07 +0100, Marek Szyprowski wrote: > > > >> On 1/15/2013 10:13 AM, Geert Uytterhoeven wrote: > > > >> > Marek? > > > >> > > > > >> > On Tue, Jan 15, 2013 at 5:16 AM, Vineet Gupta > > > >> > wrote: > > > >> > > On Monday 14 January 2013 09:07 PM, Mark Salter wrote: > > > >> > >> On Sun, 2013-01-13 at 11:44 +0100, Geert Uytterhoeven wrote: > > > >> > >>> c6x/allmodconfig (assumed): > > > >> > >>> > > > >> > >>> drivers/media/v4l2-core/videobuf2-dma-contig.c: In function ‘vb2_dc_mmap’: > > > >> > >>> drivers/media/v4l2-core/videobuf2-dma-contig.c:204: error: implicit declaration of function ‘dma_mmap_coherent’ > > > >> > >>> drivers/media/v4l2-core/videobuf2-dma-contig.c: In function ‘vb2_dc_get_base_sgt’: > > > >> > >>> drivers/media/v4l2-core/videobuf2-dma-contig.c:387: error: implicit declaration of function ‘dma_get_sgtable’ > > > >> > >>> > > > >> > >>> For architectures using dma_map_ops, dma_mmap_coherent() and > > > >> > >>> dma_get_sgtable() are provided in . > > > >> > >>> > > > >> > >>> C6x does not use dma_map_ops, hence it should implement them as inline > > > >> > >>> stubs using dma_common_mmap() and dma_common_get_sgtable(). > > > >> > >>> > > > >> > >> So are dma_mmap_coherent() and dma_get_sgtable() part of the DMA API > > > >> > >> now? I don't them in Documentation/DMA*.txt anywhere. > > > >> > >> > > > >> > >> Why does the default dma_common_mmap() for !CONFIG_MMU return an > > > >> > >> error? > > > >> > >> > > > >> > >> Wouldn't it be better to provide default implementations that an arch > > > >> > >> could override rather than having to patch all "no dma_map_ops" > > > >> > >> architectures? > > > >> > >> > > > >> > > Speaking for the still-reviewed ARC Port, I completely agree with Mark. > > > >> > > > >> dma_mmap_coherent() was partially in the DMA mapping API for some time, but > > > >> it was available only on a few architectures (afair ARM, powerpc and avr32). > > > >> This caused significant problems for writing unified device drivers or some > > > >> device helper modules which were aimed to work on more than one > > > >> architecture. > > > >> > > > >> dma_get_sgtable() is an extension discussed during the Linaro meetings. It > > > >> is required to correctly implement buffer sharing between device driver > > > >> without hacks or any assumptions about memory layout in the device drivers. > > > >> > > > >> I have implemented some generic code for both of those two functions, > > > >> keeping > > > >> in mind that on some hardware architectures (like already mentioned VIVT) > > > >> it might be not possible to provide coherent mapping to userspace. It is > > > >> perfectly fine for those functions to return an error in such case. > > > > > > > > It's not possible on VIPT either. This means that the API is unusable > > > > on quite a large number of architectures. Surely, if we're starting to > > > > write drivers using this, we need to fix the API before more people try > > > > to use it. > > > > > > > > For PA-RISC (and all other VIPT, I assume) I need an API which allows me > > > > to remap the virtual address of the kernel component (probably using the > > > > kmap area) so the user space and kernel space addresses are congruent. > > > > > > So what are we gonna do for 3.8? I'd like to get my allmodconfig build > > > green again ;-) > > > > > > Change the API? > > > > Well, if we want the API to work universally, we have to. As I said, > > for VIPT systems, the only coherency mechanism we have is the virtual > > address ... we have to fix that in the kernel to be congruent with the > > userspace virtual address if we want coherency between the kernel and > > userspace. > > > > However, if it only needs to work on ARM and x86, it can stay the way it > > is and we could just pull it out of the generic core. > > > > Who actually wants to use this API, and what for? > > > > > Keep the API but do a best-effort fix to unbreak the builds? > > > - Apply my patches that got acks (avr32/blackfin/cris/m68k), > > > - Use static inlines that return -EINVAL for the rest > > > (c6x/frv/mn10300/parisc/xtensa). > > > I still have a few m68k fixes queued for 3.8, for which I've been postponing the > > > pull request to get this sorted out, so I could include the above. > > > > > > Any other solution? > > > > If it's an API that only works on ARM and x86, there's not much point > > pretending it's universal, so we should remove it from the generic arch > > code and allow only those architectures which can use it. > > There might be a simple solution: just replace void *cpu_addr with void > **cpu_addr in the API. This is a bit nasty since compilers think that > void ** to void * conversion is quite legal, so it would be hard to pick > up misuse of this (uint8_t ** would be better). That way VIPT could > remap the kernel pages to a coherent address. This would probably have > to change in the dma_mmap_attr() and dma_ops structures. > > All consumers would have to expect cpu_addr might change, but that seems > doable. I still don't get how this can help having a coherent buffer between DMA (devices) and CPU (kernel and user space mappings). The main purpose of the dma_mmap_coherent() call is to provide a common API for mapping a coherent buffer between DMA (device) and userspace. It is not about creating a coherent mapping for sharing memory between userspace and kernel space. Best regards -- Marek Szyprowski Samsung Poland R&D Center