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Message-ID: <20130122114221.02bb54b3@redhat.com> Date: Tue, 22 Jan 2013 11:42:21 -0200 From: Mauro Carvalho Chehab <mchehab@...hat.com> To: James Bottomley <James.Bottomley@...senPartnership.com> Cc: Geert Uytterhoeven <geert@...ux-m68k.org>, Marek Szyprowski <m.szyprowski@...sung.com>, Mark Salter <msalter@...hat.com>, Vineet Gupta <Vineet.Gupta1@...opsys.com>, linux-arch@...r.kernel.org, linux-c6x-dev@...ux-c6x.org, linux-kernel@...r.kernel.org, linux-media@...r.kernel.org, Tomasz Stanislawski <t.stanislaws@...sung.com> Subject: Re: [Linux-c6x-dev] [PATCH 3/9] c6x: Provide dma_mmap_coherent() and dma_get_sgtable() Em Tue, 22 Jan 2013 10:32:07 +0000 James Bottomley <James.Bottomley@...senPartnership.com> escreveu: > On Tue, 2013-01-22 at 10:16 +0000, James Bottomley wrote: > > [adding Mauro and v4l since they're the only non-arm consumers] > > On Tue, 2013-01-22 at 10:13 +0000, 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 > > > > > <James.Bottomley@...senpartnership.com> 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 > > > > > >> > <Vineet.Gupta1@...opsys.com> 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 <asm-generic/dma-mapping-common.h>. > > > > > >> > >>> > > > > > >> > >>> 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. > > > > Mauro, will this work for you and the v4l guys? You've got a use > > embedded in > > > > drivers/media/v4l2-core/videobuf2-dma-contig.c > > > > Which I can't tell how extensive it might be. > > Also, this implementation is really not done well. > > ppc has this: > > arch/powerpc/include/asm/dma-mapping.h:#define ARCH_HAS_DMA_MMAP_COHERENT > > But no other architecture does. > > Nothing is gated on this except pcm_native.c which has this wonderful > snippet: > > #ifndef ARCH_HAS_DMA_MMAP_COHERENT > /* This should be defined / handled globally! */ > #ifdef CONFIG_ARM > #define ARCH_HAS_DMA_MMAP_COHERENT > #endif > #endif > > Since there's no documentation at all, it's hard to tell what's going > on. > > However, I'd suggest > > 1. Move to an API that is at least capable of implementation on all > architectures > 2. Utilize ARCH_HAS_DMA_MMAP_COHERENT to fix the compile problems > within the architectures > 3. Document whatever is chosen so we don't end up in this mess > again. Agreed. I remember some efforts were done in the past years moving toward a common interface, when DMABUF were introduced (and people explicitly mentioned ARCH_HAS_DMA_MMAP_COHERENT on such discussions), like on this thread: http://www.mail-archive.com/linux-media@vger.kernel.org/msg30487.html While I followed part of that discussion, I didn't actually track the arm-specific patches. So, I'm not sure why ARCH_HAS_DMA_MMAP_COHERENT is not defined on arm. Maybe Tomasz or Marek may have more details. Regards, Mauro -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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