Messages in this thread | | | Date | Thu, 24 Jan 2013 11:49:52 +0100 | From | Marek Szyprowski <> | Subject | Re: [Linux-c6x-dev] [PATCH 3/9] c6x: Provide dma_mmap_coherent() and dma_get_sgtable() |
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On 1/21/2013 9:00 PM, Geert Uytterhoeven wrote: > On Tue, Jan 15, 2013 at 5:56 PM, James Bottomley > <James.Bottomley@hansenpartnership.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@synopsys.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? > > 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.
I think this would be the best solution for fixing v3.8. I would like to put those patches in linux-next asap and then after a week or so send a pull request for merging them to v3.8-rc6.
EINVAL approach can be later fixed/extended with real implementation (if there are any driver candidates, which will use such interface).
Best regards -- Marek Szyprowski Samsung Poland R&D Center
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